Party Cohesion in Westminster Systems
Inducements, Replacement and Discipline in the House of Commons,
1836–1910∗
Andrew C. Eggers†

Arthur Spirling‡

April 29, 2014

Abstract

We consider the historical development of a characteristic crucial for the functioning
and normative appeal of Westminster systems: cohesive legislative parties. To do this,we gather the universe of the twenty thousand parliamentary divisions that took place
between 1836 and 1910 in the British House of Commons, construct a voting record
for every Member of Parliament serving during this time, and carry out analysis thataims to both describe and explain the development of cohesive party voting. In linewith previous work, we show that—with the exception of a chaotic period in the 1840s
and 1850s—median discipline was always high and increased throughout the century.
We use novel methods to demonstrate that much of the rise in cohesion results fromthe elimination of a rebellious ‘left tail’ from the 1860s onwards, rather than central
tendency shifts. In explaining the aggregate trends, we use panel data techniques andnote that there is scant evidence for ‘replacement’ explanations that involve new intakesof members behaving in more disciplined ways than those leaving the chamber. We offer
evidence that more loyal MPs were more likely to obtain ministerial posts, and speculatethat this and other ‘inducement’-based accounts offer more promising explanations ofincreasingly cohesive parties.
∗

First version: March 2013. This version: April 29, 2014. Kathy Wang, Tania Amarillas, and Jack Blumenau
provided excellent research assistance. We are grateful to the History of Parliament Trust for data. We also gratefully
acknowledge the financial support of STICERD and IQSS. Michael Gill, Brandon Stewart, audiences at the IQSSApplied Statistics Seminar, the Australian National University and Midwest Political Science Association provided
helpful comments on an earlier draft. All data used for this paper will be made available on the DVN network on
publication.
†
Lecturer, London School of Economics. email: a.c.eggers@lse.ac.uk‡
Associate Professor, Department of Government, Harvard University. email: aspirling@gov.harvard.edu (submitting and corresponding author for journal)

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1

Introduction

Strong party discipline is a defining feature of legislative politics in Westminster systems (Lijphart,
1999; Spirling and McLean, 2007; Kam, 2009b). In modern times, parliamentary divisions (‘rollcalls’ in American parlance) are characterized as predictable affairs insofar as members of parliament (MPs) can be expected to vote in line with their parties’ explicit orders, and rebellion
against the ‘whip’ is a relatively rare event (see Cowley, 2002, for an overview). One consequenceof party cohesion, in tandem with an electoral system that yields a ‘manufactured majority’ ofseats (Bogdanor and Butler, 1983), is that governing parties almost never lose votes important totheir legislative agenda. This arrangement facilitates the particular vision of “responsible” government (Birch, 1964; Gamble, 1990; Campbell and Wilson, 1995; Richards and Smith, 2002) and“accountability” (in the sense of Powell, 2000) that has allowed unusual longevity and diffusionof the Westminster system from its home in Britain to other polities (Rhodes and Weller, 2005;
Rhodes, Wanna and Weller, 2009; Godbout and Hoyland, 2013).

Given the centrality of cohesive parties to both the functioning of the Westminster system andto the normative justification of its use, it is unsurprising that a great deal of political science attention has been lavished upon them. Starting as early as Lowell (1902), scholarship has focussed
on the 19th century as a transitional time during which party cohesion—and thus the Westminster
system—took hold in the House of Commons (e.g. Bagehot, 1873/2011; Trevelyan, 1922; Aydelotte,
1963; Berrington, 1968; Cromwell, 1982; Cox, 1987; Jenkins, 1996; Rush, 2001; Schonhardt-Bailey,2003). While researchers have come to a clear consensus that discipline did indeed increase, thecentral question of why MPs became more cohesive has lacked a convincing answer. Though weare not without theories (with Cox, 1987, being perhaps the most comprehensive), demonstrating conclusively which particular mechanism explains the patterns we see has remained an elusive
goal. To the extent that understanding parliamentary discipline in modern times is an important
goal in political science (e.g. Norton, 1978; Diermeier and Federsen, 1998; Cowley, 2005; Benedettoand Hix, 2007) and to the extent that modern politics relies upon it (e.g. Economist, 2012), there
is obvious interest in improving on this state of affairs. This paper represents such an improvement.
2

At its core, the question is this: did parties become more cohesive primarily because MPs who
were previously less disciplined became more disciplined as the century progressed; or was the increase in cohesion due to more disciplined ‘types’ of individuals entering the Commons while lessdisciplined ‘types’ left it? The former story implies that change was mostly due to factors acting
on already existing members—like increasingly efficient whipping, a greater sense of solidarity, better agenda control by leaders, or responsiveness to a party orientated electorate. We denote thisan ‘inducements’ account, insofar as extant members were encouraged to behave differently. The
latter story suggests that it is newcomers to the chamber who brought with them cohesion, whilea rebellious ‘old guard’ were replaced by natural wastage. We denote this a ‘replacement’ account,insofar as it is turnover of latent traits that drives any changes in cohesion. It is both fundamental
and difficult to determine how much each of these mechanisms contributed to the developmentof cohesive parties in the House of Commons. That previous work has been unable to decisively
resolve the issue is in part due to methodological shortcomings, but mostly due to issues of data
availability. Here, we solve both problems using novel statistical methods, panel data analysis, anda new collection of roll call records.

Our data consists of every one of the 20,262 roll calls cast between 1836 and 1910. We have arecord of every member’s decision on every division. By combining this voting information with a
new data set that incorporates extensive individual-level covariates on the thousands of MPs serving during this time, we are able to explore discipline in more nuanced ways than previous studies
that used aggregated statistics (e.g. Lowell, 1902; Berrington, 1968; Cox, 1987) ever could. Similarly, we are able to chart the changes to cohesion over a much broader period than the intensive,
but time-limited studies (e.g. Adyelotte, 1954; Cromwell, 1982; McLean, 2001; Schonhardt-Bailey,2003) preceding ours. In short, we have the best of both worlds: micro-data as a long time series. In
the first part of the paper, we characterize cohesion over time using ‘relative distribution methods’(Handcock and Morris, 1998, 1999) that give a more complete account of how cohesion changed
from parliament to parliament than is possible with means and standard deviations; these methods

3are both non-parametric and robust to outliers (a common occurrence in histories of legislative voting). In the second part of the paper we use panel data methods to assess the possible mechanisms
by which the changes in cohesion actually occurred.

We find that voting cohesion began high in the 1830s, waned in the 1840s, and then grew thereafter. This observation is commensurate with earlier accounts, though we are able to be much more
precise in our discussion of changes. Importantly, we show that cohesion was always relatively highin absolute terms for the average MP, and that, by the 1860s, almost all growth in aggregate partyline voting is due to the disappearance of a rebellious ‘left tail’ from the distribution of cohesion.
We then show that this ‘folding in’ of the rebel tail is not due to new blood entering the Houseof Commons or from older less-malleable types retiring out. Instead, we demonstrate that sitting
MPs became more cohesive as the Victorian age wore on, examining ministerial promotion as one‘inducements’-based explanation for increasingly cohesive voting. This pattern is consistent withtheories of ‘endogenous’ change, whereby internal reforms and incentives (in the sense of e.g. Cox,
1987; Kam, 2009b) matter more than sociological adjustments to MP stock (in the sense of e.g.
Rush, 2001).

Our paper continues as follows: in Section 2 we review the literature pertinent to the questionat hand and set up our empirical tasks; in Section 3 we introduce the data with which we willaddress those tasks. In Section 4 we use relative distribution methods to characterize the change
in cohesion over time. In Section 5 we assess the role of replacement and in Section 6 we evaluate
one possible ‘inducements’ channel. Section 7 concludes.

2

Literature and Orientation

The period between the first Reform Act of 1832 and the fourth Reform Act of 1918 has been citedby scholars as the key period of development for the both the Westminster system and the British
polity as a whole (e.g. Cox and Ingram, 1992; Jenkins, 1996; McLean, 2001; Rush, 2001). Central tothis story is increased party cohesion in legislative voting, and scholarly interest has focussed on two
4

distinct but related questions therein. First, researchers have sought to document how roll call behavior changed in a descriptive sense; second, they have sought to understand exactly why it did so.

Scholars have described changing cohesion using different tools (e.g. Lowell, 1902; Berrington,1968), typically concentrating on aggregate measures like ‘proportion of divisions in which the two(major) parties opposed one another’, or on simple averages—like the proportion of divisions inwhich given MPs voted with their whips (e.g. Cox, 1987). Cox (1987, 26) summarizes the consensus
finding of the literature when he notes that “For the Conservatives, there is a sharp decline [in the
1830s and 1840s]. . . a plateau in the 1850s; and a sharp recovery [later]. . . ” Meanwhile, “[f]or the
Liberals, there is a much smaller decline. . . followed by an erratic increase”. While univariate averages are helpful for outlining general trends, using them alone conceals other potentially important
distributional features that characterize the nature of the change over time. For example, measuresof central tendency say nothing per se about the relative proportion of the most and least cohesive
members, nor about the magnitude of their loyalty and rebeliousness respectively. This means, forinstance, that we cannot know whether a period of ‘low’ party unity is due to a chamber of mostly‘independent’ voting haphazardly from the whips’ perspective (Jenkins, 1996, Ch3) (resulting in ahigh variance spread around the mean), or due to a small but persistent group of ‘rebels’—such as
the ‘Fourth Party’—who actively seek to vote against the party line (a left tail that drags meancohesion down). In short, being able to describe the evolving features of roll call voting in a more
complete, subtle and precise manner allows researchers to know both ‘where to look’ for possible
causal relationships, and how to interpret any evidence they do find.

Whatever the descriptive statistics, it is unclear exactly why cohesive parties increasingly became
the norm. Explaining the pattern obviously requires understanding how MPs both sorted themselves into parties and why they behaved the way they did once sorted. As previewed above, one
way to organize work on emerging unity is into ‘inducement’ or ‘replacement’ accounts, the former
dealing with encouragements for MPs with given (possibly latent) characteristics to behave in different ways, the latter focussing on the role of new MPs entering the chamber over time. Scholars

5in the ‘inducement’ tradition have, for example, emphasized the importance of material interests
as prompts for voting in the 1840s (Adyelotte, 1954; Schonhardt-Bailey, 2003) with others noting
the importance of rhetorical manipulation in the same period (McLean, 2001). Related literaturespoint to electoral reform as ushering in the “triumph of partisan politics” in the later Victorian era,with polarization of the electorate having knock-on consequences for the British House of Commons
itself (Jenkins 1996, see also Cox and Ingram 1992). With a view to the broader time period andmore attention to internal reform, Cox (1987) provides a rare example of work that relies on datacollected over a relatively long period to link external changes to suffrage and individual motivations for division behavior. His particular emphasis is on Cabinet promotion opportunities as areturn for good behavior in the Commons, and is perhaps the prototypical ‘inducement’ account(see e.g. Benedetto and Hix, 2007, for modern work echoing the central point). Connected to suchreasoning is a broader literature emphasizing the importance of campaign finance arrangements ina more modern democratic age (Hanham, 1978; Kam, 2009a; Stokes, 2011) and the need to avoid
falling into bad odor with a central party administration.

In contrast to these researchers, scholars of ‘replacement’ have pursued a different tack, notingchanges in the personal socioeconomic circumstances of MPs during this period, without particular attention to ‘interests’ per se. Rush (2001), for example, describes the pronounced decline
in members with aristocratic links as the century wore on (see also Cannadine, 1996), and theircommensurate replacement with businessmen and representatives of the (legal) professions (see also
Lawrence, 1998, on the changing demographics of radical MPs). The fundamental logic of suchwork appears in some of the accounts above too, albeit implicitly: Cox (1987, 68–79), for example,
notes the importance of ‘ministerial ambition’ in explaining why men would seek to enter parliament in the second half of the Victorian age, with obvious implications for ‘types’ of MPs likely toappear over time. Note too that accounts that rely on period-specific or class-specific inducements,
like Schonhardt-Bailey (2003), would presumably accept that as different individuals with different
interests enter the Commons, voting behavior is likely to change. In part due to its obvious simplicity and plausibility, ‘replacement’ stories have featured in accounts for roll call behavior change

6well beyond Westminster systems: the ‘conditional party government’ model (Aldrich and Rohde,
2000) relies in part on a post-Watergate surge of liberal Democrats in the House replacing moreconservative southern representatives.

For foregoing scholars, deciding which account of the changes is more plausible (or in what proportion)—‘inducements’ vs ‘replacement’—was a tall order since it implicitly required records of individual
MP behavior, over a long time period. That is because the debate can only be resolved by knowing
whether MPs became more disciplined over their career in the House, and whether or not thatevolution outweighs (in a statistical sense) the effect of new members entering the Commons who
were ‘already’ more cohesive in type. Put more simply, we need panel data and previous (sometimes Herculean) efforts have either limited the scope to one or two parliaments with records at the
MP-level (e.g. Adyelotte, 1954; McLean, 2001; Schonhardt-Bailey, 2003), or have looked at a longer
time period in aggregate terms without attention to unit (MP) level variation (e.g. Lowell, 1902;
Berrington, 1968; Cox, 1987). Studies that collect disaggregated voting data over a considerable
period (e.g. Cromwell, 1985) do so by covering only certain years or certain divisions.

As we explain in the next two sections, we address shortcomings of previous work in terms ofdescriptive analysis and inference: we bring to bear the entire universe of MP decisions on roll calls
for the period 1836–1910 and new methods to study them. This allows us to better present thechanging pattern of aggregate behavior, and to model cohesion ‘within’ MP careers. Thus, if we
believe that latent preferences are essentially fixed, and we can show that cohesion increases are not
the product of replacement effects, we can go some way to showing that an inducements story—in
which whipping, rhetoric, agenda control, leadership and other such factors became more powerfulmotivational tools—is more convincing. More formally, our intention is to
1. fully characterize the changes in party division behavior during this period, using methods
that allow us to comment on the ‘overall’ nature of unity over time, and the relative andabsolute solidarity of the most and least cohesive Members;
2. examine the association between being entering or exiting the House of Commons and voting
7cohesion. If the replacement story has merit, fresh entrants ought to, ceteris paribus, be moredisciplined than the mass of colleagues they join who are already incumbents and departing
MPs ought to be less so;
3. examine the association between seniority and cohesive voting for individual MPs over time.
‘Inducements’ theories imply that the average individual MP should vote more in line withhis party as his career progresses from parliament to parliament; if ‘replacement’ theories
have merit, this seniority effect should be small relative to the increase in average cohesion
across members from parliament to parliament;
4. examine with more complete data a question previously raised and studied by Cox (1987):
did party cohesion increase partly because ministerial posts were offered to more loyal party
voters? 1
We explain each of these tasks more fully in subsequent sections. Before doing so, note that while
we recognize there are other inducement mechanisms for good behavior, and in the Discussion
we return to what those might be, this paper focusses particularly on ministerial promotion forseveral reasons. First, because both the theory (e.g. Dowding, Berlinski and Dewan, 2012) andempirics on cabinet personnel management (e.g. Benedetto and Hix, 2007) are well studied, andpart of a mature literature thought to explain much of the discipline seen in the House of Commons.
Second, despite some pioneering efforts to measure ‘leadership’ via less observable channels like theuse of rhetoric (e.g Moser and Reeves, 2013), obtaining reasonable estimates of the effects of otherinducement mechanisms is prohibitively difficult—especially for historical data.
1

In modern politics, the threat is not simply that an MP may not be promoted for shows of disloyalty, it is that
one may be deselected as a candidate. For the period under study, assessing such a threat is very difficult because we
do not have the quantity or quality of historical records necessary to know why candidates ran (or did not run) for
office at particular elections. Nonetheless, it is perhaps worth noting as per e.g. Cox (1987) that the effect of local or
central party organizations on candidate selection was not a strong influence until the start of the twentieth century(see also Hanham, 1978, ch 7).

8

3

Data

The current project requires roll call votes of MPs in the House of Commons, or ‘divisions’ as they
are known for this particular legislature. As noted, several authors have collected roll calls for the
19th century prior to our efforts, but we are much more comprehensive in both depth and breadth.
In particular, we gathered every individual roll call vote for the entire period 1836–1910.

Our raw data was supplied by the History of Parliament Trust, which had commissioned a major
digitization project to convert published division lists into machine-readable data. We linked MPs
to covariates in a relational database, as described in Eggers and Spirling (Forthcominga), and wewere compelled to undertake substantial disambiguation of names such that the roll calls could be
used for the purposes below. Appendix A provides more details.

For reasons that will become clear below, we required the identity of the head (‘Chief’) whips
for the two major parties (Liberals and Conservatives for this period) at any given time. We foundannual listings of their identities in Cook and Keith (1975) and Butler and Butler (1994) but the
precise resignation and promotion dates for these officers is unknown. While it is generally the casethat government Chief Whips are made parliamentary secretary to the Treasury, obtaining dates
for this office proved no easier. And, in any case, there is no such aliased information for Chief
Whips of non-government parties at this time. Some scholars have made efforts to expand andrefine this information (e.g. Sainty and Cox, 1997), though in such schemes (educated) speculationis required to ‘fill in’ missing details, especially for secondary whips assisting the head official. As
a result, we occasionally found months of roll calls within years in which the particular whip citedin aggregate records did not appear to be serving as an MP (and we therefore assume he was notin fact whip during this period). To circumvent this problem, we simply assumed that anyone who
at some point served as a Chief Whip during the parliament in question represented the ‘party
line’ when voting in the way we specify below—that is, as a ‘teller’. Thus, if there were two Tories
serving as Chief Whips in Parliament A, we check the names of both against the relevant divisionlists (actually, the tellers) in order to update our understanding of the Conservative party position9

on a given vote. Empirically, treating all Chief Whips who served in a period as embodying theparty line on a given day may be correct or not (there is no way of knowing with current data), but
it is unlikely to cause problems for our measure below since, presumably, those who become Chief
Whip vote in a way congruent to that office immediately preceding or after their occupation of it.2

A natural concern for scholars of parliaments pertains to the selection process by which propositions make it to a vote at all. By definition, our division records from the History of Parliament
Trust do not include those unobserved cases which were put to a voice-vote (only), or were neverpursued in the lobbies. It is certainly the case that there are both more divisions over time, andthat there are more divisions that the government (and the opposition) whip over time: startingwith around 30% of the 300-700 divisions in the early period, rising to around 70% of the 700
or so votes per parliament by the 1850s, and up to 90% of 1500-odd divisions by the end of thedata. Ultimately though, notice that our approach compares only like-with-like: that is, we arecalculating the cohesion of parties based on those votes on which the Chief Whip acts as a teller.
Thus, while it is true that ‘private members’ found it much harder to introduce business after the
Cabinet’s agenda reforms during the 19th and early 20th centuries (see Cox, 1987, ch6), any private
member bill without explicit government backing should not appear as one of our data points.

3.1

Measuring Cohesion

Measuring the cohesiveness of voting in British parliamentary parties is a long-standing methodological problem (see, e.g., MacRae, 1970, for an overview). In contrast to most work on the subject,
our goal in the current paper is to shed light on the way that individuals behave with respect totheir parties, and not on the aggregate behavior of parties per se.3 . We begin from the premise thatan MP votes in line with his party when he votes in line with his party Chief Whip acting as a‘teller’ for that division (see Cox, 1992, for discussion of this indicator of discipline). Our measure
2

In principle, we might have situations where Chief Whips ‘tell’ in opposite directions, though this doesn’t seemto be the case from inspection of our records, so we are not concerned with miscodings in this sense.
3
In this literature, the Rice Index is the most commonly used such approach, defining the discipline on division i|ayes −noes |as ayesi +noesii (see Desposato, 2003, 2005; Hix, Noury and Roland, 2005, also). In early work, Lowell (1902) usesithe proportion of roll calls in which at least 90% of one party are voting in opposition to 90% of another.

10

of cohesion for a given MP at a given time is a simple proportion: the number of times he votes
the same way as his whip (‘aye’ when the whip votes ‘aye’, ‘no’ when the whip votes ‘no’), divided
by all the times both he and his whip state a preference (i.e. we ignore missing values in termsof calculating both the numerator and denominator of this measure. A ‘discipline distribution’ or‘cohesion distribution’ as alluded to below is then just the aggregated set of all these individual
scores, for all the relevant MPs in a session. We give more details of our measurement procedurein Appendix B.

Implicit here is the assumption that the Chief Whip receives orders directly (or as if directly)
from his party leader. It could conceivably be the case that a particular roll call was not whipped,
yet the Chief Whip chose to vote as a teller. In that case, voting with the Chief Whip overstates
the degree of discipline pertaining to a specific MP. But such occurrences are likely to be extremely
rare, since Chief Whips are agents of leadership throughout this period and unlikely to waste timeon more frivolous matters that come up for roll call votes. Alternatively, it may be the case that,on a particular whipped roll call, the Chief Whip (or one of his close associates) fails to act asteller for some idiosyncratic reason, yet gives instructions to the MPs (which we cannot observe).
To the extent that voting behavior in such circumstances is similar to that in other whipped votes(and/or these circumstances are rare), excluding these cases does not affect our results.

As our comments above imply, we restrict the analysis that follows to the two major governing
parties at this time: the Conservatives and Liberals. While we have information on MPs fromother parties, we typically do not know their whipping arrangements, and the process is especiallycomplicated for some ‘break-away’ factions like the Liberal Unionists who spend some time in the
19th century voting with the Conservative whip. Thus our restriction avoids such ambiguities.

In Figure 1, we report boxplots representing the distribution of cohesion scores for MPs overtime. Recall that the fundamental unit of analysis is a given MP’s agreement with his whip (i.e.
we are not plotting aggregated division-based scores here). The thick black bars represent medians,

11and it is clear that while median behavior becomes less disciplined in the 1840s,4 it approaches
1 thereafter, reaching its zenith at the end of the data.5 The grey points represent outliers who
are least disciplined: MPs whose record puts them into the tail of their parliament’s distribution.
Looking more closely, we see something of a discrete shift in cohesion around 1868, wherein the
median ‘jumps’ to just shy of its maximum, while the implied spread of discipline begins to packmuch more tightly around the average. While the pattern observed—more disciplined MPs packed
more tightly around a higher median—is relatively intuitive, we wish to be more formal about thechanges to the distributions. In the next section, we do just that.

4

Describing the Rise in Party Cohesion

Our first task is to compare the distribution of MPs, in terms of their obedience, unity or cohesiveness, over time. To do this, we employ ‘relative distribution’ methods, in the sense of Handcock andMorris (1998). The essence of the approach is to transform the data from two distribution—here,
sets of MP agreement scores—into one distribution, such that we can compare them clearly andeasily. The resulting ‘relative distribution’ consists of the percentile rank that every member inthe first dataset would have in the second. Suppose, for example, that we broke up our data from
Figure 1 into two parts: all the cohesion scores for the parliaments prior to 1874 as the first part,and the cohesion scores for the parliament of 1874 as the second part. Inspection suggests that,generally speaking, those in the earlier period would have relatively low percentile ranks in thesecond. We can be formal about this comparison and Handcock and Morris (1998) suggest two
ways to do so.6 First, we can decompose any distribution change between two sets of data into
a location and shape change component—and we can compare the relative contribution of each4

The 1840s is a well documented time of relative tumult in the House of Commons, as Peelites left the ruling Tory
party to join Whigs and Radicals in the Liberal party, just after the passage of the corn laws (see McLean, 2001, for
discussion). One consequence was that for a period, nominally Conservative MPs could in principle be part of morethan one voting bloc in the chamber, with obvious consequences for cohesion as we measure it.
5
For comparison, the Blair government of 1997–2001 and the Coalition government after 2010 has a similar median(where agreement is measured as proportion of times MPs vote on the same side of the issue as their whip, rather
than via the Chief-Whip-as-teller metric) to the latter part of the data set under study here.
6
Readers are likely familiar with more basic techniques for comparing distributions such as the Kolmogorov–
Smirnov test. While useful for determining whether, in fact, distributions differ, such approaches can say little about
how distributions differ.

1213Figure 1: Aggregate discipline in the House of Commons over time: distributions of individual
scores. Measure bounded between 0 and 1, with 1 being most disciplined.

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part. The ‘location’ component refers to the notion of a ‘shifted’ distribution, and answers thequestion “how much of the change between this era of MPs and another era of MPs is due to anincreased or decreased median cohesion score?”. Any variation in the distributions that remains
after taking into account this location change is due to changes in ‘shape’: that is, changes to theway that the distribution looks as a whole—including its spread, its range and so on. In Appendix
C we give some more intuitive details on these ideas. The other summary measure Handcock andMorris (1998) suggest pertains to the ‘polarization’ of the distribution over time. In our context,
their ‘polarization index’ refers to the extent to which there is more mass in the tails of the current
parliament distribution than in the previous parliament. Thus, when the index is rising, we arein a situation where MPs are increasingly different from one another: spread over the support of
the whip scores increasingly thinly. By contrast, when the index is falling, MPs are increasingly
gathering around the central tendency, whatever that location measure might be. Here we will
focus on the polarization in the left tail specifically, since this represents the least cohesive, most‘rebellious’ MPs in our data. A nice feature of these summary measures is that they are robust to
outliers—an important characteristic in the context of voting in parliament. Such approaches are
implemented in the R statistical environment by Handcock and Aldrich (2002), and we use theirsoftware in what follows.

Note that the approach here is non-parametric, meaning that one does not need to commit to,for example, looking at mean and variance shifts under the very dubious assumption that whip
agreement scores are normal. A drawback, as with other non-parametric approaches, is that relative distribution methods tend to require large amounts of data. A second caution is that relative
distribution approaches are sensitive to ‘heaping’ (Handcock and Morris, 1998, 89), which refers
to the tendency of social scientific data to ‘pile up’ in certain places on the continuous spectrum.
Here, as time goes on, more and more MPs appear at, or close to, a whip agreement score of 1 (thatis, full agreement). As a result, the accuracy of the entropy calculations—a measure of the distance
between two distributions that can be decomposed into location and shape components—can suf-

14

fer.7 To ameliorate this problem, we compare a shifting set of parliaments in cohesion terms, thus
maximizing our sample size and helping to smooth across transitions.

In Table 1 we report the estimated entropy difference between distributions, and then the locationand shape components that contribute to it. Technically, the location and shape components should
sum to the entropy for that transition, though here this is not quite the case for all entries. Likely,
this discrepancy is due to the heaping and ‘small’ data problems we mention above. In each casehere, we are comparing two sets of parliaments: all those before a certain date, to all those afterthat date. For example, the entry for 1847 treats the parliaments of 1835, 1837, 1841 and 1847 as
one distribution, and then the parliaments of 1852, 1857, 1859, 1865, . . . , 1900, 1906 as the second
distribution. Similarly, the entry for 1900 compares the distribution of all parliament up to andincluding that one with the parliament of 1906.

In Figure 2, we provide a barplot of the same information as in Table 1 except that we impose the rationality constraint that all negative values (of location contribution) are in fact zero,and then treat the sum of the location and shape contributions as the total entropy. The [red]broken line is the proportion of the entropy that is due to the shape distribution alone. The black
horizontal line marks the point at which half of any change is due to the shape. When the broken
line is above this point, as it is after the 1850s, the majority of any alteration to MP disciplinearises from the shape rather than median shifts. As can be seen from the plot, at the beginning of
the century, location and shape changes have approximately equal responsibility for any changes
to the cohesion distribution. After the 1850s, the shape contribution rapidly increases; by the endof the century, only the shape is changing.

7
Note that, as used here, the ‘entropy’ is simply the Kullback-Leibler divergence, written D, between two distributions. We can denote the reference distribution (which is the ‘original’ set of scores to which the newer ones
will be compared) as F0 , the comparison distribution (the newer set of scores) as F , and the location adjusted
distribution (simply the reference distribution which has been ‘moved up’ to have same median as the comparison
distribution) as FA . Then, in principle, the entropies we need for the location and shape components can be obtained
via D(F ; F0 ), D(F ; FA ) and D(FA ; F0 ). In fact, this cannot be done directly, but there are related identities thatallow one to proceed in practice.

15

parliament
of
1835
1837
1841
1847
1852
1857
1859
1865
1868
1874
1880
1885
1886
1892
1895
1900

Total
entropy
1.26
1.15
1.13
1.16
1.18
1.27
1.34
1.45
1.58
1.55
1.64
1.78
1.58
1.55
1.45
1.09

locationcontribution
1.13
1.07
1.05
1.06
1.01
1.03
1.02
1.09
1.09
1.05
1.05
0.93
0.75
0.49
0.28
-0.15

shape
contribution
0.82
0.81
0.78
0.86
1.03
1.16
1.03
1.38
1.46
1.47
1.56
1.71
1.66
1.73
1.75
1.51

Table 1: Relative contribution of median and shape to changing distribution of whip agreementscores over time. Note that the location and shape contributions do not always sum to the (total)
entropy. This arises from the data requirements of relative distribution methods not being fully
met: note though, that the relative contribution of the components can be still be compared inabsolute terms.

16

2.5
1.0

1.5

2.0

location
shape

●
●

●

●

●

●

1835

1837

1841

1847

●

●

●

1852

1857

1859

●

●

●

1865

1868

1874

1880

●

●

0.0

0.5

●
●

1885

1886

1892

1895

1900

Figure 2: Barplot showing relative contribution of location and shape components in relative distributions for parliaments in the data. Red broken line demarcates proportion of entropy due toshape contribution: note that it rises over time. The solid black horizontal line shows the ‘50%’
mark for shape contribution: when the [red] broken line crosses this horizontal line, more than halfof the distributional change is due to shape changes. Notice that the bar for a given year reflects
the comparison of all parliaments to that date, with all that come after.
As noted above, we can also study the ‘polarization’ of a distribution: essentially a measure of what
proportion of the distribution’s mass is in the center relative to the tails. Handcock and Morris(1998) suggest several such measures, wherein a positive value implies more mass in the tails, anda negative value implies more concentration around the median. Here, we focus on the polarizationof the left tail. In terms of data arrangement, we now compare each ‘current’ parliament to all
previous ones.8 Thus, 1847 is compared to the parliaments of 1835, 1837 and 1841 considered as
one set of observations. Our idea here is to describe what happened to the most rebellious parts ofthe parties in the new parliament. Figure 3 presents the results. There, points represent the lower
tail index. When black, the points imply that the index is statistically significant (meaning that any
observed polarization of the distribution—i.e. deviation in shape from the previous parliament—issufficiently unlikely to have occurred by chance). As the solid lowess line combined with the points
makes clear, after an initial period of flatness or possible increase, rebellion is decreasing over time.
8

The findings do not change substantively if we alter how many parliaments are being compared at once, but the‘performance’ of the nonparametric techniques is better—in terms of the issues mentioned above concerning accuracyof the entropy calculations if we proceed this way.

17

0.4

●

●

0.2

●

●

0.0

●

●

●

●

−0.4

−0.2

●

●

●

−0.8

−0.6

left tail polarization

●

●
1840

1850

1860

1870

1880

●

●
●
1890

1900

Figure 3: Share of the discipline distribution change in the left tail, over time. Comparison iscurrent parliament to all previous ones at that point. Points represent index; black symbols imply
effects with p < 0.01, gray symbols imply otherwise. Note the solid lowess line and points imply
first increasing (or at least non-decreasing), then decreasing over time and implying that the left
tail is ‘emptying out’.
In practice, this means that rebeliousness becomes more common early on, but the rate of change
no longer increasing by the 1850s, and is actively decreasing (i.e. there is less mass in the rebeltail) by the 1860s.

Figure 3 and Table 1 tell a consistent story: as the 19th century progressed, the distribution of
the discipline in the House of Commons became increasingly concentrated around its average andaway from the left tail. Put otherwise: for whatever reason, over the period we study the norm wasalmost always to vote as the whips did; what changed was that a smaller and smaller proportionof MPs did not obey this norm. In the next section, we delve into possible mechanisms for thischange.99Note that as a robustness check, we disaggregated our cohesion data to a year-by-year basis, and conducted
standard structural break tests for various quantiles of the distributions in question. Our results are discussed in18

5

Assessing the Role of ‘Replacement’

If the decline of the rebellious left tail documented in the previous section is due to the replacementof undisciplined MPs by other MPs who were more inclined to vote with their party, then analysis
of MPs’ cohesion scores should reveal several patterns. In this section we assess the evidence forthese patterns using panel data. We first compare the cohesion scores of new entrants to Parliamentand MPs who were on their way out of the chamber (‘joiners’ and ‘leavers’) to that of other MPs;
we then use member fixed effects to decompose the long-term increase in cohesion into the partthat could plausibly be due to replacement and the part that must be due to other factors.

5.1

Comparing “joiners” and “leavers” to other MPs

If the ‘replacement’ story explains the decreased number of ‘rebels’ in the left tail, we ought tosee that (1) ‘joiners’ (that is, MPs who have recently entered the chamber) vote with the party
whips more diligently than other MPs of their party, and (2) ‘leavers’ (that is, MPs leaving thechamber) vote with the party whips less diligently than other MPs of the party.10 We now carry out
some simple tests of that prediction. In what follows, recall that each MP contributes a separate
observation to the data; thus the analysis below deals with multiple observations for each MP, whois (possibly) serving in multiple parliaments. Before reporting our results, we acknowledge that
our models below are ‘stripped down’ to core variables, primarily pertaining to fixed effects formembers and parliaments. Of course, we acknowledge that underlying such fixed effects, and in
addition to them, other more subtle forces are likely at work in determining how cohesive particular MPs are—including their district characteristics and personal connections and histories in thelegislature. Other work has made in-roads into studying such associations (see Eggers and Spirling,
Forthcominga), and this is not our focus here.

Appendix D and are exactly consistent with our account here insofar as we see the median and 95th percentile closeto (or converging) on one throughout the entire period, while the 5th percentile rapidly converges towards one fromthe 1860s onwards.
10
We recognize that differences among ‘joiners’, ‘leavers’, and other MPs could be explained in other ways. For
example, some ‘leavers’ may have been encouraged to leave because of their disloyal voting; some ‘joiners’ may bevote in a disciplined way because they hope to impress party leaders. Still, we find this a useful starting point forassessing the role of replacement.

19

Table 2 reports regression results in which each MP’s per-parliament cohesion score is regressed on
indicators for whether an MP is a ‘joiner’ or a ‘leaver’ in a given parliament as well as an indicator
for the parliament, an indicator for the MP’s party, and the interaction of the two.11 In column (1)the only RHS variable aside from parliament and party dummies is an indicator for whether the
MP is in his first parliament; the coefficient indicates that new MPs were if anything less cohesive
overall than other MPs (controlling for the parliament, the MP’s party, and the interaction between
the two). When we interact our “Joiner” indicator with a dummy for pre-1852 (column 2), we seethat joiners were no more or less likely to vote cohesively during the post-1852 period in which
average cohesion increased; if joiners voted less cohesively on average, it was only during the early
period when average cohesion was stable or declining. Columns (3) and (4) indicate that MPs in
the last parliament of their career (“Leavers”) voted somewhat less cohesively than other MPs; incolumn (4), the “main effect” of being a leaver is negative and significant at the .05 level. When we
include both indicators in the regression (Columns 5 and 6), we cannot reject the null hypothesisthat joiners and leavers were indistinguishable from other MPs in terms of voting cohesion. Not
only are the effects not statistically significant, but the magnitudes of the point estimates are verysmall, suggesting that the cohesion scores of joiners and leavers did not differ from other MPs by
more than about half a percentage point on average.

In analysis reported in Appendix E, we carry out the same regressions separately by party to allow
for the possibility that replacement may have explained rising cohesion in one party but not theother. We find some evidence that Conservative joiners were marginally more cohesive than otherConservative MPs in the later period (with a coefficient of .01), but this is counteracted by evidence
that Liberal joiners were less cohesive. In neither party are leavers less cohesive than those who
continued on. Taken together, these regressions do not tend to support a replacement story: only
for Conservatives do we find that joiners voted more cohesively, but the opposite is true for Liberals
and these effects (and all other point estimates) are fairly small in magnitude.
11

Thus the comparison is whether a given MP who is e.g. a ‘joiner’ votes more cohesively than other MPs of thesame party in a given parliament.

20

Table 2: Are new MPs (joiners) more disciplined? Are exiting MPs (leavers) less disciplined?

Joiner(1)-0.0051†[0.0027](2)-0.002[0.0028]Pre-1852-0.0356∗∗∗[0.0067]

Joiner × pre-1852-0.0124†[0.007](3)(5)-0.0043[0.0033]-0.0382∗∗∗[0.0071]-0.0341∗∗∗[0.0075]-0.0064∗[0.0031]-0.0048[0.0036]0.0035[0.0072]

Leaver × pre-1852-0.0056[0.004]0.0036[0.0088]-0.0019[0.0054]

Joiner × leaver
Joiner × leaver
× pre-1852
Adj. R2
N
Parliament dummies, party
dummies, and interactions(6)-0.0011[0.0035]-0.0129[0.0089]-0.0054†[0.0029]

Leaver(4)-0.0022[0.0058]
0.0015[0.0146]

0.222
10350

0.222
10350

0.217
10086

0.217
10086

0.217
10086

0.217
10086

X

X

X

X

X

X

Note: Dependent variable is a member’s cohesion score in a parliament. Heteroskedasticity-robust standard errors(in brackets) are clustered by member and parliament. Guide to significance codes: ∗∗∗ indicates p < .001; ∗∗ indicates.001 < p < .01; ∗ indicates .01 < p < .05; and † indicates .05 < p < .1.

21

5.2

Assessing the role of replacement through member fixed effects

We now adopt a slightly different approach to assessing the role of replacement in explaining theincrease in party cohesion in this period. Above we looked looked for more or less cohesive voting
among MPs who enter or exit the Commons. Here our approach is to use member fixed effects tosoak up MP-specific variation in cohesive voting; if replacement is an important part of the story,these member fixed effects should explain much of the long-term increase in average cohesion. In
this section we assess whether this is the case.

Our first approach is to see if the inclusion of member fixed effects reduces the magnitude ofestimated time effects. We ran two regressions; in the first, members’ cohesion scores (parliamentby-parliament) are regressed only on parliament dummies to get a measure of average cohesion ineach parliament; in the second we carry out the same regression but we add member fixed effects.
If replacement helps explain the increase in cohesion over time, the parliament dummies should besmaller in magnitude in the second regression, indicating that some of the over-time variation in
average cohesion is explained by the replacement of undisciplined MPs by more disciplined ones.
Figure 4, which plots the parliament dummies for the two regressions, indicates that this is not thecase. If anything, the magnitude of the parliament effects is larger when member fixed effects areincluded. Consistent with the above analysis, this indicates that the sustained increase in party
cohesion in the second half of the 19th century is not due to replacement effects. More formally,
we can show that the average magnitude of the parliament dummies is actually about 4% larger
when we include member fixed effects than when we do not, and we are unable to reject the null
hypothesis that the magnitude of the parliament dummies is unaffected by the inclusion of member
fixed effects.12

As an additional way of clarifying the role of replacement in explaining the change in disci12
For each of m repetitions and for each of the two regressions, we draw a set of parliament effects from a multivariate
normal distribution that is centered at the regression estimates with the corresponding variance-covariance matrix.
(This is a quasi-Bayesian procedure similar to the one used in King, Tomz and Wittenberg (2000).) Across simulations,the average size of the parliament effects is 4% larger on average in the regressions with member fixed effects; in only
about 1/3 of the simulations are the parliament effects smaller in the regressions with fixed effects.

22Figure 4: Do member fixed effects soak up the over-time variation in cohesion?

1837

●

●

1841

●

●

●
●

1847

●

1852

●
●

1857
●

1859

●

●

1865

●
●

1868

●

●
●
●

1874
●

1880
●

1885

●

●

1886

●

1892

●

1895

●

●

●
●

1900
1906

●

●
●

●

w/o member fixed effects
w/ member fixed effects

−0.05

0.00

0.05

●
●

0.10

Parliament coefficient
Note: Point estimates and .95 confidence intervals plotted for the “parliament effect” corresponding to each year
shown. The white circles show the estimates for a regression in which cohesion scores are simply plotted on parliament
dummies. The black circles show the estimates for the same regression with member fixed effects added. The factthat member fixed effects do not soak up much of the over time variation is an indication that replacement does notaccount for the long-term increase in cohesion.

23

pline over this period, we carry out regressions to see how much of the overall change in cohesion
can be linked to changes within an MP’s career. The reasoning is similar to that in the comparison of parliament effects above. If replacement is an important part of the explanation for the
long-term increase in cohesion in the House of Commons, then we might expect to find that thecohesion score of the average MP does not change much within his career; if replacement is the key,then the important thing is the replacement of MPs who were intrinsically rebellious by MPs who
were inclined to vote with their party. One implication of the replacement story, then, is that any
increase in cohesion within an MP’s career should occur at a lower rate than the increase in overall
cohesion. If this is not the case – that is, if the average individual MPs became more cohesiveat the same annual rate as the legislature overall – then this would seem to leave little room for
replacement as an explanation of the long-term increase in cohesion.

To carry out this test, we regress members’ cohesion scores (one for each parliament) on theirseniority at the beginning of the parliament. By including member fixed effects, we measure the“effect” of seniority within an MP’s career; the MP’s baseline level of cohesiveness is soaked up by
the fixed effect. These regression coefficients are reported in columns (1) and (2) of Table 3. For
comparison, in columns (3) and (4) we report parliament level regressions where, having calculated
average cohesion for MPs in each parliament, we regress cohesion on “seniority” (meaning years
since 1835) in the same way as in the member-level regression.

The remarkable result of this analysis is that during the second half of the 19th century (theperiod during which overall cohesion increased) the change in cohesion within MPs’ careers is essentially the same as the change in average overall cohesion during the same period: the coefficientson “Seniority” in Table 3 in columns (1) and (2) are basically the same as the corresponding coefficients in columns (3) and (4). One implication of this is that all of the increase in average cohesion
between 1852 and 1906 can be explained by changes that happen within individual MPs’ careers.
Between 1852 and 1906 the mean cohesion score in parliament increased from 0.87 to 0.98, a gainof 0.11. But that is almost exactly equal to the implied gain from our model for an MP serving

24

Table 3: How does the change in cohesion within an MP’s career compare to the change in aggregate
cohesion over time?
Member level(1)(2)
Seniority

0.0016∗∗∗[0.0002]0.0024∗∗∗[0.0002]

Parliament level(3)(4)
0.0017∗∗∗[0.0004]

0.0027∗∗∗[0.0005]Pre-1852

0.0469∗∗∗[0.007]0.1069∗∗[0.0322]

Seniority × pre-1852-0.0037∗∗∗[0.0005]-0.0091∗∗[0.0033]

Adj. R2
N
Member fixed effects

0.007
10350
X

0.013
10350
X

0.495
17

0.694
17

Note: Dependent variable in columns (1) and (2) is a member’s cohesion score in a parliament; “Seniority” inthose columns refers to the time between the MP’s entry into Parliament and the beginning of a given parliament.
Dependent variable in columns (3) and (4) is the average cohesion score of MPs (Liberals and Conservatives only) ina given parliament; “Seniority” in those columns refers to the time elapsed between 1835 and the beginning of theparliament. Standard errors (heteroskedasticity-robust and clustered by member and parliament in columns 1 and 2)
in brackets. Guide to significance codes: ∗∗∗ indicates p < .001; ∗∗ indicates .001 < p < .01; ∗ indicates .01 < p < .05;
and † indicates .05 < p < .1.

(hypothetically) the entire period under study: that is, β̂post−52 × (1906 − 1852) = .13, with a .95
confidence interval of [.10, .15].13

To be clear, our argument is not that cohesion increased in the 19th century because averageseniority increased. In fact, average seniority was if anything declining over the period we examine,with steady increases only in the period before the 1850s when cohesion was falling. Rather, ourclaim is that, because individual MPs got more disciplined at about the same rate as the bodyas a whole, replacement effects must have been minimal. To see the point more clearly, consider
an alternative situation in which all of the increase in cohesion over time is due to replacement.
In that case, if we ran a regression like the one reported in columns (1) and (2) of Table 3 wewould find a coefficient of zero on “Seniority”, with all of the variation in cohesion being soaked
13
Note that based on the interaction term in Table 3 (“Seniority × pre-1852”) the drop in cohesion up to 1852 may
have been due in part to replacement, as the drop in cohesiveness within MPs’ careers appears to have been slower
than that of cohesiveness overall.

25

up by the member fixed effects. What we find instead is that the member fixed effects evidently
explain relatively little of the change in cohesion over time, and thus that cohesion in the House ofCommons appears to have increased because MPs in the House of Commons voted more cohesively
over the course of their careers (for whatever reason), not because undisciplined MPs were replacedby those who voted more cohesively.14

6

Assessing an Inducements Story: Promotion to the Cabinet

We now turn to assessing one channel through which MPs may have been induced to vote with
their party whips: the reward of office. Cox (1987) addresses this question by comparing the priorvoting records of MPs who were awarded ministerial posts to those of MPs who were not awarded
ministerial posts. In analysis of Liberal ministers 1880-1885 and Conservative ministers 1874-1880,
he obtains mixed results, concluding that if ministerial ambition explains increasing party cohesion
it likely does so only for the 1890s and early 1900s. In this section we revisit the question with far
more extensive data—every parliament, as opposed to one parliament per party.

As an initial cut, we compare the distribution of cohesion scores over time among those appointed
to cabinet positions and those not appointed to cabinet positions. Figure 5 depicts, for each parliament, three quantiles (.5, .25, and .1) of cohesion scores for those subsequently appointed to thecabinet (left panel) and those not appointed to the cabinet (right panel). In particular, we lookat all cases where an MP has not previously served in a cabinet office before that parliament andthe MP’s party formed a government during that parliament; we then calculate the median (or25th percentile or 10th percentile) cohesion score for these cases in the previous parliament (toreflect the idea that MPs may be promoted in a given parliament for loyal voting in the previous
parliament). To smooth out the presentation, we compute the quantiles depicted for a given year
based on data from all parliaments elected within 15 years of that year.

14

It should be noted that the foregoing analysis adopts a fairly restrictive view of how replacement effects mightwork. A broader view would recognize that an MP might vote more cohesively over the course of his career in part
because of changes in who else was sitting the legislature.

26

Figure 5: Are more cohesive voters more likely to be promoted to the cabinet?

0.9
0.8
0.6

0.7

Lagged cohesion score

0.8
0.7
0.6

Lagged cohesion score

0.9

1.0

MPs NOT appointed to cabinet

1.0

MPs appointed to cabinet

1835
1837
1841
1847
1852
1857
1859
1865
1868
1874
1880
1885
1886
1892
1895
1900
1906

0.5

Median25th pctile
10th pctile

1835
1837
1841
1847
1852
1857
1859
1865
1868
1874
1880
1885
1886
1892
1895
1900
1906

0.5

Median25th pctile
10th pctile

Parliament

Parliament

Note: Figure indicates quantiles of cohesion scores for MPs who were appointed to the cabinet in a given parliament(left panel) and those who were not (right panel). To smooth the data each estimate is based on all parliaments
within 15 years of the parliament indicated. MPs are included in the analysis only for parliaments in which they havenot previously occupied a cabinet office and their party formed a government at some point during the parliament.

Figure 5 clearly shows that MPs who were not appointed to the cabinet voted less cohesively.
Up until the 1870s at least, the median non-appointed MP consistently had a lower cohesion scorethan the median appointed MP (solid lines), and the same is even more clearly true for the otherquantiles. (By the end of the century cohesion is high enough that it is difficult to distinguishbetween the two groups of MPs.)

The initial comparison in Figure 5 of MPs who were and were not appointed to the cabinet suggeststhat party leaders may have rewarded loyal voting with subsequent appointments to cabinet office.
Of course, these comparisons of distributions do not control for other differences between MPs who27are and are not appointed.15 It could be, for example, that MPs who vote more cohesively are also
more senior or have served in other official capacities, and that controlling for these differences theappointees and the non-appointees may not differ in voting cohesion. We address some of theseconcerns in Table 4, which reports regressions in which a binary indicator for whether an MP wasappointed to a cabinet position in a parliament is regressed on the MP’s lagged cohesion scores,
other characteristics of the MP, and (in column 4) parliament dummies.16 (Again, we restrict
attention to MPs who had not previously held cabinet office and who belonged to a party thatformed a government in a given parliament.)

The results of Table 4 are consistent with party leaders offering cabinet positions somewhat
more readily to MPs who had previously voted more loyally with party whips. The lagged cohesion
score is a statistically significant predictor of the appointment decision in all specifications at the.1 level or better. (Note also the low adjusted R2 across models.) The best predictor of whether an
MP will be appointed to cabinet office is whether the MP was previously appointed to non-cabinet
office;17 the coefficient on age indicates that, conditional on an MP’s seniority, younger MPs are
somewhat more likely to be promoted to the cabinet.

It should be noted that the magnitude of the “effect” of cohesion on subsequent promotion to
the cabinet is modest: a rebellious MP with a cohesion score of .5 who subsequently votes withperfect loyalty would increase his predicted probability of promotion by just 1 or 2 percentage
points depending on the model. Put differently, those who were and were not promoted differed inprevious cohesion by a substantively small amount. Still, the lure of a ministerial post was large
enough that even a small preference for MPs with relatively loyal voting records may have been
15
They also do not control for the ideology or policy preferences of MPs: it could be that more cohesive voters weremore likely to be promoted because prime ministers sought cabinet members who were more ideologically alignedwith themselves. We currently have no measure of ideology independent of voting, and as such it is impossible todistinguish ideological alignment from cohesive voting.
16
These are OLS regressions. The results are almost the same in terms of substantive magnitude and statisticalsignificance if we instead perform logistic regressions and transform the results to marginal effects calculated at themeans of RHS variables.
17
In separate analysis (reported in Appendix F), we fail to find a similar relationship between disciplined voting
and subsequent promotion to a non-cabinet office.

28

Table 4: Are more cohesive voters more likely to be promoted to the cabinet?
(1)

(2)

(3)

(4)

Lagged cohesion score

0.0436∗∗∗[0.0117]0.0409∗∗∗[0.0105]

0.0241∗[0.0103]

0.0212†[0.0111]

Seniority

0.0011∗∗∗[0.0002]0.0011∗∗∗[0.0002]0.0002[0.0002]0.0001[0.0002]Age-0.001∗∗∗[0.0002]-0.001∗∗∗[0.0002]-0.0005∗∗[0.0002]-0.0005∗∗[0.0002]

Post-1868

0.0573[0.0494]

0.0648[0.0489]

0.0586[0.0577]

Lagged cohesion ×
post-1868-0.0482[0.0515]-0.0577[0.0509]-0.0519[0.0575]

0.0991∗∗∗[0.0133]

0.1003∗∗∗[0.0134]

0.068
3695

0.069
3695
X

Previously held
non-cabinet office
Adj. R2
N
Parliament dummies included

0.007
3695

0.009
3695

Note: Dependent variable is 1 if an MP occupied a cabinet office in a given parliament. MPs are included only ifthey have not previously occupied a cabinet office and only if their party formed a government at some point during
the parliament. Heteroskedasticity-robust standard errors (in brackets) are clustered by member and parliament.
Guide to significance codes: ∗∗∗ indicates p < .001; ∗∗ indicates .001 < p < .01; ∗ indicates .01 < p < .05; and †indicates .05 < p < .1.

29

enough to apply decisive pressure on MPs to fall in line behind the party whips. A fuller record ofdata on voting and ministerial promotion thus indicates that ministerial posts were indeed awarded
to more loyal MPs and, contrary to the specifics (though in line with the thrust) of Cox (1987)’s
conjecture, the difference is particularly evident before the 1870s when MPs were more rebellious
on average.

However modest the effect size, given that the evidence for replacement is dubious, the fact thatcohesion increased over time suggests that ministerial promotion became more valuable—or more
responsive to behavior—during the period under study. Explaining exactly why this was is beyond
the scope of our current efforts. Nonetheless, several reasons may be posited, all interrelated: as
the electorate became increasingly ‘party orientated’ in the sense of Cox (1987), it was the party
leaders (rather than local MPs) who were de facto at the forefront of voters’ minds when they went
to the ballot box. One consequence presumably was that Prime Ministers became more powerfulas previously local power bases were eroded: able to pick members for their ministerial ‘team’ subject to their performance in divisions, without hindrance from notable backbenchers or their allies.
Simultaneously, party voting also encouraged a reorganization in Commons procedure, with more
power residing in the cabinet and the opposition front bench (Eggers and Spirling, Forthcomingb).
It is to be expected that being a minister (rather than a relatively impotent backbencher) was
increasingly a prize worth seeking—whatever behavior it required.

6.1

Summary

The foregoing sections showed that
1. average cohesiveness—measured as the proportion of times an MP voted with his whip—was
always relatively high, with a baseline of around 0.9 in 1830s. It dipped in the 1840s, andincreased to present day levels thereafter;
2. almost all the aggregate increase in cohesion in the second half of the 19th Century is a
product of the left tail of discipline scores receding into the main mass of the distribution:that is, rebels became fewer and further between over time;30

3. contrary to a ‘replacement’ story, incoming MPs in this period were not more disciplined thanother MPs nor were exiting ones less so;
4. contrary to a ‘replacement’ story and consistent with a number of ‘inducement’ explanations,the average annual increase in cohesion within an MP’s career after 1850 is about the sameas the average annual increase in cohesion of Parliament as a whole in the same period.
5. consistent with one ‘inducements’ story, MPs who were promoted to the cabinet in a given
parliament tended to have voted more cohesively in the prior parliament.

7

Discussion

Legislative party cohesion is important for the functioning of Westminster democracies. Without
it, the link between government policy-making and voter preferences is broken. Understanding how
this institutional arrangement came about has therefore been of critical importance to scholars ofhistory and political science (e.g Cox, 1987; Jenkins, 1996; Adelman, 1997). Here, we undertooka massive data collection task to shed light on the central question of why members became moredisciplined. We gathered over 20,000 roll calls from the period 1836–1910, and we matched themto individual MP records so that intra-career changes in cohesion could be calculated for the firsttime. We compared accounts focussing on ‘inducement’ effects—deriving as we think they do froma possibly large number of different sources, and encouraging (already serving) MPs to behave
differently in roll calls—to more sociological ‘replacement’ effects, by which new personnel withnew characteristics might drive the patterns we saw. In practice, we were able to reject essentially
all claims that the latter school of thought might offer; by implication, endogenous sources of party
cohesion offer more promising explanations. We showed that, at a descriptive level, what drove
cohesion was the ‘folding in’ of the rebellious left tail of the cohesion distribution, and that thisgeneral pattern was consistent over time and was not a product of cohort entry and exit dynamics.

Our findings have several broader implications. To the extent that an inducement account is correct
and that parties’ changing strategies played a major role here, there is evidence of ‘institutional
31

learning’, in the sense that actors both learn from and effect the rules and procedures around them.
Our focus here on the importance of formal and informal rules of behavior, albeit within a broader
context of a changing electorate and other constraints, places this paper within the ‘rational choice
institutionalism’ literature (see Shepsle, 2006, for an overview). Second, we find that the House ofCommons has been remarkably robust to new entrants in terms of procedure; this may mean thatfor certain ‘new’ types of individuals serving as MPs (e.g. women, (Norris and Lovenduski, 1995;
Lovenduski, 2005)) to have the hoped-for impact on the norms of behavior there, other changes
to structural incentives may be required (see also Cowley, 2002, on this point). Third, whatever
the electoral consequences of the Second Reform Act (1867) (see Smith, 1967; Feuchtwanger, 1968;
Jenkins, 1996; Berlinski and Dewan, 2011), the legislative effects of this surge in the franchise appear to have been minimal in terms of ‘average’ behavior as per Figure 1; parties were already on
the way to modern practice by the 1850s. This is in line with suggestions from others Cox (1987),
albeit we have more comprehensive data to make the point.

We have documented one channel through which backbench troops may have been brought intoline (the lure of ministerial promotion), and the evidence we found was modest but tangible. Of
course, there are numerous other plausible theories, ranging from socialization (Rush and Giddings,2011), to partisanship (Jenkins, 1996), to increasing rhetorical skill by leaders (Moser and Reeves,2013), to agenda control by the executive (Dewan and Spirling, 2011) or some combination of theabove (Kam, 2009b). It is difficult to investigate such accounts with the kind of individual-level
analysis we have done here because these mechanisms tend to operate on entire cohorts of MPs;
this makes it difficult to isolate the effects of a given mechanism from those of other over-timechanges that took place simultaneously. Such aggregate-level forces are undoubtedly importantfor explaining the patterns we have documented here, however, and future scholars may draw on
our data to make progress. One option is to use parliamentary speech to get a sense of how MPs
talked about votes over time, and possibly to see how this language compared with actual voting
behavior for individual MPs. Here, extant qualitative historical work on leaders and the cultureof the Commons at the time may provide a useful resource (e.g. Cooke and Vincent, 1974; Blake,32

1985; Coleman, 1988; Hawkins, 2007). We leave such efforts for future work.

33

A

The Division Data

We received the data as a set of page images and corresponding xml files (one for each division);as in the original printed source, each file included basic information about the division along with
the names of the MPs voting ‘Aye’ and ‘No’ and the names of tellers for each side. There aresome 20,262 of these roll calls. Although the xml provides useful structure to the raw data, theoriginal source did not identify the party of each MP, nor are MPs identified in a standardized way
in either the original source or the xml markup. Thus without further processing this remarkabledata source is of limited use in systematically analyzing voting cohesion or anything else. We
therefore endeavored to link the voting data to our broader database, which includes every MP
serving in this period along with his electoral records, speeches in parliament, and key backgroundinformation including party, dates of birth and death, dates of service, constituency names, offices
held, etc. Merging the named MPs in the division lists to their records in our database required a
combination of custom automated processing and manual attention (ultimately requiring hundredsof research assistant hours); the merging is not straightforward because MPs are not identified instandard ways in the division lists (whether due to irregular conventions, misspellings, or OCR
errors). The covariates themselves come from various standard sources, such as Craig (1989), Craig(1974), Walker (1978), Cook and Keith (1975) and Butler and Butler (1994). In this way, each
vote in each division list becomes part of a record on a given individual. By the end of the cleaning
process, both automatic and manual, we had identified the MP being around 99% of the votes castin each parliament; that is, we were uncertain who the MP cast was in around one percent of the
4,791,282 vote decisions in our data.18

B

Measurement of Discipline

To clarify how our cohesion measure works, consider the following example. Let w be the composite
whip voting vector expressed as the side he ‘tells’ for. That is, the vector of decisions on all the rollcalls in a given parliament. The whips may ‘tell’ in one of three ways: for the ‘aye’ side (coded as1), for the ‘no’ side (coded as 0), or not at all (coded as NA). In what follows, suppose there are atotal of five roll calls and that the whips’ voting profile is
w = [1, 0, NA, 1, 1 ].
| {z } |{z}
whip1

whip2

Recall that we sometimes have more than one MP serving as a Chief Whip, and we infer thewhipping vector from whomever holds that position and acts as teller at some point during theparliament. Here, the vector is implicitly divided into two parts: one MP serving as Chief Whip for
the first three votes, another for the last two. Suppose a Member of Parliament i’s voting vector ismi = [1, NA, 0, 1, 0].
Since MP i only votes in four divisions, the denominator for any agreement scores is four. Here, ivotes in the same way as the whip ‘tells’ on two of the four roll calls on which he attends, and thus
18

In principle, essentially all of the votes can be linked to an individual; we viewed 1% missingness as acceptablefor our purposes.

34

i’s agreement score is 24 . Notice that missingness in the MP’s vote vector does not count against the
MP’s disciplinary record. Similarly, when the whips’ votes are missing, that roll call is removed fromconsideration in terms of agreement. The distribution of all such MP’s agreement scores is denotedas the “discipline distribution” or “cohesion distribution”. Note that this aggregate distribution
must be bounded between zero and one. Importantly, while the whip vector is party-specific, theaggregate discipline distribution merges the parties. That is, the aggregate distribution has as its
ith component the agreement score between the i MP and his party-specific whip, though thatparty can be different from MP to MP. This means that a Tory MP with the same discipline scoreas a Liberal MP in a given parliament may have voted in exactly the same way in practice as
the Liberal (‘ayes’ and ‘noes’ in the same places), or in the exactly converse way, or something in
between. What matters is how often he follows his whip, and whips in different parties may insome circumstances agree (in voting terms) on specific issues. Finally, note that we do not require
both party whips to vote in a given division for it to be included in our analysis—so long as one
enters a preference, that division enters the data for his party.

C

Relative Distributions: Intuition

Relative distribution methods allow changes in the distribution of cohesion to be due to location
shifts, shape changes, or some combination of the two. In Figure 6 we make these ideas clearer
in graphic form. In the top panel (a), the distributions of cohesion differ only in their location:that is, the light colored density is exactly the same as the dark colored one, but shifted ‘up’ to
the right. The proportion of members in the left or right tails is identical in both: there has been
no change in shape. In the middle panel, labelled (b), the locations (the medians) of the densities
are the same, but the shapes are different: literally, the proportion of members in the tails in thelight colored density is obviously larger than for the dark colored one. Finally, in the bottom panel,labeled (c), we report two cohesion densities that differ in terms of both location and shape: thedark colored one has a lower spread, and its median is clearly lower also. The idea of the methods,
intuitively, is that if we see a change as pictured in panel (c) we may break it down into two setsof movements: a location shift (as in the top panel), and a shape change (as the middle panel).
The entropy measurements tell us how much a move like that pictured in panel (a) or panel (b)captures the total movement as seen in panel (c).

D

Verifying Changes in Disaggregated Data

Here we verify that the changes seen in the boxplots hold in less aggregated data. In Figure 7 we
consider division discipline on a year-by-year basis, using the same whipping and coding rules asabove. There, we show three indicative quantiles—the 5th, 50th and 95th percentile—over time.
As can be seen very obviously from the lowess lines at the top of the graph, the most loyal (the95th percentile) and the median are both close to one throughout our period. The least loyal MPs,
the 5th percentile, start out around 0.6, fall a little in the 1840s, and then surge in cohesion fromthe 1860s onwards: by the close of the century they are virtually indistinguishable from the 95th
percentile. For both the 5th and 50th percentile, a structural break test (in the sense of Bai andPerron, 2003) finds a (single) change point around 1873–1874 (that is, the beginning of the firstparliament after the Second Reform Act).

35

(a)

0.0

0.2

0.4

cohesion

0.6

0.8

1.0

(b)

0.0

0.2

0.4

cohesion

0.6

0.8

1.0

(c)

0.0

0.2

0.4

cohesion

0.6

0.8

1.0

Figure 6: Relative distribution methods: if we observe a change from the dark to the light densityin panel (c), we can estimate the extent to which this is the consequence of a location shift (panel(a)), and/or a shape change (panel (b))

36

1.0

●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●●● ●● ●●● ●●● ● ●●
●
● ●
●●
●
●●
●●
●

0.8

●

●●
●

●

●

●

●

●
●

●
●

●

●

●
●

●
●

●

●

●

●

0.6
cohesion

●
●

●●

●

●

●

●

●

●

●

●

0.4

●
●●

●

●
●●
●

●

●●

●

●●
●

0.2

●

●
●
●

0.0

●

●

1840

1860

1880

5th percentile
median95th percentile
5th percentile
median95th percentile
1900

Figure 7: Cohesion in parliament on a year-by-year basis. Lines (lowess fit) trace 95th, medianand 5th percentile over time. Note that the 5th percentile (dotted [red] line, solid points) increases
markedly during the 1860s.

37

E‘Joiners’ and ‘Leavers’ by Party

Table 5 reports analysis in which each Conservative MP’s per-parliament cohesion score is regressedon parliament dummies and indicators for whether an MP is a ‘joiner’ or a ‘leaver’ in a given parliament. In column (1) the only RHS variable aside from parliament dummies is an indicator forwhether the MP is in his first parliament; the coefficient of .005 indicates that new Conservative
MPs’ cohesion scores were only very slightly higher than those of other Conservative MPs (controlling for the parliament), and the effect is not statistically significant at the .1 level. When weinteract our “Joiner” indicator with a dummy for pre-1852 (column 2), we see that Conservative
joiners in fact voted somewhat more cohesively during the post-1852 period during which cohesion
increased overall in Parliament. Columns (3) and (4) indicate that MPs in the last parliament of
their career (“Leavers”) were not more or less likely to vote with party whips. (The point estimate
is negative but not significant at the .1 level.) When we include both indicators in the regression(Columns 5 and 6) the patterns hold: “Joiners” were somewhat more likely to vote with party
whips (after 1852), but we cannot reject the null that “Leavers” were just as likely as other MPs
to vote with the whips.
Table 6 reports the same analysis for Liberal MPs. The main takeaway is that Liberal “joiners” tended to vote less cohesively throughout the period we examine, which is the opposite of thefinding for Conservatives and contrary to the replacement story. In column (4) we find suggestive
evidence that “Leavers” were somewhat less likely to vote with party whips during the period after
1852 when overall cohesion was increasing. When both indicators are included (columns 5 and 6)
we again find that Liberal joiners voted less cohesively but we fail to reject the null that leavers
voted as cohesively as other MPs.

F

Non-Cabinet Promotion As Inducement?

In Figure 8 and Table 7 we extend the analysis of Section 6 by focusing on promotion to non-cabinet
offices such as Junior Lord of the Treasury, Solicitor-General, or Under-Secretary of State for theHome Department. We use the same procedures as above, except here we restrict attention to
MPs who had never held any office before (cabinet or otherwise). Comparison of the distributionsof cohesion in Figure 8 shows far less of a difference between appointees and non-appointees.
Regression analysis in Table 7 similarly indicates a more modest relationship between cohesion andappointment than was true for the cabinet, with a borderline significant “effect” only when we
focus on the period before 1868.

38

Table 5: Are new MPs (joiners) more disciplined? Are exiting MPs (leavers) less disciplined?
Conservatives only

Joiner(1)
0.005[0.003](2)0.0118∗∗∗[0.0029]Pre-1852-0.0355∗∗∗[0.0067]

Joiner × pre-1852-0.0252∗∗[0.0086](3)(5)
0.0046[0.0038]-0.0342∗∗∗[0.0073]-0.0292∗∗∗[0.0079]-0.0038[0.0037]-0.0062[0.0047]-0.0072[0.0095]

Leaver × pre-1852-0.0051[0.005]-0.0014[0.0118]0.0011[0.0065]

Joiner × leaver
Joiner × leaver
× pre-1852
Adj. R2
N
Parliament dummies(6)0.01∗∗[0.0037]-0.0186†[0.0105]-0.0058[0.0036]

Leaver(4)

0.0043[0.0062]-0.0153[0.0189]

0.125
5225
X

0.126
5225
X

0.123
5134
X

0.123
5134
X

0.123
5134
X

0.124
5134
X

Note: Dependent variable is a member’s cohesion score in a parliament. Heteroskedasticity-robust standard errors(in brackets) are clustered by member and parliament. Guide to significance codes: ∗∗∗ indicates p < .001; ∗∗ indicates.001 < p < .01; ∗ indicates .01 < p < .05; and † indicates .05 < p < .1.

39

Table 6: Are new MPs (joiners) more disciplined? Are exiting MPs (leavers) less disciplined?
Liberals only

Joiner

(1)-0.0157∗∗∗[0.0044](2)-0.0157∗∗[0.0048]

Pre-1852-0.133∗∗∗[0.0092]

Joiner × pre-1852-0.0001[0.0113](3)(5)-0.014∗[0.0056]-0.1485∗∗∗[0.0091]-0.1478∗∗∗[0.0096]-0.0089†[0.0049]-0.0036[0.0055]

0.0143[0.0107]

Leaver × pre-1852-0.006[0.0061]0.0081[0.013]-0.0038[0.0088]

Joiner × leaver
Joiner × leaver
× pre-1852
Adj. R2
N
Parliament dummies(6)-0.0123∗[0.0061]-0.0098[0.015]-0.0051[0.0044]

Leaver(4)-0.0077[0.0097]
0.022[0.0226]

0.2
5125
X

0.2
5125
X

0.186
4952
X

0.186
4952
X

0.187
4952
X

0.187
4952
X

Note: Dependent variable is a member’s cohesion score in a parliament. Heteroskedasticity-robust standard errors(in brackets) are clustered by member and parliament. Guide to significance codes: ∗∗∗ indicates p < .001; ∗∗ indicates.001 < p < .01; ∗ indicates .01 < p < .05; and † indicates .05 < p < .1.

40

Figure 8: Are more cohesive voters more likely to be promoted to non-cabinet offices?

0.9
0.8
0.6

0.7

Lagged cohesion score

0.8
0.7
0.6

Lagged cohesion score

0.9

1.0

MPs NOT appointed to non−cabinet office1.0

MPs appointed to non−cabinet office

1835
1837
1841
1847
1852
1857
1859
1865
1868
1874
1880
1885
1886
1892
1895
1900
1906

0.5

Median25th pctile
10th pctile

1835
1837
1841
1847
1852
1857
1859
1865
1868
1874
1880
1885
1886
1892
1895
1900
1906

0.5

Median25th pctile
10th pctile

Parliament

Parliament

Note: Figure indicates quantiles of cohesion scores for MPs who were appointed to non-cabinet office in a given
parliament (left panel) and those who were not (right panel). To smooth the data each estimate is based on all
parliaments within 15 years of the parliament indicated. MPs are included in the analysis only for parliaments inwhich they have not previously occupied a cabinet office and their party formed a government at some point during
the parliament.

41

Table 7: Are more cohesive voters more likely to be promoted to non-cabinet offices?
(1)

(2)

(3)

Lagged cohesion score0.0015[0.0085]0.0101†[0.0054]0.0101†[0.0056]

Seniority

0.0003∗[0.0002]0.0003∗[0.0002]0.0003†[0.0002]Age-0.0003∗∗[0.0001]-0.0003∗∗[0.0001]-0.0003∗∗[0.0001]

Post-1868

0.0621[0.0439]0.0686[0.051]

Lagged cohesion ×
post-1868-0.0612[0.0448]-0.0694[0.0519]

0.004
3150

0.005
3150
X

Adj. R2
N
Parliament dummies included

0.001
3150

Note: Dependent variable is 1 if an MP occupied a cabinet office in a given parliament. MPs are included only ifthey have not previously occupied a cabinet office and only if their party formed a government at some point during
the parliament. Heteroskedasticity-robust standard errors (in brackets) are clustered by member and parliament.
Guide to significance codes: ∗∗∗ indicates p < .001; ∗∗ indicates .001 < p < .01; ∗ indicates .01 < p < .05; and †indicates .05 < p < .1.

42

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