Ruggedness:
The blessing of bad geography in Africa
Nathan Nunn∗ ‡Harvard University, NBER and BREAD

Diego Puga∗ §IMDEA Social Sciences Institute and CEPRFirst version, September 2007
This version, May 2010
Abstract: We show that geography, through its impact on history,
can have important effects on current economic development. The
analysis focuses on the historic interaction between ruggedness andAfrica’s slave trades. Although rugged terrain hinders trade and mostproductive activities, negatively affecting income globally, within Africa
rugged terrain afforded protection to those being raided during theslave trades. Since the slave trades retarded subsequent economic development, within Africa ruggedness has also had a historic indirect
positive effect on income. Studying all countries worldwide, we estimate the differential effect of ruggedness on income for Africa. We
show that the differential effect of ruggedness is statistically significant
and economically meaningful, it is found in Africa only, it cannot be
explained by other factors like Africa’s unique geographic environment,
and it is fully accounted for by the history of the slave trades.

Key words: terrain ruggedness, slave trades, Africa, geography, economic development
jel classification: o11, o13, n50, n40∗ We are grateful for valuable comments from Ann Carlos, Edward Glaeser, Larry Katz, James Robinson, the Editor
Dani Rodrik, two anonymous referees, and as well as seminar participants. Both authors thank the Canadian Institutefor Advanced Research (cifar) for its support. Funding from the Social Sciences and Humanities Research Council of
Canada, the Comunidad de Madrid (prociudad-cm, s2007/hum/0448), the the European Commission’s Seventh Research Framework Programme (grant agreement 225343, collaborative project hi-pod), Spain’s Ministerio de Educacióny Ciencia (sej2006–09993), and the Centre de Recerca en Economia Internacional is gratefully acknowledged. All thedata necessary to reproduce the results of this paper are available from http://diegopuga.org/data/rugged/.
‡ Department of Economics, Harvard University, Littauer Center M-29, Cambridge, ma 02138 (e-mail:nnunn@fas.harvard.edu).
§ Madrid Institute for Advanced Studies (imdea) Social Sciences, Antiguo pabellón central del Hospital de Cantoblanco, Carretera de Colmenar Viejo km. 14, 28049 Madrid, Spain (e-mail: diego.puga@imdea.org; website: http://diegopuga.org).

1. Introduction
Rugged terrain is tough to farm, costly to traverse, and often inhospitable to live in; yet in Africa,
countries with a rugged landscape tend to perform better than flatter ones. This paper uncovers
this paradox and explains it by reaching back more than two centuries — to the slave trades.
In Africa, between 1400 and 1900, four simultaneous slave trades, across the Atlantic, the Sahara
Desert, the Red Sea and the Indian Ocean, led to the forced migration of over 18 million people,
with many more dying in the process (Africa’s total population was roughly stable over this period
at 50–70 million). The economies they left behind were devastated: political institutions collapsed,and societies fragmented. For African people fleeing the slave trade, rugged terrain was a positive
advantage. Enslavement generally took place through raids by one group on another, and hills,
caves and cliff walls provided lookout posts and hiding places for those trying to escape. Today,
however, the same geographical ruggedness is an economic handicap, making it expensive totransport goods, raising the cost of irrigating and farming the land, and simply making it more
expensive to do business.
We use the historical importance of terrain ruggedness within Africa to inform the debate thathas arisen about the importance of geography for economic development. While it is commonly
agreed that geography can have important consequences for economic outcomes, there is a growing debate over the channel of causality. The traditional focus has been on direct contemporaneous
effects of geography on economic outcomes (e.g, Kamarck, 1976, Mellinger, Sachs, and Gallup,
2000, Sachs, 2001, Gallup and Sachs, 2001, Sachs and Malaney, 2002, Rappaport and Sachs, 2003).1
Recently, others have argued for a more nuanced effect of geography on economic outcomes,
which works through past interactions with key historical events (Diamond, 1997, Engerman
and Sokoloff, 1997, Sokoloff and Engerman, 2000, Acemoglu, Johnson, and Robinson, 2001, 2002,
Engerman and Sokoloff, 2002). For instance, Acemoglu et al. (2001) argue that the importance ofa disease-prone environment for current income levels lies in the effect that it had on potential
settler mortality during colonization. In areas where high mortality discouraged Europeans fromsettling, colonizers implemented poor institutions which adversely affected subsequent economic
development.
Generally, it is difficult to estimate the historic indirect effects of geography. The difficultyarises because locations are generally not only affected by the historic effect of a geographicalcharacteristic, but also by any direct effects that may exist today. Since geographic features areconstant over time, disentangling the two channels is difficult. Our analysis exploits the fact thatthe long-term, positive effect of ruggedness, through fending off slave-raiders, is concentrated in
African countries, where the trades took place. Thus, we are able to identify the indirect historiceffect of terrain ruggedness that works through the slave trade. We further test for this channel byusing estimates, constructed by Nunn (2008), of the number of slaves taken from each country in
1 The geographical characteristics that have been linked to economic outcomes include a disease-prone environment,

proximity to the coast, and the prevalence of desert or tropical climate.

1

Africa.2
We describe in section 2 how we measure terrain ruggedness (data sources for all other variables
employed in the analysis are detailed in the appendix). Then, after introducing the econometricframework in section 3, we investigate the relationship between ruggedness and income in section
4. We find strong evidence for a differential positive effect of the ruggedness in Africa that isboth robust and highly significant. Looking within Africa, in section 5 we provide evidence thatthe positive effect of ruggedness operates through the slave trades. We also estimate each of thecoefficients for each of the channels implicit in the indirect effect of ruggedness. We find support
for each of the underlying relationships: ruggedness negatively affects slave exports, and slave
exports negatively affect the quality of domestic institutions, which is an important determinantof per capita income.

2. Terrain ruggedness
There are a variety of effects of ruggedness on income that are experienced by all regions of theworld. The most well-established of these are the contemporary negative effects of ruggedness.
Irregular terrain makes cultivation difficult. On steep slopes erosion becomes a potential hazard,
and the control of water (e.g. irrigation) becomes much more difficult. According to the Food andAgriculture Organization (1993), when slopes are greater than 2 degrees, the benefits of cultivation
often do not cover the necessary costs, and when slopes are greater than 6 degrees cultivation
becomes impossible. In addition, because of the very high costs involved in earthwork, building
costs are much greater when terrain is irregular (Rapaport and Snickars, 1999, Nogales, ArchondoCallao, and Bhandari, 2002). As well, transportation over irregular terrain is slower and morecostly.3
Our hypothesis is that within Africa ruggedness had an additional historic effect resulting
because of Africa’s history of the slave trades. In Africa, we expect terrain ruggedness to also
have beneficial effects by having helped areas avoid the negative long-term consequences of theslave trades. The most common method of enslavement was through raids and kidnapping bymembers of one ethnicity on another, or even between members of the same ethnicity (Northrup,
1978, Lovejoy, 2000). Rugged terrain afforded protection to those being raided. It provided caves
for hiding and the ability to watch the lowlands and incoming paths. African historians havedocumented many examples of this. For instance, Bah (1976) describes how “[t]hroughout time,
caverns, caves and cliff walls have served as places of refuge for people. [...] There are many
examples of this defensive system in Africa. At Ebarak (south-eastern Senegal), there are still traces
left of a tata wall near a cave in which the Bassaris, escaping from Fulani raids, hid.” Writing about
what is now Mali, Brasseur (1968) explains that “[h]idden in the uneven terrain, they [the Dogon]
2 The figures are constructed by combining historical shipping records with slave inventories reporting slave ethnici-

ties. Nunn finds that the slave trades had adverse effects on subsequent economic development because they weakened
indigenous political structures and institutions, and promoted ethnic and political fragmentation.
3 A recent study by Allen, Bourke, and Gibson (2005) highlights these negative effects of irregular terrain within
Papua New Guinea. The authors show that steep terrain not only makes the production of cash crops very difficult, but
it also makes it much more costly or even impossible to transport the crops to the markets. The end result is that the
populations living in these parts of Papua New Guinea have lower incomes and poorer health.

2

bc

bc

bc

bc

bc

bc

bc

bc

bc

bc

bc

bc

b

bc

bc

bc

bc

bc

bc

bc

bc

bc

bc

bc

bc

Figure 1: Schematic of the terrain ruggedness calculation

were able to use the military crests and, as far as the techniques of war at the time were concerned,
were impregnable.”4When measuring terrain ruggedness, our purpose is to have a measure that captures small-scaleterrain irregularities, such as caverns, caves and cliff walls that afforded protection to those being
raided during the slave trades. We do so by calculating the terrain ruggedness index, originallydevised by Riley, DeGloria, and Elliot (1999) to quantify topographic heterogeneity in wildlife
habitats providing concealment for preys and lookout posts. The main benefits of this measure
are that it quantifies small-scale terrain irregularities and it was designed to capture precisely
the type of topographic features we are interested in. Other measures that have been used byeconomists and political scientists are typically constructed to capture the presence of large-scaleterrain irregularities, and mountains in particular.5 Nevertheless, we show below that the results
are robust to the use of alternative measures of terrain ruggedness.
Our starting point is gtopo30 (us Geological Survey, 1996), a global elevation data set developed through a collaborative international effort led by staff at the us Geological Survey’s Center
for Earth Resources Observation and Science (eros). Elevations in gtopo30 are regularly spaced at
30-arc seconds across the entire surface of the Earth on a map using a geographic projection. The
4 For additional evidence, see Marchesseau (1945), Podlewski (1961), Gleave and Prothero (1971), Bah (1985, 2003),

Cordell (2003), and Kusimba (2004).
5 For example, Gerrard (2000) constructs a measure of the percentage of each country that is covered by mountains,which is used by Fearon and Laitin (2003), Collier and Hoeffler (2004) and others in studies of civil war and conflict. Ramcharan (2006) uses data from the Center for International Earth Science Information Network (2003) on thepercentage of each country within different elevation ranges in an instrumental-variables analysis of how economic
diversification affects financial diversification. An exception to the focus on large-scale terrain irregularities is the article
by Burchfield, Overman, Puga, and Turner (2006), who construct measures of both small-scale and large-scale irregularities and show that they have opposite effects on the scatteredness of residential development in us metropolitan areas.
Burchfield et al. (2006) measure small-scale terrain irregularities using the same terrain ruggedness index of Riley et al.
(1999) that we use in this paper. Olken (2009) also uses small-scale terrain irregularities to compute a predicted measureof the signal strength of television transmissions to Indonesian villages in his study of the effects of television on social
capital.

3sea-level surface distance between two adjacent grid points on a meridian is half a nautical mileor, equivalently, 926 metres.
Figure 1 represents 30 by 30 arc-second cells, with each cell centred on a point from the gtopo30
elevation grid. The ruggedness calculation takes a point on the Earth’s surface like the one marked
by a solid circle in the centre of figure 1 and calculates the difference in elevation between thispoint and the point on the grid 30 arc-seconds North of it (the hollow circle directly above it in the
figure). The calculation is performed for each of the eight major directions of the compass (North,
Northeast, East, Southeast, South, Southwest, West, and Northwest). The terrain ruggedness index
at the central point of figure 1 is given by the square root of the sum of the squared differences inelevation between the central point and the eight adjacent points. More formally, let er,c denoteelevation at the point located in row r and column c of a grid of elevation
q points. Then the terrain

+1
c +1
2
∑ri=
r −1 ∑i =c−1 ( ei,j − er,c ) .
We then average across all grid cells in the country not covered by water to obtain the averageruggedness index of Riley et al. (1999) at that point is calculated as

terrain ruggedness of the country’s land area. Since the sea-level surface that corresponds to a 30
by 30 arc-second cell varies in proportion to the cosine of its latitude, when calculating the averageterrain ruggedness — or the average of any other variable — for each country, we weigh each cellby its latitude-varying sea-level surface.
The units for the terrain ruggedness index correspond to the units used to measure elevationdifferences. In our calculation, ruggedness is measured in hundreds of metres of elevation difference for grid points 30 arc-seconds (926 metres on a meridian) apart. Examples of countries with anaverage ruggedness that corresponds to nearly level terrain are Netherlands (terrain ruggedness
0.037) and Mauritania (0.115). Romania (1.267) and Zimbabwe (1.194) have mildly rugged terrain
on average. Countries with terrain that is moderately rugged include Italy (2.458) and Djibouti(2.432). Highly rugged countries include Nepal (5.043) and Lesotho (6.202). Basic summary
statistics for our ruggedness measure and correlations with other key variables are reported in
an online appendix.

3. Econometric framework
We now develop our estimation strategy for investigating the relationship between ruggedness,the slave trade, and income. Our starting hypothesis is that ruggedness has an effect on current
income that is the same for all parts of the world. This relationship can be writtenyi = κ1 − αri + βqi + ei ,(1)where i indexes countries, yi is income per capita, ri is our measure of ruggedness, qi is a measureof the efficiency or quality of the organization of society, κ1 , α and β are constants (α > 0 and β > 0),and ei is a classical error term (i.e., we assume that ei is independent and identically distributed,
drawn from a normal distribution, with a conditional expectation of zero).
In equation (1), we assume that the common impact of ruggedness on income is negative. This
is not important for the exposition. It simply anticipates our empirical findings of a negative
common effect of ruggedness. In reality, an important part of −α is the effect of ruggedness on
4income through increased costs of cultivation, building, and trade. However, −α may also containpersistent historic effects of ruggedness that are similar across regions. For example, rugged terrain
may be correlated with historic deposits of precious minerals, which may have had long-term
effects by impacting historic institutions (Dell, 2011).
Historical studies and the empirical work of Nunn (2008) have documented that Africa’s slave
trades adversely affected the political and social structures of societies. We capture this effect ofAfrica’s slave trades with the following equation(
κ2 − γxi + ui
qi =ui

if i is in Africa,
otherwise,(2)

where xi denotes slave exports, κ2 and γ are constants (γ > 0), and ui is a classical error term.
Historical accounts argue that the number of slaves taken from an area was reduced by theruggedness of the terrain. This relationship is given by
xi = κ3 − λri + vi ,(3)

where κ2 and λ are constants (λ > 0), and vi is a classical error term.
Equations (1), (2) and (3) are the core relationships in our analysis. Our first approach is tocombine all three by substituting equations (2) and (3) into (1), which gives(κ1 − αri + βγλri + κ4 + ζ i + ξ i if i is in Africa,yi =κ1 − αri + ζ iotherwise,(4)

where κ4 ≡ β(κ2 − γκ3 ), ζ i ≡ ei + βui and ξ i ≡ − βγvi . Equation (4) summarizes the relationships
between ruggedness, the slave trades, and current income. It illustrates the core hypothesis of thepaper: that for African countries, there is an additional positive historic effect of ruggedness on
income that works through the slave trades βγλ.
Guided by equation (4), we estimate the following relationship between ruggedness and income:yi = β 0 + β 1 ri + β 2 ri IiAfrica + β 3 IiAfrica + ε i ,(5)where IiAfrica is an indicator variable that equals 1 if i is in Africa and 0 otherwise.
Combining our predictions about the relationships between ruggedness, the slave trades, andincome yields a first hypothesis, which is core to our paper:6
Hypothesis 1. β 2 > 0 (in Africa ruggedness has an additional positive effect on income).
We have assumed throughout that the conditional expectation of each of the error terms inequations (1)–(3) is equal to zero. In this case, estimating equation (5) provides a consistentestimate of the historic effect of ruggedness in Africa. In practice, our assumptions rely on there
not being variables that belong in any of the structural equations (1)–(3), but are omitted from our
reduced form estimating equation (5). More specifically, in order for an omitted variable to bias ourcoefficient of interest (β 2 in equation (5)), it must be the case that, either the relationship between
6 We have implicitly assumed that β

2 is the same for all African countries. In section 4, we relax this assumption andallow the indirect effect of ruggedness to differ across the regions of Africa.

5income and the omitted factor is different inside and outside of Africa, or that the relationshipbetween the omitted factor and ruggedness is different inside and outside of Africa. For this reason, in our empirical analysis, we pay particular attention to identifying and including potentiallyomitted factors for which the relationship with either income or ruggedness is potentially different
inside and outside of Africa.
Equation (5) illustrates the relationship between income and ruggedness, leaving slave exports
in the background. Recall that we arrived at this equation by substituting both (3) and (2) into (1).
In section 5 we bring slave exports to the foreground by instead only substituting (2) into (1) andestimating (3) separately. This gives us a relationship between income and both ruggedness (nowonly incorporating its common effect) and slave exports:(
κ1 − αri + βκ2 − βγxi + ζ iyi =κ1 − αri + ζ i

if i is in Africa,
otherwise.

(6)

We test this relationship and (3) by estimating the following equations (note that for all non-Africancountries slave exports are zero, xi = 0):yi = β 6 + β 7 ri + β 8 ri IiAfrica + β 9 IiAfrica + β 10 xi + ε i ,(7)xi = β 11 + β 12 ri + ei .

(8)

Estimating (7) and (8) allows us to test four additional hypotheses:
Hypothesis 2. β 12 < 0 (ruggedness negatively affects slave exports).
Hypothesis 3. β 10 < 0 (slave exports negatively affect income).
Hypothesis 4. β 8 = 0 (once slave exports are taken into account, the effect of ruggedness is nodifferent in Africa).
Hypothesis 5. β 7 = −α (once slave exports are taken into account, the coefficient on ruggedness
provides a consistent estimate of the common effect of ruggedness).
Hypothesis 2 and 3 are that ruggedness deterred slave exports and that slave exports are negatively
related to current income. Hypothesis 4 provides a way of testing whether the slave trades canfully account for the positive indirect effect of ruggedness within Africa. If the ruggedness-income
relationship is different for Africa only because of the slave trades, then once we control for theeffect of the slave trades on income, there should no longer be a differential effect of ruggedness for
Africa. Hypothesis 5 states that taking into account the indirect effect of ruggedness by controlling
for the slave trades, yields a consistent estimate of the common effect of ruggedness.

4. The differential effect of ruggedness in Africa
As a first step in our empirical analysis, we now estimate the common effect of ruggedness on
income per person and its differential effect for Africa. Our baseline estimates of equation (5) are
given in table 1. Looking first at column (1), when we estimate equation (5) by regressing income
per person on ruggedness while allowing for a differential effect in African countries, we find that6

Table 1: The differential effect of ruggedness in Africa
Dependent variable: Log real gdp per person 2000
(1)
Ruggedness
Ruggedness · I Africa
I Africa
Diamonds-0.203(0.093)∗∗0.393(0.144)∗∗∗-1.948(0.220)∗∗∗

(2)

(3)

-0.196(0.094)∗∗0.404(0.146)∗∗∗-2.014(0.222)∗∗∗0.017-0.203(0.094)∗∗0.406(0.138)∗∗∗-1.707(0.325)∗∗∗(4)-0.243(0.092)∗∗∗0.414(0.157)∗∗∗-2.066(0.324)∗∗∗0.302(0.130)∗∗-1.615(0.295)∗∗∗(0.012)-1.562(0.415)∗∗∗0.028-0.002(0.003)-0.008(0.006)% Tropical climate% Tropical climate · I Africa-0.009Distance to coast(0.002)(0.002)

0.004

0.006

Distance to coast · I Africa
9.204
∗∗∗

(0.148)

170
0.367

9.221
∗∗∗

(0.200)170
0.363(0.007)-0.007∗∗∗(0.004)

170
0.357

0.321

(0.127)∗∗-0.0260.000% Fertile soil · I Africa(0.143)-0.231(0.077)∗∗∗(0.011)∗∗(0.003)

9.223
∗∗∗

(6)

(0.010)∗∗∗-0.014% Fertile soil

Observations
R2-0.193(0.081)∗∗(0.012)

Diamonds · I Africa

Constant

(5)

9.514
∗∗∗

(0.164)170
0.405-0.009∗∗∗(0.004)-0.657∗∗∗(0.177)(0.193)-0.291-0.194

9.388
∗∗∗

9.959
∗∗∗

(0.360)(0.134)170
0.421-1.039∗∗∗(0.386)(0.195)170
0.537

Notes: Coefficients are reported with robust standard errors in brackets. ∗∗∗ , ∗∗ , and ∗ indicate significance at the 1, 5,
and 10 percent levels.

the coefficient for ruggedness is negative and statistically significant (i.e., β 1 < 0 in equation (5)).
This indicates a negative common effect of ruggedness for the world as a whole. This is consistent
with ruggedness negatively affecting income by increasing the costs of trade, construction andagriculture. While we cannot rule out the existence of some positive common consequences ofruggedness, we find that the net common effect is negative.
The coefficient estimate for ruggedness interacted with an indicator variable for Africa is positive and statistically significant (i.e., β 2 > 0 in equation (5)). This differential effect for Africa isconsistent with hypothesis 1. Within Africa, there is an additional positive effect of ruggedness on
income.
Robustness with respect to omitted geographical variables
When interpreting our core results regarding the relationship between ruggedness and current
economic outcomes, a possible source of concern is that the estimated differential effect of ruggedness within Africa may be driven, at least in part, by other geographical features. However, for an
omitted variable to bias our estimated differential effect, it is not enough that the omitted variable
is correlated with income and ruggedness. It must be the case that, either the relationship between
7the omitted factor and income is different within and outside of Africa, or the relationship between
the omitted factor and ruggedness is different within and outside of Africa. Thus, to deal withpotentially omitted differential effects, we include in our baseline specification of column (1) boththe control variable and an interaction of the control variable with our Africa indicator variable.
By doing this we allow the effect of the control variable to differ for Africa.
A potentially confounding factor, which may have differential effects within and outside of
Africa, is the curse of mineral resources (Sachs and Warner, 2001, Mehlum, Moene, and Torvik,
2006). If diamond deposits are correlated with ruggedness, and diamond production increases
income outside of Africa, but decreases income within Africa because of poor institutions, thenthis could potentially bias the estimated differential effect of ruggedness.7 Column (2) adds to ourbaseline specification of column (1) a control variable measuring carats of gem-quality diamonds
extracted per square kilometre 1958–2000 (see the appendix for details of how this and othergeographical controls are constructed), as well as an interaction of this control with the Africa
indicator variable. We find weak evidence that the effect of diamonds is positive in general, butthat for African countries there is a differential negative effect that nearly wipes out the general
positive effect (however, neither effect is statistically significant unless we include other controls
as in column (6)). The inclusion of this control variable and its interaction with the Africa indicatorvariable does not alter our results regarding the relationship between ruggedness and current
economic outcomes. We have also tried controlling for other mineral resources, such as oil reserves
and gold (together with an African interaction term). The results are unaffected by the inclusionof these additional controls.
It is also possible that in general rugged areas have worse soil quality, but within Africa rugged
areas have better soil quality. For example, the Rift Valley region of Africa is rugged but hasvery fertile soil. To control for this possibility, we construct a measure of the percentage of fertile
soil in each country. This is defined as soil that is not subject to severe constraints for growing
rain-fed crops in terms of either soil fertility, depth, chemical and drainage properties, or moisturestorage capacity, and is based on the fao/unesco Digital Soil Map of the World. In column (3),we add the measure of soil fertility and its interaction with the Africa indicator variable to ourbaseline specification. The results show that the differential effect of ruggedness remains robust tocontrolling for soil quality.
A related argument can be made about tropical diseases. If rugged areas are less prone totropical diseases within Africa but not in the rest of the world, then this could potentially bias theestimated differential effect of ruggedness. To check for this possibility, in column (4) we add toour baseline specification a variable measuring the percentage of each country that has any of thefour tropical climates in the Köppen-Geiger climate classification, as well as an interaction of thisvariable with the Africa indicator variable. We see that there is a statistically significant negative
relationship between tropical climate and income, but that the effect is no different for Africancountries. Our core results are, once again, unchanged.
We recognize that alternative proxies for tropical diseases are also possible. For example, one
7 See Mehlum et al. (2006) and Robinson, Torvik, and Verdier (2006) for theory and empirical evidence supporting

such a differential effect of resource endowments.

8can focus specifically on malaria and include an index of the stability of malaria transmission from
Kiszewski, Mellinger, Spielman, Malaney, Sachs, and Sachs (2004) and the corresponding Africaninteraction. When we do this our core results remain unchanged. The same is also true if wecontrol for the distance to the equator and the corresponding African interaction.
Motivated by the arguments of Rappaport and Sachs (2003) and others that coastal access is afundamental determinant of income differences, in column (5) we control for the average distance(measured in thousands of kilometres) to the nearest ice-free coast for each country. As before,
we also include an interaction of the distance variable with the African indicator variable. Our
results remain robust. Finally, in column (6) we include all of the geographic controls and theircorresponding interaction terms. We find again that our baseline results from column (1) are robust
to controlling for other geographic characteristics that could have a differential effect in Africa.8
Robustness with respect to alternative income and ruggedness measures
We next consider a number of sensitivity checks to ensure that the findings documented to thispoint are in fact robust. First, one can think of many alternative measures of ruggedness. We
have chosen to use a well-established measure of terrain ruggedness that was developed by Riley
et al. (1999) to quantify topographic heterogeneity that creates hiding places and outlook posts inwildlife habitats. The first robustness check that we perform is to ensure that our results hold using
other measures of ruggedness. The first alternative measure we consider is the average absolutevalue of the slope of the terrain. Thus, using the same gtopo30 elevation data, we calculate theaverage uphill slope of the country’s surface area.9 Our second alternative measure is the average
standard deviation of elevation within the same eight-cell neighbourhood. The third measure ismotivated by the possibility that what matters is having a large enough amount of sufficiently
rugged terrain nearby, even if some portions of the country are fairly flat. To capture this logic,
we calculate the percentage of a country’s land area that is highly rugged.10 All of these measurestreat land uniformly when averaging over cells to construct country averages. Thus, they do not
capture the possibility that ruggedness may be more important (and thus should be given moreweight) in areas that are more densely populated today. Therefore, our final alternative measure isa population-weighted measure of ruggedness. We start by calculating the ruggedness of each 30
by 30 arc-second cell but, in averaging this for each country, we weight ruggedness in each cell by
the share of the country’s population located in that cell.11
The second robustness check that we perform is a test of whether our results are robust when
8 Of independent interest is the relationship between ruggedness and our set of control variables. We do not find asignificant relationship between ruggedness and either diamond production, soil fertility, or distance from the coast. We
do find a negative relationship between ruggedness and the fraction of a country that has a tropical climate. As well,
we do not find Africa to be significantly more or less rugged than the rest of the world.
9 For every point on the 30 arc-seconds grid, we calculate the absolute value of the slope between the point and theeight adjacent points. The absolute values of the eight slopes are then averaged to calculate the mean uphill slope foreach 30 by 30 arc-second cell. We then average across all grid cells in a country not covered by water to obtain the
average uphill slope of the country’s land area. Again, our calculations take into account the latitude-varying sea-levelsurface that corresponds to the 30 by 30 arc-second cell centred on each point.
10 We use a threshold set at 240 metres for the terrain ruggedness index calculated on the 30 arc-seconds grid, below
which Riley et al., 1999 classify terrain as being ‘level’ to ‘intermediately rugged’.
11 The population data are for 2000 and are from the LandScan data set (Oak Ridge National Laboratory, 2001).

9

Table 2: Robustness with respect to influential observations
Dependent variable: Log real gdp per person 2000
Omit 10
most rugged

Omit 10smallest

Omit if √|dfbeta| > 2/ N

Usingln(Ruggedness)

Box-Cox Trans.
of Ruggedness

(1)

(2)

(3)

(4)

(5)

Ruggedness
Ruggedness · I Africa
I Africa
All controls
Observations
R2-0.202(0.083)∗∗0.286(0.133)∗∗-1.448-0.221-0.261(0.083)∗∗∗(0.068)∗∗∗

0.188

0.223

(0.099)∗(0.116)∗-1.465-1.510-0.171(0.051)∗∗∗

0.234(0.119)∗∗-1.083-0.249(0.075)∗∗∗0.333(0.142)∗∗-1.139(0.454)∗∗∗(0.405)∗∗∗(0.406)∗∗∗(0.394)∗∗∗(0.391)∗∗∗

Yes
160
0.520

Yes
160
0.545

Yes
164
0.564

Yes
170
0.527

Yes
170
0.533

Notes: Coefficients are reported with robust standard errors in brackets. ∗∗∗ , ∗∗ , and ∗ indicate significance at the 1, 5,
and 10 percent levels. All regressions include a constant and our full set of control variables: Diamonds, Diamonds· I Africa , % Fertile soil, % Fertile soil · I Africa , % Tropical climate, % Tropical climate · I Africa , Distance to coast, andDistance to coast · I Africa . Coefficients and standard errors for the control variable are reported in an online appendix.

we consider income from other time periods. When looking at time periods earlier than 2000,
we turn to data from Maddison (2007), which has much better historic coverage than the World
Bank.12 We find that our results are robust using income from any year between 1950 and 2000, orfor average annual income between 1950 and 2000.
We re-estimate our baseline estimating equation with the full set of controls (i.e., the specification in column (6) in table 1), using all the different combination of the income and ruggedness
measures. We find that the estimated positive differential effect of ruggedness is very robust.
In all regressions, we find that the differential effect of ruggedness within Africa is positive andstatistically significant.13
Robustness with respect to influential observations
Next, we check whether the results from table 1 are driven by some particularly influential outliers.
Figure 2 shows a scatter plot of income per person against ruggedness for African countries (toppanel) and non-African countries (bottom panel). In these plots of the raw data, one observes a
positive relationship for African countries and a negative relationship for non-African countries.
However, a number of observations appear as clear outliers in terms of their ruggedness. Our first
12 In 2000, Maddison only has data for 159 countries, compared to 170 for the World Bank. But once one starts to moveback in time, Maddison’s coverage is much better than the World Bank’s. For example, prior to 1980 the World Bank
does not have data on real per capita ppp-adjusted gdp. Maddison has data for 137 countries as far back as back to 1950.
13 For brevity, we do not report the results here. See the online appendix for the complete results.

10

12

African Countries

Log Real GDP per Person 2000

(coef. = .190, t−stat. = 1.70, N = 49)

SYC

GNQ

9

MUS
BWA
GAB

ZAF

TUN
NAM
DZA

CPV

EGY

SWZ

MAR
ZWE

6

GHA CMRGIN
AGO
MRTGMBCIV SDN
TGO
SEN
UGA
CAF
KEN
BFA
BEN
COG
NGA
MOZ
MDG
TCD
MLI GNBZMBNER CODETHBDI
MWI
TZA
SLE

0

DJI

RWA

ERI

2

LSO

COM

4

Ruggedness

6

12

Non−African Countries(coef. = −.203, t−stat. = −2.20, N = 121)

Log Real GDP per Person 2000

LUX

9

NOR
USA
IRL
DNK
NLD
BELGBRCAN FRA ISLJPN ITASWE
HKG
DEU
AUSFIN
PYF
ISR
SGP
PRI
NCL
ESP
ARE
CYP
NZL
MAC
PRT
MLT
BHSBHR
SVN
KOR
KWT
CZE
BRB
HUN ARGSAUOMNSVKKNA POL
ATG
EST
CHL
MEX CRITTO
MYS HRVLTU URY
LVA
GRD
BRA
RUS
TON
TUR
DOM
THA
COL
PAN
DMA
ROUBGRBLZ
IRN MKD
VEN
LCA
BIH
FJI WSM
BLR
PER
KAZ
SLV
PRY
UKR
GUY
PHL
GTM
CHN
JOR
TKM LKAJAM
NIC ECUSYRIDN
VUT
AZE
HND
ARM
IND
BOL
PNG
VNM
PAK
SLB
KHM
HTI
MNG
LAO
BGD UZB
MDA

CHE

AUT
GRC

VCT
LBN

ALB

GEO

KGZ

YEM

NPL

6

TJK

0

2

Ruggedness

4

6

Figure 2: Income and ruggedness among African and non- African countries

11

12

African Countries

Log Real GDP per Person 2000

(coef. = .242, t−stat. = 1.82, N = 49)

SYC

GNQ

9

MUS
BWA
GAB

DZA

ZAF

TUNNAM
CPV
EGY

SWZ

MAR
ZWE

6

GHA
CMR GIN
AGO
MRT
GMB
SDN
CIV
TGO
SEN
UGA
CAF
KEN
BEN
COGBFANGA
MOZ
MDG
TCD
GNB
MLI
ZMB
NER
COD
ETH
BDI
MWI
TZA
SLE

−3

DJI

ERI

LSO

COM
RWA

0

3

Ruggedness (Box−Cox Transformed)

12

Non−African Countries(oef. = −.213, t−stat. = −2.38, N = 121)

Log Real GDP per Person 2000

LUX

SGP

9

KNA
ATG
DMA

NOR
USA
IRL
CHE
AUT
DNK
BEL GBRCAN FRA ISLJPNITA
SWE
HKG
FIN
DEU
AUS
PYF
ISR
PRI
NCL
ESP
ARE
CYP
NZL
MAC
PRTMLTBHS
KOR SVN GRC
KWT
BHR
CZE
BRB
HUN
SAUOMN
ARG
SVK
POL
EST
MYSHRV MEXCRI CHLURY TTOLTU
LVA BRA
GRD
RUS
TON
TUR
DOM
THAROU
COL
PAN
MKD
BGR
BLZ
IRN
VEN
LCA
VCT
BIH
FJI
WSM
BLR PRY KAZPER SLVLBN
GUY UKR
PHL
GTM
CHN
JOR
ALB
TKM
JAM
LKA
ECU
NIC
SYR IDN
VUT
AZE
IND
BOL
PNG HND ARM
VNMGEOSLB PAK
KHM
HTI
MNG
LAO
KGZ
UZB
BGD
MDA
NPL

NLD

TJK

6

YEM

−3

0

Ruggedness (Box−Cox Transformed)

3

Figure 3: Income and ruggedness (Box-Cox transformed) among African and non-African countries

12sensitivity check estimates our baseline specification, with our full set of control variables, after
dropping the ten most rugged countries. The results are presented in column (1) of table 2.
In the scatter plot, one can also observe that small countries (based on land area) tend tohave either unusually high (e.g., Seychelles, identified in the figure by its iso 3166-1 code syc)or unusually low ruggedness (e.g., Saint Kitts and Nevis, kna). Given this, we perform a second
robustness check where the ten smallest countries are omitted from the sample. The estimates arereported in column (2) of table 2.14
We next adopt a more systematic approach to deal with influential observations, and remove
influential observations using each observation’s dfbeta, which is a measure of the difference inthe estimated coefficient for the ruggedness interaction (scaled by the standard error) when theobservation is included and when it is excluded from the sample. Following Belsley, Kuh, and√Welsch (1980), we omit all observations for which |dfbetai | > 2/ N, where N is the number ofobservations, in our case 170.15 Results are presented in column (3) of table 2.
In all three of the regressions with omitted observations, the ruggedness coefficient remains
negative and statistically significant, and the ruggedness interaction remains positive and statistically significant, confirming the existence of a differential effect of ruggedness within Africa.
The reason why, in figure 2, a small number of observations appear as particularly influential
is because the ruggedness measure is skewed to the left, leaving a small number of observations
with large values. We remedy this in two ways. First, we take the natural log of ruggedness anduse this in the estimating equations. This draws in the outlying observations in the regression.
The estimates of interest, reported in column (4), remain robust to this transformation. However,looking at the natural log of ruggedness variable, one finds that the measure is no longer left
skewed, but it is now right skewed, with a small number of influential observations taking on verysmall values. Because of this, we pursue a second strategy where we perform a zero-skewness BoxCox power transformation on the ruggedness variable to obtain a measure with zero skewness.
The relationships between income and the zero-skewness ruggedness measure are shown in figure
3. It is evident that the income-ruggedness relationships using the zero-skewness measure do notfeature influential, outlying observations. In addition, a different relationship within Africa andoutside of Africa is still apparent in the scatter plots of the data. Estimates using the zero-skewness
measure are reported in column (5). The estimates confirm the impression given by the figures.
There is a positive and significant differential effect of ruggedness within Africa.
Do other African characteristics or colonial rule explain the differential effect of ruggedness?
A final possible source of concern is that the differential effect of ruggedness for Africa is notreally an African effect. Perhaps it arises because the effect of ruggedness on income differs forareas with some geographic characteristic that happens to be particularly prevalent in Africa. For
14 A related concern is that our results may be driven by “atypical” African countries, such as island countries orNorth African countries. Our results are also robust to omitting these “atypical” African countries from the sample.
15 Using other measures and rules for the omission of influential observations, such as dfits, Cook’s distance or Welsch
distance, provide very similar results.

13

Table 3: Considering differential effects of ruggedness by characteristics prevalent in Africa
Dependent variable: Log real gdp per person 2000
(1)
Ruggedness
Ruggedness · I Africa
I Africa
Ruggedness · % Tropical cl.
Ruggedness · % Tropical cl.
% Tropical climate
Ruggedness · % Fertile soil
% Fertile soil
Ruggedness · Colonizer FEs
Colonizer FEs
Ruggedness · Legal Origin FEs
Legal Origin FEs
Observations
R2-0.259(0.101)∗∗0.357(0.130)∗∗∗-1.814(0.213)∗∗∗

0.001

(2)
-0.322(0.160)∗∗0.400(0.155)∗∗∗-1.977(0.223)∗∗∗(0.002)

Yes
Yes
No
No
No
No
No
No
170
0.404

No
No
Yes
Yes
No
No
No
No
170
0.363

(3)
-0.374(0.161)∗∗

0.360(0.140)∗∗-1.818(4)-0.386(0.176)∗∗0.399(0.203)∗∗-1.740(5)-0.543(0.179)∗∗∗0.435(0.135)∗∗∗-1.994(0.218)∗∗∗(0.337)∗∗∗(0.216)∗∗∗(0.002)(0.002)(0.002)

0.001

Yes
Yes
Yes
Yes
No
No
No
No
170
0.408

0.002

Yes
Yes
Yes
Yes
Yes
Yes
No
No
170
0.430

0.002

Yes
Yes
Yes
Yes
No
No
Yes
Yes
170
0.559

Notes: Coefficients are reported with robust standard errors in brackets. ∗∗∗ , ∗∗ , and ∗ indicate significance at the 1, 5,
and 10 percent levels.

instance, it could be that in countries where a large fraction of the territory experiences tropicalclimates, rugged areas are cooler, dryer, or even less prone to tropical diseases. If tropical climates
are particularly prevalent in Africa (they characterize 34.0% of land in Africa compared with 19.3%of the rest of the world excluding Antarctica), perhaps the interaction between ruggedness and theAfrica indicator is proxying for an interaction between ruggedness and tropical climates. Similarly,
it could be that in countries where a large fraction of the territory is covered by dry unfertile
soil, like desert, rugged areas are less arid. If areas with poor soil are particularly prevalent in
Africa (fertile soil comprises 22.5% of the land in Africa compared with 25.3% in the rest of theworld excluding Antarctica), perhaps the interaction between ruggedness and the Africa indicatoris proxying for an interaction between ruggedness and poor soil quality.
We consider these possibilities in columns (1)–(3) of table 3, where we add to our baseline
estimating equation variables measuring the percentage of each country with tropical climates andthe percentage of each country with fertile soil (these can be seen as playing the same role as theAfrica indicator), as well as interactions between ruggedness and these two variables (these can beseen as playing the same role as the interaction between ruggedness and the Africa indicator).16
In columns (1) and (2) we include each of the two sets of controls one at a time, and in column (3)we include them together. The coefficients of interest, measuring the common effect of ruggedness
16 The results are also robust if we use a measure of the proportion of a country’s land that is desert, rather than the
proportion with fertile soil.

14and the differential effect for Africa, change little and remain statistically significant.17
We next consider the possibility that our Africa indicator variable may be picking up theprevalence of colonial rule. In areas that were colonized, rugged terrain may have provided a way
to defend against colonial rule. Since a greater proportion of countries in Africa, relative to the restof the world, experienced colonial rule (within Africa 89.5% of the countries were colonized, while
outside of Africa this figure is 44.1%), the differential effect of ruggedness in Africa may be biased
by a differential effect of ruggedness in countries that were colonized.
We control for this possibility in columns (4) and (5) of table 3. In column (4), we include
five indicator variables for the identity of a country’s colonizer, with the omitted category being
for countries that were not colonized.18 We also include the set of colonizer indicator variables
interacted with ruggedness. The differential effect of ruggedness remains positive and statisticallysignificant.
Numerous studies have shown that differences in the legal origin of the colonizing powers
is an important determinant of a variety of country characteristics, including financial development, labour market regulations, contract enforcement, and economic growth (La Porta, Lopez-deSilanes, and Shleifer, 2008). Given the particular strong impact of colonial rule that works throughlegal origin, we also control directly for each country’s legal origin, by including four legal origin
indicator variables and their interactions ruggedness. The four indicators are for French, German,
Scandinavian, and Socialist legal origins, with the omitted category being British legal origin. The
positive differential effect of ruggedness remains when accounting for differences in countries’
legal origins.
Differential effects of ruggedness across regions within Africa
One concern with the results presented to this point is that we only allow the effect of ruggedness
on economic outcomes to differ for African countries. We have also checked whether one also finds
a positive and statistically significant differential effect of ruggedness within other parts of theworld. Treating other continents in the exact same manner that we have treated Africa in equation(5) (including a continent indicator and an indicator interacted with ruggedness), we find that forno other continent is there a positive and statistically significant differential effect of ruggedness.
In other words, the positive differential effect of ruggedness is unique to Africa, and is not found in
North America, South America, Europe, Asia, or Oceania.
Having determined that the differential effect of ruggedness is specific to the African continent,
we examine whether the strength of the effect differs across the regions within Africa in a mannerthat is consistent with the known history of the slave trades. Our argument is that ruggedness
17 One could also think that certain countries, because of inferior access to technology or poor governance, are worse
equipped to mitigate the common negative effects of ruggedness. However, note that this would work against estimating
a positive differential effect of ruggedness within Africa, since access to technology and governance are likely to
be worse on average in Africa. A further concern is that the tropical climate measure is potentially endogenous toruggedness, since some areas may not be classified as tropical if they are rugged. A preferable measure would quantify
how tropical a climate would be if it were not rugged. Using a country’s distance from the equator as a proxy for thismeasure yields very similar results.
18 The five categories for the identity of the colonizer are: British, Portuguese, French, Spanish, and other European.

15

Table 4: Differential effects of ruggedness across regions within Africa
Dependent variable: Log real gdp per person 2000
(1)
Ruggedness
Ruggedness · I Africa
I Africa
Ruggedness · I West Africa
I West Africa-0.203(0.093)∗∗0.312(0.159)∗∗-1.735(0.291)∗∗∗(2)-0.203(0.093)∗∗0.408(0.161)∗∗-1.844(0.229)∗∗∗(3)-0.203(0.093)∗∗0.409(0.147)∗∗∗-2.008(0.230)∗∗∗(4)-0.203(0.093)∗∗0.406(0.147)∗∗∗-2.046(0.222)∗∗∗(5)-0.203(0.093)∗∗0.448(0.179)∗∗-2.054(0.232)∗∗∗0.532(0.154)∗∗∗-0.635(0.283)∗∗

Ruggedness · I East Africa

0.162

I East Africa-0.760(0.274)(0.532)

Ruggedness · I Central Africa

0.575

I Central Africa

0.020(1.197)(0.597)

Ruggedness · I North Africa-0.404(0.131)∗∗∗I North Africa

1.465(0.241)∗∗∗

Ruggedness · I South Africa-0.200

I South Africa

0.592

Constant
Observations
R2(0.195)9.223∗∗∗(0.144)

170
0.367

9.223
∗∗∗

(0.144)

170
0.368

9.223
∗∗∗

(0.144)

170
0.359

9.223
∗∗∗

(0.144)170
0.375(0.519)9.223∗∗∗(0.144)170
0.363

Notes: Coefficients are reported with robust standard errors in brackets. ∗∗∗ , ∗∗ , and ∗ indicate significance at the 1, 5,
and 10 percent levels.

has a differential positive effect within Africa because no other continent was subject to the slave
trades that devastated Africa between 1400 and 1900. However, the exposure to the slave trades
was not uniform across the continent. West Africa was the region most severely impacted by theslave trades, whereas North Africa was barely touched.19 Thus, the logic of our core argument
suggests that ruggedness should have a more beneficial effect within West Africa, where the threatof being enslaved was greatest, but within North Africa, where slave capture was nearly absent,
the effect should be much smaller, and not very different from that in the rest of the world. To
check this, we examine the five regions of Africa defined by Bratton and van de Walle (1997):
West Africa, Central Africa, North Africa, South Africa, and East Africa. We construct an indicatorvariable for each region and then individually include each indicator variable and its interaction
19 The correlation between our measure of slave exports, described in detail in the next section, and a West Africa

indicator variable is 0.53 and is statistically significant. The correlation between slave exports and a North Africa
indicator variable is −0.30 and is also statistically significant. For all other African regions, the correlation between
slave exports and a region indicator variable is not statistically different from zero.

16

with ruggedness in equation (5). The estimates are reported in table 4. The results show thatfor West Africa and North Africa there is a statistically different effect of ruggedness relative tothe average for all of Africa. Within West Africa, the positive effect of ruggedness is significantly
larger. This is consistent with the positive effect of ruggedness working through the slave trades,
and with West Africa being the region most severely impacted by the slave trades. In North Africa,
where slave capture was almost completely absent, there is no positive effect of ruggedness.20 The
results also show that the other three regions lie between these two extremes. For these regions,the positive differential effect of ruggedness is not statistically different from that for Africa as awhole.
Our finding that, across regions within Africa, the magnitudes of the differential effects of theruggedness aligns closely with the intensity of the slave trades provides suggestive evidence thatthe differential effect of ruggedness within Africa is intimately linked to the slave trades. In thefollowing sections, we examine this directly, and provide additional evidence that this is in fact thecase.

5. Do slave exports account for Africa’s differential effect?
We now examine whether the slave trades can account for the differential effect of ruggedness
within Africa. Our first step is to check for direct evidence that ruggedness provided protection
against slave raiding. We do this using data from Nunn (2008) on the number of slaves taken fromeach country between 1400 and 1900 during Africa’s four slave trades. The figures are constructedby combining historical shipping records with slave inventories reporting slave ethnicities (seethe appendix and Nunn, 2008, for details). Because the variable is very skewed to the left andsome countries have zero slave exports, we take the natural logarithm of one plus the measure, i.e.
ln(1 + slave exports/area). Using these data, we estimate equation (8) from section 3. Results arereported in columns (5)–(7) of table 5.
Column (5) of table 5 reports the unconditional relationship between ruggedness and slave
exports among the 49 African countries in our sample. The estimate shows that there is a negativeand statistically significant relationship between ruggedness and slave exports, and that ruggedness alone explains almost 30% of the variation in slave exports. This confirms hypothesis 2 (i.e.,
β 12 < 0 in equation (8)). In columns (6) and (7), we include additional variables to address several
potential concerns regarding the relationship between ruggedness and slave exports. We firstinclude our baseline set of control variables. Among the four controls, the fraction of fertile soil
is the only covariate that is statistically significant. The positive coefficient likely reflects the factthat soil fertility was an important determinant of having a dense and sedentary initial population,
which led to more slaves being captured. In column (7), we include additional controls for otherfactors that may be important determinants of slave exports. We control directly for log population
density in 1400. This is a particularly important characteristic, since it is possible that the reason
fewer slaves were taken from countries with greater terrain ruggedness is that there were fewer
20 This is calculated by adding the coefficient of the North Africa interaction to the coefficient of the Africa interaction.
This gives: 0.406 + −0.404 = −0.002.

17

people living in more rugged areas, and not just because rugged terrain provided protection. The
variable has a positive and statistically significant coefficient. Since Nunn (2008) shows that slave
exports are decreasing in the distance from each country to the closest final destination in each of
the four slave trades, we also include the sailing distance from each country’s coast to the closest
final destination for the trans-Atlantic and Indian Ocean slave trades and the overland distance tothe closest final destination for the trans-Saharan and Red Sea slave trades (measured in thousandsof kilometres). For the trans-Atlantic and Indian Ocean slave trades, in addition to the voyage by
ship, slaves captured inland would have to be brought to the coast. Our distance to coast variableaccounts for this.21 The ruggedness coefficient remains negative and significant at the 1% leveleven after controlling for these additional factors.
Having established that rugged terrain deterred slave exports, we now turn to showing thatslave exports are negatively related to current economic outcomes and that this fully accounts forthe differential effect of ruggedness within Africa. In column (1) of table 5, we estimate equation(7) from section 3. This is identical to equation (5) (for which we reported estimates in column(1) of table 1), except that slave exports are also included in the estimating equation. Column (3)reports the same estimation as column (1) except that we also include our baseline set of control
variables from table 1 in the estimating equation. With or without the full set of controls, whenslave exports are controlled for, the differential effect of ruggedness within Africa disappears. The
estimated coefficient on ruggedness · I Africa is close to zero, and is no longer statistically significant.
This confirms hypothesis 4 (i.e., β 8 = 0 in equation (7)), and provides support for the explanationthat the differential effect of ruggedness arises because of the slave trades.
In columns (2) and (4), we re-estimate the specifications of, respectively, columns (1) and (3),leaving out the interaction between ruggedness and the Africa indicator variable. The estimatesconfirm hypothesis 3, which states that current economic outcomes in Africa are worse in places
more affected by the slave trades (i.e., β 10 < 0 in equation (7)). We can also see in columns (2)and (4) that the common effect of ruggedness, once slave exports are accounted for, is negative andvery close to the magnitudes from columns (1) and (6) of table 1. This confirms hypothesis 5, whichstates that β 7 in equation (7) provides a consistent estimate of the common effect of ruggedness on
income for the world as a whole.22
The estimates from table 5 can be used to calculate an alternative estimate of the indirect historiceffect of ruggedness on income. The coefficients for slave export intensity from columns (1) and(3) provide estimates of the effect of slave exports on income, i.e., βγ from equation (6). The coefficients for ruggedness from columns (5)–(7) provide estimates of λ from equation (1). Therefore,
the product of the two coefficients provides an alternative estimate of the indirect historic effect of
21 It is possible that land sufficiently distant from the coast was not exposed to the slave trades, and that therefore ameasure that places greater emphasis on the amount of land below a threshold distance from the coast is a more precise
determinant. If we use the percentage of a country’s land area that is more than 100 kilometres from the coast insteadof the distance to the nearest coast, the results are qualitatively identical.
22 If we estimate equation (7) without controlling for slave exports, then we estimate a small negative coefficient forruggedness that is not significantly different from zero (coefficient −0.067 with standard error 0.082). This is as expected.
The negative common effect of ruggedness is biased upwards (i.e., towards zero), since the positive effect of ruggedness
within Africa is not being taken into account.

18

Table 5: The impact and determinants of slave exports
Dep. variable:
Log real gdp
per person 2000
(1)
Slave export intensity
Ruggedness-0.203-0.222(3)-0.206(4)(0.035)∗∗∗(0.036)∗∗∗(0.034)∗∗∗(0.093)∗∗(0.077)∗∗(0.077)∗∗∗(0.066)∗∗∗-0.2030.124

I Africa-0.819∗∗∗-0.169-0.231(6)-0.220-1.330(0.262)∗∗∗-1.326-0.825∗∗(0.358)∗∗∗-0.005-0.001

0.042

0.031

0.013

0.003

0.154

-1.939-0.728(0.356)(0.354)∗∗(0.010)∗∗∗(0.010)∗∗∗(0.222)0.028-0.027% Fertile soil

0.028

(0.010)∗∗∗(0.003)(0.003)-0.002-0.002

0.000

0.001

% Tropical climate-0.009∗∗∗(0.002)-0.009∗∗∗(0.002)

0.009
∗∗∗

0.008
∗∗∗

(0.006)% Tropical climate · I Africa
Distance to coast(0.003)(0.194)(0.194)∗∗∗-0.162-0.191Distance to coast · I Africa(0.321)

Log pop. density 1400

(0.015)∗∗∗(0.006)(0.003)-1.039∗∗∗(0.006)(0.005)-0.027(0.010)∗∗% Fertile soil · I Africa(0.009)-1.039(1.174)(0.343)(0.019)(0.010)(1.694)0.326(0.179)∗Dist. Saharan slave market-1.670∗(0.914)Dist. Atlantic slave market-0.973(0.480)∗∗Dist. Red Sea slave market-0.082(0.635)Dist. Indian slave market
Observations
R2-0.989(0.274)∗∗∗(0.143)-0.591∗∗∗

Diamonds · I Africa

Constant

(7)

0.047

(0.152)(0.317)(5)-0.214(0.037)∗∗∗

Ruggedness · I Africa

Diamonds(2)

Dep. variable:
Slave export
intensity-0.925∗9.223∗∗∗(0.144)

170
0.418

9.175
∗∗∗

(0.127)

170
0.415

9.959
∗∗∗

(0.195)

170
0.586

9.943
∗∗∗

(0.195)

170
0.585

5.572
∗∗∗

(0.503)

49
0.289

3.575
∗∗∗

(1.251)49
0.448(0.486)22.359∗∗(10.008)49
0.587

Notes: Coefficients are reported with robust standard errors in brackets. ∗∗∗ , ∗∗ , and ∗ indicate significance at the 1, 5,
and 10 percent levels.

ruggedness βγλ. Because this is a direct estimate of the effect of ruggedness that works through theslave trades, it is potentially more precise than our reduced form estimate (i.e., β 2 from equation(5)), which is based solely on the differential effect of ruggedness within Africa.
Consider the estimates with our baseline set of control variables, reported in columns (4) andc = −0.206 and λ̂ = −1.326. Therefore, βγc λ̂ = −0.206 × −1.326 = 0.273.
(6) of table 5. They give βγ
We can compare this estimate to our reduced-form estimate reported in column (6) of table 1, which
is 0.321. The indirect effect of ruggedness working through slave exports is almost identical to the
reduced-form differential effect of ruggedness within Africa estimated in section 4. This provides
19reassuring confirmation that the reduced form differential effect of ruggedness within Africa is infact being driven by the historic effect of ruggedness working through Africa’s slave trades.
Economic magnitude of the effects
To this point we have been focusing on the statistical significance of our estimated coefficients,
ignoring the magnitude of their effects. Using the estimates from table 5, we now undertakea number of counterfactual calculations to show that the economic magnitudes of the indirect
historic impact of ruggedness, working through the slave trades, is substantial.
We first consider the estimated magnitude of the impact of the slave trades on income. For
context, consider a hypothetical African country with the mean level of slave exports and meanlog real gdp per person among African countries. According to the estimates from column (3) oftable 5, if this country was instead completely untouched by the slave trades, then its per capita
income would increase by $2,365, from $1,784 to $4,149.23
We next consider the magnitude of the historic benefit of ruggedness, which occurs throughreduced slave exports. Consider the benefit of a one standard deviation increase in ruggedness
from the average of 1.110 to 2.389. According to the estimates from column (6) of table 5, thisreduces slave exports by 1.326 × 1.279 = 1.70, which is a 0.54 standard deviation decline in slave
export intensity. This in turn increases log real gdp per person by $747, from the average $1,784 to$2,531, which is a 0.37 standard deviation increase in log income per person.24
These effects are substantial, particularly given that we are considering the historic impact of
one very specific geographic characteristic — terrain ruggedness — working through one historic
event — the slave trades.
The effect of slave exports on income through rule of law
We have so far estimated the indirect effect of ruggedness on income, βγλ, in two ways. First,
by estimating the reduced-form relationship between income and ruggedness from equation (4)d Second, by estimating
to obtain the combined differential effect of ruggedness within Africa βγλ.
separately the effect of ruggedness on slave exports from equation (1) to obtain λ̂ and the effect of
c A third alternative is to estimate the threeslave exports on income of equation (6) to obtain βγ.
equations (1)–(3) separately to obtain λ̂, β̂, and γ̂ independently. One problem with this third
alternative is that it is difficult to obtain an appropriate measure for qi , which summarizes thedifferent aspects of the organization of societies that are negatively affected by the slave trades. As
a partial step in this direction, we use the “rule of law” variable from the World Bank’s Worldwide
23 This is calculated from: ln y0 = ln 1,784 − 0.206 × (−4.09), where 4.09 is the mean slave export intensity measure

among African countries, −0.206 is the estimated impact of slave exports on income (from column (3) of table 5), and y0
denotes the counterfactual income, had the slave trades not occurred in the hypothetical country. Solving for y0 gives$4,149.
24 This is calculated from: ln y0 = ln 1,784 − 0.206 × (−1.326 × 1.279), where 1.279 is the standard deviation ofruggedness among African countries, −1.326 is the estimated impact of ruggedness on slave exports (from column(6) of table 5), and −0.206 is the estimated impact of slave exports on income (from column (3) of table 5). Solving for y0
gives $2,531.

20

Table 6: The effect of slave exports on income through rule of law
Dep. variable:
Log real gdp
per person 2000
(1)
Rule of law 1996-2000
Ruggedness
I Africa
Slave export intensity

0.871

(2)
(0.059)∗∗∗(0.041)(0.039)-0.699(0.131)∗∗∗(3)-0.051-0.109(0.352)

Diamonds

0.009

Diamonds · I Africa-0.009% Fertile soil

0.000

% Fertile soil · I Africa-0.015∗∗-0.147(0.067)∗∗-0.509-0.885-0.935(0.344)∗∗∗(0.031)∗∗∗(0.034)∗∗∗(0.033)∗∗∗(0.009)∗∗∗(0.008)∗∗-0.086-0.0980.028-0.026(0.015)(0.009)∗∗∗(0.002)(0.003)-0.0020.019-0.017(0.008)∗∗0.003(0.003)0.006-0.010∗∗∗-0.011∗∗∗(0.006)(0.005)-0.002-0.1000.011∗∗(0.001)(0.002)

0.003

0.004

Distance to coast-0.221Distance to coast · I Africa(0.005)(0.002)0.006(0.003)(0.003)∗∗(0.174)(0.189)∗∗∗(0.162)∗∗∗-0.576∗0.233-0.340(0.003)-0.984(0.347)(0.296)I French civil law-0.427(0.270)-0.528∗∗∗(0.157)

I French civil law · I Africa

0.463(0.230)∗∗

I Socialist law-1.183(0.192)∗∗∗I German civil law

0.640(0.331)∗

I Scandinavian law
Observations
R2-0.156(0.049)∗∗∗(0.306)∗∗∗% Tropical climate · I Africa

Constant(5)(0.188)∗∗∗(0.014)% Tropical climate

(4)

0.813

(0.044)∗∗∗-0.034

Dep. variable:
Rule of
law

0.774

8.783

(0.076)∗∗∗

169
0.746

8.922

(0.159)∗∗∗

169
0.776

0.218

(0.087)∗∗

169
0.191

1.113

(0.198)∗∗∗

169
0.449

(0.209)∗∗∗1.244(0.226)∗∗∗

169
0.644

Notes: Coefficients are reported with robust standard errors in brackets. ∗∗∗ , ∗∗ , and ∗ indicate significance at the 1, 5,
and 10 percent levels.

Governance Indicators database (Kaufmann, Kraay, and Mastruzzi, 2008). Estimates of equations (1)and (2) using this variable are reported in table 6.
The first two columns of the table report estimates of equation (1), which captures the effects ofinstitutional quality, as proxied by the rule of law, on real per capita income in 2000. In column (1),we control for the Africa indicator variable only, and in column (2) we also control for our standard
set of control variables and their interactions with the Africa indicator variable. The estimatesshow a strong negative, and statistically significant, relationship between the rule of law and per
capita income. This result confirms the findings from a number of previous studies that stress the21

importance of governance and domestic institutions for long-term economic development (e.g.,
Acemoglu et al., 2001).
Columns (3)–(5) of table 6 report estimates of equation (2), which models the relationshipbetween slave exports and the quality of the organization of societies. The estimates of column(3) control for the Africa indicator variable only. We include the Africa indicator to ensure that
our estimated effect of slave exports on institutional quality is not estimated from the difference
between Africa and the rest of the world. Because slave exports are zero for all countries outside of
Africa, and because we always control for an Africa fixed effect, the estimated coefficient for slave
exports is estimated from the relationship between slave exports and institutional quality within
Africa only. In columns (4) and (5), we include additional control variables. We first include ourbaseline set of control variables and their interactions with the Africa indicator variable. Then, incolumn (5), we also add our legal origin fixed effects and their interactions with the Africa indicatorvariable.25 The estimates provide strong support for the slave trade adversely affecting domestic
institutions today. The coefficient for slave exports is negative and statistically significant.
Combining the estimated coefficients λ̂ = −1.326 from column (6) of table 5, β̂ = 0.813 fromcolumn (2) of table 6, and γ̂ = −0.065 from column (4) of table 6 yields λ̂ × β̂ × γ̂ = 0.070.
Like the reduced-form estimate from column (6) of table 1, the indirect effect of ruggedness isfound to be positive. However, the magnitude from the structural estimates is just under one
fourth of the magnitude implied by the reduced-form estimate. This occurs because our structural
estimates implicitly assume that the only effect of slave exports on income is through the ruleof law. Any effect of the slave trade on per capita income that does not occur through ourmeasured rule of law will not be captured when we estimate β and γ individually. This is not truec = −0.206.
however, for our estimate of the relationship between slave exports and income, βγ
The relationship between slave exports and income implied by the individual estimates of β andγ is β̂ × γ̂ = 0.813 × −0.065 = −0.053. The difference between the two estimated magnitudes
is consistent with the slave trade affecting income through channels other than the rule of law.
Exploring such channels is the subject of ongoing research. For instance, the recent results ofNunn and Wantchekon (2008), which show that the slave trades had a negative effect on levels oftrust 100 year after the end of the trade, provide evidence that the slave trades likely affect current
income levels through a variety of additional channels other than the rule of law.26
25 Because our regression includes an Africa indicator variable, a full set of legal origin indicator variables, and interac-tions between them, non-African British common law countries constitute the omitted baseline category. Therefore, thedifferential effect (relative to this baseline) of the other legal origins for non-African countries is given by the coefficientsof the legal origin indicator variables, while the differential effect of the other legal origins for African countries is givenby the interaction of the legal origin indicators with the Africa indicator variable. Because African countries are only ofeither British or French legal origin, and none are of Socialist, German, or Scandinavian legal origin, indicators variables
for these later three groups interacted with the Africa indicator variable are dropped from the regression.
26 An additional channel through which the slave trades can negatively affect income today is ethnic conflict. Slaves
were almost exclusively captured by other Africans (Koelle, 1854, Nunn and Wantchekon, 2008). This triggered conflicts
between neighbouring ethnicities, which may still persist today (e.g., Azevedo, 1982, Inikori, 2000, Hubbell, 2001).
Identifying this channel empirically is difficult, because it is possible that the seeds of ethnic conflict were planted
before the slave trades and we do not have data on ethnic conflict prior to the slave trades. However, we note that our
results are robust to controlling for years of violent civil conflict 1975–1999, using data from Fearon and Laitin (2003).

22

6. Conclusions
The study provides evidence showing that geography can have important effects on income
through its interaction with historical events. By focusing on a dimension of geography, terrain
ruggedness, which varies throughout the world and on a historical event, the slaves trades over theperiod 1400-1900, which is geographically confined to Africa, we are able to estimate the indirect
historic effect of ruggedness on income. For the world as a whole, we find a negative relationshipbetween ruggedness on income. We also find that rugged terrain had an additional effect in Africa
during the 15th to 19th centuries: it afforded protection to those being raided during Africa’s
slave trades. By allowing areas to escape from the detrimental effects that the slave trades had on
subsequent economic development, ruggedness also creates long-run benefits in Africa throughan indirect historic channel. We show that this differential effect of ruggedness is found in Africa
only, it cannot be explained by Africa’s unique geographic environment, and it is fully accounted
for by Africa’s the slave trades. On the whole, the results provides one example of the importanceof geography through historic channels.

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Data appendix
Data availability
All the data necessary to reproduce the results of this paper are available from http://diegopuga.
org/data/rugged/.
Country boundaries
We assign geographic features to countries using digital boundary data based on the fifth editionof the Digital Chart of the World (us National Imagery and Mapping Agency, 2000), which we haveupdated to reflect 2000 country boundaries using information from the International Organizationfor Standardization iso 3166 Maintenance Agency and other sources. We exclude areas coveredby inland water area features contained in the same edition of the Digital Chart of the World. Whencalculating the percentage of each country’s land surface area with certain characteristics or the
average value of a variable for a country, we perform all calculations on a 30 arc-second geographic
grid, correcting for the fact that the actual land area covered by a 30 arc-second cell varies withlatitude.
Land area
The land area data are from the Food and Agriculture Organization (2008), except for Macau andHong Kong where it is taken from the Encyclopædia Britannica.
Income per person
We measure average country-level income by the natural logarithm of real gross domestic productper person in 2000. The data are from the World Bank World Development Indicators (World Bank,
2006). Units are 2006 international dollars, with purchasing power parity conversions performed
using the Elteto-Koves-Szulc method.
To check the robustness of our results to the use of income data from other time periods andfrom an alternative source, in the text we refer to results using the natural logarithm of real grossdomestic product per person in 1950 and in 2000, and its annual average from 1950–2000, withdata from Angus Maddison (Maddison, 2007, updated October 2008). Units are 1990 international
dollars, with purchasing power parity conversions performed using the Geary-Khamis method.
Gem-quality diamond extraction
Data on carats of gem-quality diamond extracted by each country between 1958–2000 are obtained
from the 1959–2004 editions of the Mineral Yearbook, published first by the us Bureau of Mines(us Bureau of Mines, 1960–1996) and then by the us Geological Survey (us Geological Survey,
27

1997–2007). We use the most recent data for each country-year in Volume i (Metals and Minerals),
completed with data from Volume iii (Area Reports: International) of the 1997–2000 editions. For
countries that have split or changed boundaries, we assign diamond extraction on the basis ofmine location with respect to current boundaries. The variable is then normalized by land area toobtain carats of gem-quality diamond per square kilometre.
Percentage of each country with fertile soil
On the basis of the fao/unesco Digital Soil Map of the World and linked soil association composition table and climatic data compiled by the Climate Research Unit of the University of East
Anglia, Fischer, van Velthuizen, Shah, and Nachtergaele (2002) identify whether each cell on a
5-minute grid covering almost the entire land area of the Earth is subject to various constraints
for growing rain-fed crops. Based on plates 20 (soil moisture storage capacity constraints), 21(soil depth constraints), 22 (soil fertility constraints), 23 (soil drainage constraints), 24 (soil texture
constraints), and 25 (soil chemical constraints) in Fischer et al. (2002) and the country boundaries
described above, we calculate the percentage of the land surface area of each country that has fertile
soil (defined as soil that is not subject to severe constraints for growing rain-fed crops in terms ofeither soil fertility, depth, chemical and drainage properties, or moisture storage capacity). Cape
Verde, French Polynesia, Mauritius and Seychelles are not covered by the Fischer et al. (2002) data,
so for these countries we use instead the percentage of their land surface area that is classified by
the Food and Agriculture Organization (2008) as arable land or permanent crop land.
Percentage of each country with tropical climate
Using detailed temperature and precipitation data from the Climatic Research Unit of the University of East Anglia and the Global Precipitation Climatology Centre of the German Weather Service,
Kottek, Grieser, Beck, Rudolf, and Rubel (2006) classify each cell on a 30 arc-minute grid covering
the entire land area of the Earth into one of 31 climates in the widely-used Köppen-Geiger climateclassification. Based on these data and the country boundaries described above, we calculate thepercentage of the land surface area of each country that has any of the four Köppen-Geiger tropicalclimates.
Average distance to the nearest ice-free coast
To calculate the average distance to the closest ice-free coast in each country, we first compute thedistance to the nearest ice-free coast for every point in the country in equi-rectangular projection
with standard parallels at 30 degrees, on the basis of sea and sea ice area features contained in
the fifth edition of the Digital Chart of the World (us National Imagery and Mapping Agency, 2000)
and the country boundaries described above. We then average this distance across all land in eachcountry not covered by inland water features. Units are thousands of kilometres.

28

European colonial origin indicators
European colonial origin indicators are based on Teorell and Hadenius (2007). They distinguishbetween British, French, Portuguese, Spanish, and other European (Dutch, Belgian and Italian)colonial origin for countries colonized since 1700. For countries under several colonial powers,
the last one is counted provided that it lasted for 10 years or longer. Since Teorell and Hadenius(2007) exclude the British settler colonies (the United States, Canada, Australia, Israel and New
Zealand), we code theses as having a British colonial origin. We complete their data using thesame rule to determine the European colonial origin of French Polynesia (French), Hong Kong
(British), Macau (Portuguese), New Caledonia (French), Nauru (British), Philippines (Spanish),
Puerto Rico (Spanish), and Papua New Guinea (British).
Legal origin indicators
Legal origin indicators (common law, French civil law, German civil law, Scandinavian law, andSocialist law) are from La Porta, Lopez-de-Silanes, Shleifer, and Vishny (1999). Some of ourregressions include French Polynesia, absent from their data, which we have coded as French civil
law.
African region indicators
Region indicators for Sub-Saharan Africa (East Africa, Central Africa, West Africa, and South
Africa) are from Bratton and van de Walle (1997). We assign African countries North of the Saharandesert, which were not classified by Bratton and van de Walle (1997), to the region of North Africa.
Slave exports
Estimates of the number of slaves exported between 1400 and 1900 in Africa’s four slave trades arefrom Nunn (2008). The data are constructed by combining shipping data with data from various
historic documents reporting the ethnicities of slaves shipped from Africa. Combining the twosources, Nunn is able to construct an estimate of the number of slaves shipped from each countryin Africa between 1400 and 1900 during Africa’s four slave trades. We normalize the export figures
by a country’s land surface area, computed as explained above. Because some country’s havezero slave exports, we take the natural logarithm of one plus the number of slaves exported per
thousand square kilometres. See Nunn (2008) for more information on the nature of the data,
including why it is appropriate to use the natural logarithm of slave exports.
Quality of governance
To measure the quality of governance in each country, we use the composite variable “rule of law”
from version vii of the World Bank’s Worldwide Governance Indicators database (Kaufmann et al.,2008). It consists of “perceptions of the extent to which agents have confidence in and abide by therules of society, and in particular the quality of contract enforcement, property rights, the police,
and the courts, as well as the likelihood of crime and violence” (Kaufmann et al., 2008, p. 7).
29

Distance to export markets
Four variables measuring the distance from each country to the closest final destination slavemarket in each of Africa’s four slave trades are taken from Nunn (2008). For the trans-Atlantic
and Indian Ocean slave trades, the measure is the sailing distance from the point on the coast thatis closest to the country’s centroid to the closest final export destination for slave trade. For thetrans-Saharan and Red Sea slave trades, the measure is the great-circle overland distance from thecountry’s centroid to the closest final export destination for that slave trade. Units are thousandsof kilometres.
Population density in 1400
The data are constructed using historic population estimates from McEvedy and Jones (1978). For
countries grouped with others in McEvedy and Jones (1978), we allocate population to countries in
the group according to the distribution of population in 1950, obtained from United Nations (2007).
We normalize total population in 1400 by the land area of each country, calculated as describedabove. Because the variable is extremely skewed to the left and because the territory covered bysome countries today had zero population density in 1400, we take the natural logarithm of one
plus population density (measured in people per square kilometre).

30