The Economic Case for Probablity-Based Sentencing
Ron Siegel and Bruno Strulovici∗
September 6, 2019

Abstract
Evidence in criminal trials determines whether the defendant is found guilty, but is
usually not one of the factors formally considered during sentencing. A number of legal
scholars have advocated the use of sentences that reflect the strength of evidence. This
paper proposes an economic model that unifies the arguments put forward in this literature
and addresses three of the remaining objections to the use of evidence-based sentencing:i) political legitimacy (the impact on the coercive power of the state), ii) robustness todetails of the environment, and iii) incentives to acquire evidence.

∗

We thank Robert Burns, Andy Daughety, Eddie Dekel, Louis Kaplow, Fuhito Kojima, Adi Leibovitz, Aki

Matsui, Paul Milgrom, Jennifer Reiganum, Ariel Rubinstein, Kathy Spier, Jean Tirole, and Leeat Yariv for
their comments. The paper benefited from the reactions of seminar participants at UC Berkeley, Seoul National
University, the NBER, the World Congress of the Econometric Society, the Harvard/MIT Theory workshop,
Caltech’s NDAM conference, Duke, Penn State, Johns Hopkins, the Pennsylvania Economic Theory Conference,
Bocconi University, Oxford University, Kyoto University, Tokyo University, the Toulouse School of Economics, theHarris School of Public Policy, and the Summer School of the Econometric Society 2019. David Rodina provided
excellent research assistance. Previous versions of the paper were circulated under the name “Multiverdict
Systems.” Strulovici acknowledges financial support from an NSF CAREER Award (Grant No. 1151410) anda fellowship form the Alfred P. Sloan Foundation. Siegel: Department of Economics, The Pennsylvania State
University, University Park, PA 16802, rus41@psu.edu. Strulovici: Department of Economics, Northwestern
University, Evanston, IL 60208, b-strulovici@northwestern.edu.

11Introduction

Criminal cases traditionally end with one of two outcomes: acquittal or conviction. An acquittal
entails no punishment, regardless of the strength of evidence in the case, whereas a conviction
requires that guilt be established “beyond a reasonable doubt” (or some other demanding criterion) and entails a sentence that does not (officially, at least) depend on any residual doubt
concerning the defendant’s guilt.
Recently, a number of scholars (Bray (2005), Lando (2005), Fisher (2011, 2012), Teichman(2017), and Spottswood (2019)) have advocated the use of a probabilistic sentencing model thatwould reflect the strength of evidence and residual doubt more gradually than the thresholdmodel that underlies the binary verdicts used in practice.
These authors put forth several arguments in favor of probabilistic sentencing, which range
from achieving more effective deterrence to improving outcome equality for similar trials andto reducing the effect of biases on judicial outcomes. This recent development in legal thoughtparallels an earlier literature concerning probability-based damages, pioneered by Coons (1966)
and further explored by Kaye (1982), Shavell (1985), Fischer (1993), and Davis (2003), among
others.1 Legal scholars have also noted that binary verdicts may owe their persistence to idiosyncratic historical roots rather than to deep philosophical principles and refuted a number ofphilosophical objections to the use of probabilistic sentencing.2
1

Similar ideas have recently found their way into actual legal practice, such as the “loss of chance” doctrine

in medical malpractice lawsuits (Fischer (2001)) and the concept of market share liability in tort law (Rostron(2004)), a theory under which a plaintiff unable to identify the manufacturer of the product that caused his
injury can recover on a proportional basis from each manufacturer that might have made the product.
2

Fisher (2012) provides an in-depth account and defense of the philosophical implications of probabilistic

sentencing, which includes the expressive role of verdicts (i.e., the message sent to society about a particulardefendant and a particular crime) and the retributive role of verdicts, which requires that, as a moral principle,
guilty defendants must be punished and innocent defendants must be acquitted regardless of any utilitarianconsideration. In particular, Fisher observes (p. 875) that “Retributivists would criticize the probabilistic modelfor eroding the evidence threshold and thereby weakening the moral legitimacy of inflicting criminal punishment.
Imposing punishment based on evidence with a probative weight falling below the beyond-a-reasonable-doubt
standard (i.e., below the standard of moral certainty) results in the defendant’s objectification, causing moral
harm that is incommensurable with deterrence utilities,” Fisher answers (p. 876) that “retributivist theories
cannot, in fact, justify any type of decision regime, the present threshold model included.” She quotes Reiman
and van den Haag (1990), who assert that “the retributivist theories cannot justify the beyond-a-reasonable-

2The recent advocacy of probabilistic sentencing has been weaker along three dimensions.
The first one concerns political legitimacy: the fact that probabilistic sentencing would givethe state the right to incarcerate citizens of “less than certain” guilt. Fisher (2012, p. 882)
describes this concern as follows: “the threshold model, and its pivotal requirement of proof
beyond a reasonable doubt, are, thus, a matter of political morality and serve as one of the main
limitations on state power in modern systems,” and acknowledges (p. 883) that “the challenge
posed by probabilistic decision making to the liberal state model is, indeed, a strong case against
its implementation.”3
Second, probabilistic sentencing may have an adverse effect on the incentives to acquire
evidence. In particular, Lando (2005, p. 285) expresses a concern that “graduating sanctions
may affect the strategies and incentives to search for evidence of both defense lawyers andprosecutors” but notes that “a it is not obvious what the effect will be.”Third, the robustness of improvements offered by probabilistic sentencing have yet to be
explored. Beyond qualitative arguments and Bayesian computations requiring minute knowledgeof all institutional details concerning a case, are there robust ways of improving welfare withprobabilistic sentencing? One reason that little is known about robustness is that most legal
theories of probabilistic sentencing have, despite the quantitative nature of probabilities, been
discussed qualitatively, with the exception of Lando (2005), and to some extent Fisher (2011),
who anchors her analysis with some numerical examples. This has led to some confusion as
to which insights of this literature were actually correct. In particular, Fisher (2012, p. 856)
suggests that Lando’s analysis may in fact justify, rather than discredit, the threshold model,
and argues that defendants’ risk attitudes, as well as the adverse effect of wrongful acquittals on
deterrence, must be taken into account in order to justify the probabilistic sentencing model.4
doubt threshold, as it is inconsistent with the retributivist commitment to punishing the guilty: ”Should we try
to convict fewer innocents and risk letting more of the guilty escape, or try to convict more of the guilty, and,unavoidably, more of the innocent? Retributivism (although not necessarily retributivists) is mute on how highstandards of proof ought to be...”3

Fisher does propose arguments in favor of probabilistic voting even in this case, the most prominent of whichechoes some of Lando’s (2005) analysis. As we explain below, however, these arguments are problematic because
they entail raising the sentence to some defendants who are found guilty.
4

Another confusion, which underlies this literature, concerns the view that deterrence is achieved more effec-

tively by threshold rules than by probabilistic ones. This view can be traced to Kaye (1982) who, in the context
of tort law, argues that the preponderance of evidence criterion (which is a threshold rule) provides more effective

3

This paper offers an economic analysis of probabilistic sentencing that addresses these threeissues. To simplify the exposition, we focus our analysis on the addition of a third, intermediate
verdict to reflect more finely the strength evidence against a defendant, and later discuss anextension to more verdicts.
In our framework, citizens benefit heterogeneously from committing a crime and abstain
from its commission if the expected disutility from punishment exceeds their benefit from thecrime. We consider a welfare objective function that captures deterrence, type I/type II error
considerations, and the cost of generating evidence.
Our first set of results shows how an intermediate verdict can be used to improve welfare
while addressing fully the political legitimacy concern. We show that, starting from the best
possible binary verdict (i.e., one for which the sentence is a 2-step function of the strength ofevidence), it is always possible to construct a three-verdict system that i) increases expectedwelfare, ii) does not increase the probability of conviction of any defendant, and iii) does notincrease the sentence length of any defendant. We show that such an improvement exists even
if deterrence plays a major role in the social objective. Part iii) guarantees that probabilistic
sentencing improves welfare, even under the constraint that the state cannot be more coercive
than under the binary verdict system.
Moreover, we show that the form of the three-verdict improvement does not depend on
the minute details of the welfare and utility functions used for the analysis, on the particularthreshold used by the binary verdict, or on the particular technology used to gather evidence.
Our construction thus addresses both the political legitimacy and the robustness concerns.
Next, we exploit the expressive function of verdicts to provide a second answer to the politicallegitimacy concern: when criminal trials carry some stigma for the defendant, we show that
acquittals too can be split into two distinct verdicts that entail no direct punishment but carrydifferent stigmas. Our result provides an explicit illustration of Fisher’s (2012) argument that theexpressive function of trials can in fact be enhanced by finer evidence-based verdicts and a formal
deterrence than probabilistic damages. This view has been developed further by Shavell (1987) and is invokedin criminal law to suggest that the social costs arising from type I and type II errors (wrongful convictions andwrongful acquittals) would be better addressed by a threshold rule than by probabilistic sentencing. Thus, Fisher(2012, p. 861) is concerned that “Incorporating the ex post error-costs into the equation may, indeed, constrict
the probabilistic regime’s scope of operation.” These confusions call for a formal framework to investigate thevalidity and robustness of earlier insights.

4

foundation for Bray’s (2005) advocacy of “not proven” verdicts. Several countries, including
Israel, Italy, and Scotland distinguish among acquitted defendants based on the residual doubt
regarding their guilt. In Scotland, for example, a conviction in a criminal trial leads to a “guilty”
verdict, but an acquittal leads to either a verdict of “not guilty” or “not proven.” Neither of thetwo acquittal verdicts carries any jail time, but the latter indicates a higher likelihood that thedefendant is in fact guilty.5 The likelihood is, however, insufficiently high for conviction.6
To address the incentives for acquiring evidence, we explore the consequence of adding athird verdict for the value of acquiring evidence. As noted by Lando (2005), one may a priori beconcerned that probabilistic sentencing reduces incentives for acquiring evidence. We show that
introducing an intermediary verdict can increase the value of evidence acquisition, and in factsystematically for some regions of the belief space. To study the value of information acquisition,
we begin with a simple model of one-shot information acquisition, and then embed the analysis
into a modern, tractable model of continuous information acquisition, for which the shape of thevalue function is easy to characterize and the systematic positive impact of a third verdict on
the value of evidence is more easily presented.
In our framework, a defendant can have an arbitrary risk attitude with respect to his sentence
and an arbitrary prior probability of guilt,7 the social costs due type I and type II errors follow
arbitrary specifications, the risk of wrongful acquittal is fully taken into account, and the relative
weights of deterrence and error considerations are also arbitrary. We show that probabilistic
sentencing is valuable irrespective of these dimensions, and thus unify and clarify earlier insights.
To focus on the trial aspect of criminal justice, and in line with the aforementioned literature,
our model does not allow for plea bargaining. While plea bargaining is of paramount importancein the United States, its use in many other countries is much more limited, especially for serious
5

The introduction of a not-proven verdict is considered by Daughety and Reinganum (2015a), who study how

the effect of informal sanctions on defendants and prosecutors affect the plea bargaining process and its acceptance
rate, and consider the effect of a not-proven verdict in this context. Daughety and Reinganum (2015b) consider
two implementations of a not-proven verdict. In the first one, the defendant can choose between the standard
binary verdict system and the system with a not-proven verdict. In equilibrium, all defendants choose the lattersystem. The authors also analyze an alternative implementation in which some defendants who are found not
guilty are compensated.
6This may happen, for example, if an eye-witness testimony exists, but the testimony cannot be corroborated.

7

Both features distinguish our work from Lando (2005), who assumes risk neutrality and implicity imposes a

uniform prior.

5

crimes. In Siegel and Strulovici (2019) we consider a different question of criminal trial designin a model that allows for plea bargains.
The appendix contains the continuous-time evidence gathering model and a micro-foundation
for the Bayesian formulation used in Section 4.1. This micro-foundation establishes that trial
technology conceptualized as a mapping from accumulated evidence to a verdict can always
be reformulated in Bayesian fashion: accumulated evidence is a signal that turns the priorprobability that the defendant is guilty into a posterior probability, on which the verdict is based.
Moreover, this transformation establishes a relationship between two notions of ‘incriminating’and ‘exculpatory’ evidence. One notion is based on decisions and the other on beliefs. Whatmakes a piece of evidence ‘incriminating’ is the fact that it increases the likelihood of guilt ofa defendant and, hence, results in a longer expected sentence. In particular, there is no lossof generality when one says that a guilty defendant is more likely than innocent defendant togenerate incriminating evidence.

2

Model

We consider a trial whose objective is to determine whether a defendant is guilty of committinga certain crime and to deliver the corresponding sentence. In our baseline model the trial
is summarized by two numbers: the probability πg that the defendant is found guilty if heis actually guilty, and the probability πi that the defendant is found guilty if he is actuallyinnocent.8 Corresponding to a guilty verdict is a sentence s > 0, interpreted as jail time (so ahigher value of s corresponds to a harsher punishment).9Society wishes to avoid punishing the defendant if he is innocent, and adequately punish
him if he is guilty. This dual goal is modeled by a differentiable welfare function W . Jailing an
innocent defendant for s years leads to a welfare of W (s, i), with W (0, i) = 0 and W decreasing
in s. Jailing a guilty defendant leads to a welfare of W (s, g), which has a single peak at ŝ > 0.
Thus, ŝ is the punishment deemed optimal by society if it is certain that the defendant is guilty.
The assumption that W (s, g) increases up to ŝ and then decreases is in line with US sentencing
guidelines, which state that “The court shall impose a sentence sufficient, but not greater than8

It is natural to assume that πg > πi , i.e., a defendant is more likely to be found guilty if he is actually guilty

than if he is innocent. This assumption is, however, not required for this section.
9

We leave aside such issues as mitigating circumstances, which are tangential to the focus of the paper.

6

necessary, to...reflect the seriousness of the offense... and to provide just punishment for theoffense.”10
The relative importance of punishing the defendant if he is guilty and not punishing him if
he is innocent depends on the prior probability λ ∈ (0, 1) that the defendant is guilty. This iscaptured by the interim social welfare from the defendant going to trial when the punishment
of being found guilty is s:
W2 (s) = λ [πg W (s, g) + (1 − πg )W (0, g)] + (1 − λ) [πi W (s, i) + (1 − πi )W (0, i)] .

(1)

Since W (·, i) is decreasing and W (·, g) peaks at ŝ, it is never interim optimal to choose s > ŝ.
Society’s ex-ante welfare also depends on whether the crime is committed in the first place.
To model this, we consider an individual’s decision whether to commit the crime, and assume
that at most one individual is prosecuted for the crime if it is committed.11 In a large society, theprobability that any particular innocent individual is prosecuted for the crime is infinitesimal,
so for expositional convenience we assume that an innocent individual treats this probability as
0.12 If the individual commits the crime, he obtains a benefit b (in utility terms), but faces aprobability ηg of being arrested and prosecuted.13 Thus, the individual commits the crime if
b + ηg (πg u(s) + (1 − πg )u(0)) > 0,(2)where u (s) ≤ 0 is the defendant’s differentiable utility from a sentence s, and the utility fromnot being prosecuted is normalized to 0. Denote by H(s) the fraction of individuals for whom(2) holds. The benefit b is distributed in the population according to an absolutely continuous
cdf B, so by (2) we haveH(s) = 1 − B (−ηg (πg u(s) + (1 − πg )u(0))) .
10(3)

See 18 U.S.C § 3553. These guidelines also state that another goal is “to protect the public from further crimesof the defendant.” This incapacitation reasonably increases at a rate that decreases in the sentence, whereas thedisutility a prisoner experiences increases with his sentence, which together may also give rise to single-peakedsocial welfare.
11This allows us to abstract from interdependencies between multiple defendants, an issue that is tangential

to the focus of this paper. See Silva (2018) for an analysis of such issues.
12

The probability 1 − λ > 0 that a prosecuted individual is innocent is, however, not infinitesimal.

13

This probability can be endogenized by including the amount of costly law enforcement as a decision variablewithout changing any of the results.

7

By normalizing the welfare from no crime to 0, we obtain that the ex-ante social welfare is
H (s) (ηg (πg W (s, g) + (1 − πg )W (0, g)) + ηi (πi W (s, i) + (1 − πi )W (0, i)) − h) ,(4)

where ηi is the probability that an innocent defendant is prosecuted and h is the social harmfrom the crime.14 In particular, since that sentence s determines which individuals commit thecrime and which are deterred,15 the sentence affects social welfare beyond its direct effect on
the ex-post welfare W . Finally, when an individual is prosecuted the crime has already beencommitted so the social harm h from the crime is “sunk” and the prior that the defendant is
guilty is λ = ηg /(ηg + ηi ), so we recover (1).
Because we will later consider more than two verdicts, we rewrite the interim social welfare(1) more generally as
λEg (W (s̃, g)) + (1 − λ)Ei (W (s̃, i)) ,(5)

where the sentence s̃ is a random variable whose distribution depends on whether the defendantcommitted the crime. Similarly, we rewrite (2) asb + ηg Eg (u (s̃)) > 0,

(6)

H (s̃) = 1 − B (−ηg Eg (u (s̃))) ,(7)

and rewrite (3) as

where H (s̃) is the fraction of individuals for whom (6) holds. We rewrite (4) as
H (s̃) (ηg Eg (W (s̃, g)) + ηi Ei (W (s̃, i)) − h) .

(8)

Throughout the analysis we assume that all sentences are interior, in the sense that they canbe made more severe.16 We also assume that the harm caused by the crime exceeds the social
welfare from punishing its perpetrator, i.e.,W (ŝ, g) − h < 0.
14

(9)

The benefit from committing the crime can be considered explicitly as well without affecting any of theresults.
15

Guidelines 18 U.S.C § 3553 state that another goal of punishment is “to afford adequate deterrence to criminal

conduct.”
16This can be done by imposing a longer or harsher imprisonment term. Even an execution can be mademore severe by making it less humane. While an extreme sentence would maximize crime deterrence, it would
also deter (or “chill”) desirable behavior (Kaplow (2011)) and excessively punish those individuals who were
not deterred and committed the crime, perhaps because they were ignorant of the possible punishment or did
not rationally assess the consequences of their crime before its commission. Formally, the optimal sentence will8

3

An intermediate ‘guilty’ verdict

We consider adding a verdict that refines the ‘guilty’ verdict from the two-verdict system.17
Those defendants who would be convicted in the two-verdict system now receive one of two“guilty verdicts,” which we denote 1 and 2. Defendants who would be acquitted in the twoverdict system are still acquitted and are released.18 The distinction between the two ‘guilty’
verdicts may be based on the evidence available before and during the trial, so that among thecollections of evidence that would lead to a conviction in the two-verdict system some lead toverdict 1 and the remaining to verdict 2.19 Denote by πi1 the probability that the defendantreceives verdict 1 if he is innocent, and define πi2 , πg1 , and πg2 similarly.20 Because the same setof defendants is acquitted as in the two-verdict case, we have
πi = πi1 + πi2

and πg = πg1 + πg2 .

Without loss of generality21
πg1
πg2
πg
>
>
,
πi2
πi
πi1
so verdict 1 is an “intermediate verdict:” a guilty defendant is more likely to receive verdict 2,
relative to an innocent defendant, than verdict 1.
Let sj denote the sentence associated with verdict j. Given s1 and s2 , the interim social
be interior, even taking deterrence into account, if i) the maximal benefit from the crime exceeds the maximal
disutility from the harshest possible sentence (e.g., benefits have an unbounded support and the defendant’s
utility is bounded below), and ii) social welfare becomes sufficiently negative as defendants’ punishment becomes
sufficiently harsh.
17

Further intermediate verdicts may similarly be added, as discussed in Section 4.2.

18

Section 8 discusses how to implement the additional verdict in a way that is likely not to affect jurors’ decision

whether to acquit the defendant. It also discusses how the analysis might change if their decision is affected.
19

Evidence leading to a homicide conviction in the two-verdict system may include, for example, the discovery,in the defendant’s house, of the gun from which the bullet was fired, a confession by the defendant, a death
threat made by the defendant to the victim shortly before the murder, or any subset of these.
20

In keeping with most of the literature on trial design, we take a reduced-form approach to modeling theseprobabilities. In particular, we do not need to assume that the judge or jury are Bayesian. However, we provide
a Bayesian micro-foundation for these probabilities in Appendix A.
21

For any a, b, c, d of R++ we have min{a/b, c/d} ≤ (a + c)/(b + d) ≤ max{a/b, c/d}, with strict inequalities if

a/b 6= c/d, a generic condition which we will assume throughout (it is easy to impose conditions to guarantee it:for example, one can rank bodies of evidence in terms of the posterior that they generate, as in Appendix A).

9welfare is given by
W3 (s1 , s2 ) = λ πg1 W (s1 , g) + πg2 W (s2 , g) + (1 − πg )W (0, g) +(1 − λ) [πi1 W (s1 , i) + πi2 W (s2 , i) + (1 − πi )W (0, i)] .

(10)

Our first result shows that if the sentence associated with a conviction in the two-verdict
system is optimal, then there is an improvement that does not increases the sentence.
Proposition 1 1. Suppose that s∗ is the optimal interim sentence in the two-verdict system,
i.e., the one that maximizes W2 (s). Then, there exists an s1 < s∗ such that the interim welfare
in the three-verdict system with sentences s1 and s∗ is higher than in the two-verdict system,
i.e., W3 (s1 , s∗ ) > W2 (s∗ ). 2. Suppose that s∗∗ is the optimal ex-ante sentence in the two-verdict
system. Then, there exists an s1 < s∗∗ such that the ex-ante welfare in the three-verdict systemwith sentences s1 and s∗∗ is higher than in the two-verdict system.
The proof of Proposition 1, below, shows that the results hold even when the original sentence
is not optimal, as long as it is not too suboptimally lenient. Thus, it may be generally possible
to improve upon the two-verdict system even under the strong restriction of not harming any
defendant more than in the two-verdict system.
Proof. Consider the first part of the proposition. By construction s∗ maximizes W2 (s) withrespect to s. In particular, s ≤ ŝ. Since all sentences are interior, s∗ must satisfy the first-ordercondition
λπg W 0 (s∗ , g) + (1 − λ)πi W 0 (s∗ , i) = 0.

(11)

Now consider the derivative of W3 (s1 , s∗ ) with respect to s1 , evaluated at s1 = s∗ . From (15),
we have∂W3 (s1 , s∗ )= λπg1 W 0 (s∗ , g) + (1 − λ)πi1 W 0 (s∗ , i).
∂s1
s1 =s∗(12)

π1Since πg1 < ππgi , W 0 (s∗ , g) > 0, and W 0 (s∗ , i) < 0, the first-order condition (11) implies thatithe right-hand side of (12) is strictly negative. This shows that decreasing s1 below s∗ strictly
improves welfare, yielding the desired improvement.
For the second part of the proposition, differentiating (4) gives the first-order condition
satisfied by s∗∗ in the two-verdict system:dH (s∗∗ )[ηg (πg W (s∗∗ , g) + (1 − πg )W (0, g)) + ηi (πi W (s∗∗ , i) + (1 − πi )W (0, i)) − h]ds
+ H (s∗∗ ) (ηg πg W 0 (s∗∗ , g) + ηi πi W 0 (s∗∗ , i)) = 0. (13)10The equivalent derivative for the three-verdict case with respect to s1 at s∗∗ is
dH1 (s∗∗ )[ηg (πg W (s∗∗ , g) + (1 − πg )W (0, g)) + ηi (πi W (s∗∗ , i) + (1 − πi )W (0, i)) − h]ds1
+ H (s∗∗ ) ηg πg1 W 0 (s∗∗ , g) + ηi πi1 W 0 (s∗∗ , i) , (14)where H1 (s1 ) denotes the fraction of individuals who commit the crime in the three-verdict
system as a function of the punishment s1 when s2 = s∗∗ (in particular, H1 (s∗∗ ) = H(s∗∗ )).
Dividing (13) by πg we obtain
1 dH (s∗∗ )[ηg (πg W (s∗∗ , g) + (1 − πg )W (0, g)) + ηi (πi W (s∗∗ , i) + (1 − πi )W (0, i)) − h]
πg
ds


πi 0 ∗∗∗∗0 ∗∗+ H (s ) ηg W (s , g) + ηi W (s , i) = 0.
πgDividing (14) by πg1 we obtain
1 dH1 (s∗∗ )[ηg (πg W (s∗∗ , g) + (1 − πg )W (0, g)) + ηi (πi W (s∗∗ , i) + (1 − πi )W (0, i)) − h]πg1 ds1
πi1 0 ∗∗∗∗0 ∗∗+ H (s ) ηg W (s , g) + ηi 1 W (s , i) .
πg
From (3) and (7) we have
1 dH1 (s∗∗ )1 dH (s∗∗ )
= 1
.
πg
ds
πg
ds1
Since W 0 (s∗∗ , i) < 0, to prove that the derivative for the three-verdict case is negative, which
would demonstrate the claim in the statement of the proposition, it suffices that πi1 /πg1 > πi /πg ,which holds by definition of verdict 1.
Proposition 1 shows the optimal two-verdict system can robustly be improved without increasing the punishment of any defendant, i.e., without increasing the coercive power of thestate.
Our next result shows that welfare improvements exist even if the initial binary verdict was
not optimal. Although the improvement we construct requires a higher sentence in one case, theprobability of conviction is unchanged and the improvement is robust in a strong sense: welfare
increases conditional on facing each type of the defendant, as stated below.
Proposition 2 For any sentence s > 0 in the two-verdict system and any verdict technologies
πi , πg , πij , etc., there exists a three-verdict system with sentences s1 and s2 in which the interim11welfare is higher than in the two-verdict system, i.e., W3 (s1 , s2 ) > W2 (s). Moreover, the welfare
is higher conditional on the defendant being innocent and conditional on the defendant being
guilty. If s ≤ ŝ, then s1 < s < s2 .
One key aspect of Proposition 2 is that it applies to all two-verdict systems, even those with asuboptimal sentence s > 0, and for any way of splitting of the conviction probabilities πi and πg .
In particular, it applies whether s was chosen with an ex-ante or an interim welfare perspectivein mind. Another key aspect of Proposition 2 is that the three-verdict system does not increase
the probability of punishing the innocent relative to the two-verdict system. Instead, it modifies
the sentence to reflect the richer information that verdicts 1 and 2 convey regarding the relative
likelihood of the defendant being guilty or innocent.
Lando (2005) derives a similar result to Proposition 2 when the defendant is risk neutral
with respect to the sentence and the prior about the defendant’s type is uniform.22
Proof. If s > ŝ, then setting s1 = s2 = ŝ suffices, since the ex-post welfare W (s, i) and W (s, g)decreases in s > ŝ. Suppose that s ≤ ŝ. First, observe that W3 (s, s) = W2 (s): if we giveverdicts 1 and 2 the sentence associated with the guilty verdict of the two-verdict case, then weclearly obtain the same welfare as in the two-verdict case. We are going to create a strict welfare
improvement by slightly perturbing the sentences s1 and s2 . Consider any small ε > 0 and let
s1 = s − ε and s2 = s + εγ. The welfare impact of this perturbation isW3 (s1 , s2 ) = W2 (s) + λεW 0 (s, g) (−πg1 + γπg2 ) + (1 − λ)εW 0 (s, i) (−πi1 + γπi2 ) + o(ε),(15)

where W 0 denotes the derivative of W with respect to its first argument. Since W (·, i) is
decreasing, W 0 (s, i) is negative. Similarly, because s ≤ ŝ and W (·, g) is increasing on thatdomain, W 0 (s, g) is positive. Since πg1 /πg2 < πi1 /πi2 , we can choose γ between these two ratios.
Doing so guarantees that −πg1 + γπg2 is positive and −πi1 + γπi2 is negative, which shows theclaim.
Proposition 2 considers interim social welfare, after the crime has taken place. The incentives
to commit the crime may a priori be influenced by the introduction of a third verdict, as (6)indicates. The proof of Proposition 2 shows that the welfare-improving sentences in the three22

The uniform prior assumption is our interpretation of Lando’s formulas, which are non Bayesian, but which

Lando microfounds in a special case in an appendix. Lando does not investigate the robustness properties of the
improvement.

12

verdict system can in fact be chosen in a way that does not increase the set of individuals who
commit the crime. To see this, recall that the range of welfare-improving ratios γ for s < ŝ is 1 2 1 2πg /πg , πi /πi , which is independent of the function W (·, g). For any s > 0, choosing γ = πg1 /πg2
would not change, to a first order, the welfare for a guilty defendant, and would increase thewelfare for an innocent defendant. Replacing W (·, g) with the individual’s utility function u (·)
and setting γ = πg1 /πg2 would make a guilty defendant indifferent between the two- and threeverdict systems, so the left-hand side of (6) would not change. Therefore, the three-verdict
system would deter all the individuals deterred by the two-verdict system.23
This observation immediately implies the following corollary of Proposition 2.
Corollary 1 For any sentence s > 0 of the two-verdict system and any verdict technologies
πi , πg , πij , etc., there exists a three-verdict system with sentences s1 and s2 in which the set ofindividuals who commit the crime is no larger, and the interim and ex-ante welfare is strictly
higher, than in the two-verdict system.
While the improvement in Proposition 2 does not increase the probability of punishing an
innocent defendant (or a guilty one), an erroneously convicted defendant may face a harsher
sentence ex-post when s2 > s.

4

Posterior-Based Verdicts

4.1

The Bayesian conviction model

The analysis of Section 3 did not impose any structure on how verdicts were determined. We
now show how to specialize the setting to a class of verdicts based on the posterior probability
that the defendant is guilty. We will use this in the remainder of the section. Starting with theprior probability λ = ηg /(ηg + ηi ), the trial generates evidence that is used to form the posterior.
This is summarized by distributions F (·|g) and F (·|i), which describe the posterior based on
23

The utility of an innocent defendant would increase, so even more individuals would be deterred if theindividual took into account the negligible probability he would be charged with the crime if he didn’t commit
it.

13whether the defendant is actually guilty or innocent.24 For expositional convenience, we assume
that F (·|g) and F (·|i) have positive densities f (·|g) and f (·|i).
In a two-verdict system based on the defendant’s posterior, it is natural to follow a cut-off
rule. Appendix A shows that any “reasonable” verdict rule based on evidence in the two-verdict
system can be formalized as a Bayesian model with posterior cut-off rule. If the posterior p is
below a threshold p∗ , then the defendant is acquitted, receiving a sentence of s = 0. If p exceedsp∗ , then the defendant receives a sentence s∗ > 0. The cutoff rule is a particular case of theprevious section, with πg = P r[p > p∗ |g] = 1 − F (p∗ |g) and πi = 1 − F (p∗ |i).
The interim social welfare is given by
W2 (p∗ , s∗ ) = λ [(1 − F (p∗ |g))W (s∗ , g) + F (p∗ |g))W (0, g)] +
∗

∗

∗

(16)

(1 − λ) [(1 − F (p |i))W (s , i) + F (p |i)W (0, i)] .
Similarly, the ex-ante social welfare is given by
H (s∗ ) (ηg ((1 − F (p∗ |g))W (s∗ , g) + F (p∗ |g)W (0, g)) + ηi ((1 − F (p∗ |i)) W (s, i) + F (p∗ |i)W (0, i)) − h) .
(17)
In what follows, we will denote by (p∗ , s∗ ) the cutoff and sentence used in the two-verdict
system. These variables may be chosen to maximize (16) or (17). In that case, they correspondto the interim or ex-ante utilitarian optimum for the two-verdict case.

4.2

Multi-verdict systems

Our analysis can be extended to more than three verdicts. Granted an arbitrary number ofverdicts, from an interim perspective one would wish to associate with each posterior belief p
the sentence s(p) that maximizes the welfare objectivepW (s, g) + (1 − p)W (s, i)with respect to s. Rewriting the objective function asW(p, s) = p[W (s, g) − W (s, i)] + W (s, i),24

In order to match the prior λ, the distributions must satisfy the conservation equation
Z 1
Z 1
λ = E[p] = λpdF (p|g) + (1 − λ)pdF (p|i).
0

0

14

(18)

we notice that it is supermodular in (p, s).25 This implies that the selection of maximizers of (18)is isotone. In particular, there exists a nondecreasing selection s(p) of optimal sentences. The
same is true when choosing sentences to maximize the ex-ante welfare.
The arguments used for Propositions 2, Corollary 1, and 1 easily generalize to yield thefollowing results. For k ≥ 2, we define a k-verdict system by a vector (p0 , s0 , p1 , s1 , . . . , pk−1 , sk−1 )of strictly increasing cutoffs and sentences, with p0 = 0, pk−1 < 1, s0 = 0 and sk−1 ≤ ŝ. In thissystem, a defendant receives sentence sk0 whenever his posterior p lies in (pk0 , pk0 +1 ).
Proposition 3 Suppose that the posterior distributions are continuous for both the guilty andinnocent defendants. Then, for any k-verdict system there is a k + 1 verdict system that strictlyincreases ex-ante and interim welfare. Moreover, if a k-verdict system is optimal among all
k-verdict systems and k ≥ 2, then there is a k + 1-verdict system that strictly increases ex-anteand interim welfare and in which any defendant receives a weakly lower sentence.

5

Intermediate “not guilty” verdict

Suppose now that those defendants who would be acquitted in the current two-verdict system
now receive one of two verdicts, which we denote 1 and 2. Both verdicts are associated with nojail time, i.e., with s = 0. Verdict 1, which we refer to as “not guilty,” obtains if the posterior
is less than some cutoff piv < p∗ , where p∗ is the threshold for conviction, and verdict 2, which
we refer to as “not proven,” obtains if the posterior is between piv and p∗ . We denote by pi theprobability that a defendant is guilty conditional on verdict i = 1, 2. A conviction leads to thesame sentence s∗ as in the two-verdict system.
We assume that society observes the verdict at the end of the trial, but not the posteriorregarding the defendant’s guilt. The stigmatization associated with being charged and tried ismodeled by a cutoff ps , such that the defendant is stigmatized if the probability he is guilty
conditional on the verdict exceeds ps . We take ps as exogenous, and assume that convicting
a defendant is more demanding than stigmatizing him, so ps < p∗ .26 We also assume that if
25

W (s, g) increases in s over the relevant range [0, ŝ] while W (s, i) is decreasing in s. This implies that∂W/∂p = W (s, g) − W (s, i) increases in s and, hence, supermodularity of W(p, s). See Milgrom and Shannon(1994).
26

This implies that the analysis of Section 3 does not change as a result of the stigma, since a defendant who

receives verdicts 1 or 2 is stigmatized.

15the defendant is completely cleared in the trial and the public were fully aware of this, then hewould not be stigmatized. That is, p < ps , where p is the lowest possible posterior. An innocent
defendant who is stigmatized lowers welfare by di > 0, and a guilty defendant who is stigmatizedincreases welfare by dg > 0.27 We are interested in the optimal cutoff piv and the conditions
under which introducing the additional verdict increases welfare. For expositional simplicity wewill consider interim welfare; the same qualitative results hold for ex-ante welfare.
The relevant part of the welfare function in the two-verdict system is
λ [W (0, g) + 1png >ps dg ] + (1 − λ) W (0, i) − 1png >ps di ,where png is the probability that a defendant is guilty conditional on being acquitted, since
whether an acquitted defendant is stigmatized depends on whether ps is lower or higher thanpng . We consider these two possibilities below.
Suppose first that png ≥ ps , so an acquitted defendant in the two-verdict system is stigmatized. For any piv , it must be that p2 ≥ png ≥ ps , so the defendant is stigmatized if he is found“not proven” in the three-verdict system. The split can have an effect on social welfare only
if p1 ≤ ps , in which case the defendant is not stigmatized if he is found “not guilty” in thethree-verdict system. Therefore, consider piv such that p1 < ps . Eliminating the stigma when
the defendant is found “not guilty” increases the relevant part of the welfare function by−λX

f (p|g) dg + (1 − λ)

p≤piv

X

f (p|i) di .

p≤piv

For a given posterior p ≤ piv the increase is−λf (p|g) dg + (1 − λ) f (p|i) di > 0 ⇐⇒f (p|g)(1 − λ) di<.
f (p|i)λdg(19)Since f (p|g) /f (p|i) increases in the posterior p, a fact we show in Appendix A.1, we obtain thefollowing result.
Proposition 4 Suppose that being acquitted in the two-verdict system carries a stigma. Then,
optimally splitting the acquittal into “not guilty” and “not proven” increases interim welfare if
f (q|g)iand only if f (q|i) < (1−λ)d.
λdg
If the condition in Proposition 4 holds, then the optimal cutoff piv is the minimum between
the highest posterior for which (19) holds and the highest posterior such that p1 ≤ ps . Notice27

A similar analysis can be conducted for di ≤ 0 and/or dg ≤ 0.

16

that the condition in Proposition 4 is satisfied more easily if the defendant is more likely to
be innocent (λ decreases), the stigma for the innocent increases, or the stigma for the guilty
decreases.
Now suppose that png < ps , so an acquitted defendant in the two-verdict system is not
stigmatized. The split can have an effect on social welfare only if p2 > ps , in which case thedefendant is stigmatized if he is found “not proven” in the three-verdict system. Therefore,
consider piv such that p2 > ps . Stigmatizing the defendant when he is found “not proven”
increases the relevant part of the welfare function by
λX

f (p|g) dg − (1 − λ)

p>piv

X

f (p|i) di .

p>piv

For a given posterior p > piv the increase isλf (p|g) dg − (1 − λ) f (p|i) di > 0 ⇐⇒f (p|g)(1 − λ) di>.
f (p|i)λdg(20)Since f (p|g) /f (p|i) increases in the posterior p, we obtain the following result.
Proposition 5 Suppose that being acquitted in the two-verdict system does not carry a stigma.
Then, optimally splitting the acquittal into “not guilty” and “not proven” increases interim
∗

i

|g)
welfare if and only if ff(p> (1−λ)d.
(p∗ |i)λdg

If the condition in Proposition 5 holds, then the optimal piv is the maximum between thelowest posterior for which (20) holds and the lowest posterior such that p2 ≥ ps . Notice that thecondition in Proposition 5 is satisfied more easily if the defendant is more likely to be guilty (λincreases), the stigma for the innocent decreases, or the stigma for the guilty increases.

6

Value of evidence with a third verdict

The previous sections have taken as given the technology that generates evidence regarding thedefendant’s guilt. Gathering evidence is costly, however, and the amount of evidence generated
in a case depends on the incentives of the agents involved in the evidence-gathering process: law
enforcement officers, prosecutors, experts, etc.
Setting aside the possible biases in these agents’ behavior, the socially optimal amount ofevidence to be gathered in a case clearly depends on the verdict structure. For example, a trial
17

system in which a single verdict is given regardless of the evidence produced clearly eliminates
any value of gathering evidence.
This section investigates the impact on evidence gathering of introducing a third verdict. For
simplicity, we focus on the setting of Section 3 with the Bayesian conviction model of Section 4.1.
We consider interim welfare, since for any particular crime investigated it is plausible thatthe agents involved in the discovery stage of the trial are primarily concerned with the facts
pertaining to the specific case and not with deterrence.
A (possibly multi-) verdict system leads to welfare
w(p) = pW (s(p), g) + (1 − p)W (s(p), i),(21)

where p 7→ s(p) is a step function that starts at zero, has two levels in a two-verdict system, andthree levels in a three-verdict system. The welfare function w(p) is piecewise linear. It starts at
0, and decreases until a kink at which the sentence jumps from 0 to a positive level. Figure 1
represents the welfare function for the optimal two-verdict system when W (·, g) and W (·, i) are
quadratic, for parameters given in the appendix.
Welfare
0.2

0.4

0.6

0.8

1.0

p

-0.05
-0.10
-0.15
-0.20
-0.25
-0.30

Figure 1: Welfare function, 2 verdicts.
The kink occurs at the cutoff p∗ = 1/3, at which the sentence jumps from 0 to 2/3. Figure 2
represents the welfare function for the optimal three-verdict system obtained by adding anintermediate verdict and keeping the highest sentence at 2/3. The first cut-off is p1 = p∗ = 1/3,
and the second cut-off is p2 = 1/2. The welfare function is discontinuous at p1 : this reflects thefact that p1 is not chosen optimally, but is rather “inherited” from the two-verdict system. In
contrast, because p2 is chosen optimally, the welfare function is kinked but continuous at p2 .
Actual evidence formation processes are complex, involving various actors of different types– forensic experts, lawyers, witnesses—and different forms of evidence. To model evidence18

Welfare
0.2

0.4

0.6

0.8

1.0

p

-0.05
-0.10
-0.15
-0.20
-0.25
-0.30

Figure 2: Welfare function, 3 verdicts.
formation, we must abstract from much of this complexity. Instead, we take the viewpoint of asocial planner who may gather information until a verdict is reached.
The tradeoff at the heart of this task is clear: effort spent gathering evidence is costly, but
provides information about the defendant’s guilt. We discuss two ways to model this tradeoff(there are, of course, many others). The first is a one-shot binary evidence-gathering decision,
which already captures the rough intuition for why two-verdict and three-verdict systems differ
in their effects on evidence gathering. The second is a continuous evidence-gathering process,which provides a more visually appealing representation of the impact of a third verdict onevidence gathering.
6.0.1

One-shot evidence gathering

Suppose the planner decides whether to gather evidence, at a cost of c > 0. Starting with aprior p0 , the evidence returns a higher probability of guilt, say p0 + ∆ with probability 1/2, anda lower probability p0 − ∆ also with probability 1/2. The belief process is a martingale: themean of the posterior p0 is equal to the prior: 12 (p + ∆) + 12 (p − ∆) = p.
When is evidence gathering socially desirable? Suppose first that the prior is close to 0, sothat the posterior p0 surely lies below the cutoff p1 . Then, the additional evidence has no value
as the defendant will be acquitted in all cases. Similarly, if p0 is high enough for p0 to lie above
the cutoff p1 no matter what, the additional evidence has no value as the defendant will be
convicted regardless of p0 . For p0 slightly below p1 and ∆ such that p0 + ∆ lies above p1 , thevalue of evidence is positive, since it can lead to a conviction and increase welfare (relative to an
acquittal) when it does. Similarly, evidence is valuable for p0 slightly above p1 . Thus, evidence19

is valuable around the kink, where the welfare function is convex.
Consider now the case of three verdicts. For p0 slightly below p1 , the value of evidence ishigher than in the two-verdict case because a positive belief update triggers a large improvement
in welfare (see Figure 2). For p in a neighborhood of p2 , the value of evidence is also positive
due to the convex kink there, whereas it is 0 (for ∆ small enough) in the two-verdict case.
For p0 slightly above p1 however, additional evidence may be more valuable in the two-verdict
case, which creates a “doughnut hole:” additional evidence is more valuable in the three-verdict
case than in the two-verdict case for more extreme beliefs, and less valuable in some intermediate
region. This result is easier to visualize in the next model, where evidence gathering is more
gradual.
6.0.2

Continuous evidence gathering

Now suppose that evidence is gathered continuously. As long as evidence is gathered, a flow costof c is incurred. During this time the belief pt that the defendant is guilty evolves continuously ina way that is consistent with Bayesian updating, so that the closer the belief is to 0 or 1 the more
slowly it evolves. Let the value function v(p) denote the social welfare that arises from stopping
optimally when the current belief is p. If it is optimal to stop immediately, then v (p) coincides
with w (p) given by (21). Otherwise, it is optimal to continue collecting evidence, which leads top changing continuously. In this case, v (p) is the expectation of the value of stopping optimallyin the future.
As in the one-shot setting, the value of gathering evidence in the two-verdict setting (formallyanalyzed in Appendix B) is high when the belief is very close to the convex kink p1 in Figure 1.
For beliefs slightly farther from the kink the value is still high because collecting evidence there
leads with high probability to beliefs that are closer to the kink, for which the value of gathering
evidence is high. The continuous evidence collection structure smooths the value of evidencecollection as a function of the belief. This value decreases continuously as the belief moves awayfrom the kink. For beliefs that are sufficiently far from the kink, it is optimal to stop immediately.
Less informative evidence and higher costs of evidence collection lead to lower value of evidencecollection, which correspond to functions v with to lower values. This is depicted in Figure 3 forparameters given in the appendix.
Consider now the case of three verdicts in Figure 2. Around the kink p2 , the value function v

20

Welfare
0.2

0.4

0.6

0.8

1.0

p

-0.05
-0.10
-0.15
-0.20
-0.25
-0.30

Figure 3: Value function, 2 verdicts, for varying cost levels.
behaves similarly to the two-verdict case. Around the discontinuity p1 the behavior is different,
just like in the one-shot setting. Immediately to the left of the discontinuity the value of collecting
evidence is high, so v substantially exceeds w, and this value decreases continuously for lower
beliefs. Immediately to the right of the discontinuity there is no value in collecting evidence,so v coincides with w. Higher beliefs have a positive value of evidence collection, because of
the kink. This value continuously decreases to 0 as the beliefs increase above the kink. This is
depicted in Figure 4.
Welfare
0.2

0.4

0.6

0.8

1.0

p

-0.05
-0.10
-0.15
-0.20
-0.25
-0.30

Figure 4: Value function, 3 verdicts, for varying cost levels.
In conclusion, the impact of switching to a three-verdict system by splitting the guilty verdict
depends on how informative the evidence is and how costly it is to collect. When evidence isvery informative, the posterior is unlikely to end up in the middle region, so the intermediate
verdict has little impact. When finding new evidence is very costly, however, the posterior may
end up in the middle region. The third-verdict system then increases the value of gathering
evidence in two regions, below p1 and around p2 , and decreases the value immediately above p1 .
Overall, because p̃0 < p̂1 and p̃2 > p̂2 , the three-verdict system results in evidence gathering at
21

more extreme beliefs, where in the two-verdict evidence gathering has already stopped.

7

Reflecting residual doubt in the current justice system

Numerous scholars including Fisher (2012), Lando (2005) have already noted the existence of ad
hoc probabilistic sentencing in existing criminal justice systems. The most explicit inclusion ofresidual doubt in the U.S. criminal justice system concerns the determination of death sentences.
In capital cases, juries must decide, after returning a guilty verdict, whether the defendant should
get the death penalty. In this penalty phase, residual or “lingering” doubt may be used as a
mitigating circumstance to reject the death penalty.28 The Capital Jury Project—an academic
survey of past jurors in capital cases—has found that lingering doubt was the most importantmitigating factor identified by jurors.
There is, however, wide variation in how residual doubt is applied. First, the U.S. penal
code (Title 18, §3592) does not explicitly mention residual doubt in its list of mitigating factors,
although it does state that mitigating circumstances are not limited to this list. In some cases,
jurors are not informed that lingering doubt is a valid mitigating circumstance.29 In Franklin v.
Lynaugh (1988), the U.S. Supreme Court rejected a defendant’s right to invoke residual doubt
at the penalty stage, while in People v. McDonald (Supreme Court of Illinois, 1995) a trial
judge refused to answer jurors’ question on the issue, a decision which was later affirmed by theSupreme Court of Illinois.
Compounding this inconsistency, there is empirical evidence that many jurors get confusedwith the voting rules used to establish aggravating and mitigating circumstances at the penalty
stage. While the unanimity rule is required to find a circumstance aggravating, no such standard
exists for mitigating circumstances. The Capital Jury Project found, however, that 45% ofjurors failed to understand that they were allowed to consider any mitigating evidence during
the sentencing phase of the trial, not just the factors listed in the instructions.30
28

The more demanding requirement of proving guilty beyond “all doubt” has been discussed in some states,

such as the bill proposed in 2003 by then Illinois House Republican leader Tom Cross. Some death penalty
advocates have countered that it was impossible to prove anything beyond all doubt, and that the bill would in
effect rule out the death penalty. Various degrees of lingering doubt have been discussed (e.g., Sand and Rose,
2003) without any mathematical formalism.
29

See, e.g., People v. Gonzales and Soliz, California Supreme Court, 2009.

30

The

CJP’s

findings

concerningjurors’understanding

22of

instructionsaresummarizedatWhen sentencing is performed by a trial judge, the invocation of residual doubt can be highly
controversial. In State v. Krone (Arizona Supreme Court, 1995) a trial judge sentenced to life
in prison a defendant found guilty of murder, citing doubt about whether he was the true killer.
In their legal textbook, Dressler and Thomas (2010, pp. 57–61) comment that this decision“borders on the unbelievable.” They do not, however, suggest an alternative solution.
In non-capital cases, only five states permit juries to make the sentencing decision. Outside ofthese states, residual doubt can thus only be expressed by the sentencing judge, whose opinion
does not necessarily reflect the views of the jury. Again, residual doubt is not listed as a
mitigating factor in sentencing guidelines.
The fact that residual doubt should only be considered in capital cases seems largely arbitrary.31 Even comparably less serious cases can carry long sentences, resulting in extreme
punishments for defendants who are found guilty but for whom residual doubt remains. For
example, in State v. May (Arizona Superior Court, 2007) a thirty-five-year-old defendant wassentenced to 75 years in jail after being found guilty of touching, in a residential swimming pool,the clothing of four children in the vicinity of their genitals (Nelson, 2013). Jurors had doubts
about the guilt of the defendant: they were twice unable to reach a verdict within the first threedays of deliberation. The explicit inclusion of residual doubt in sentencing would have likely
avoided such an extreme outcome.
Some felonies provide an indirect way of expressing doubt by using the lesser-included-offense
rule: juries can return a manslaughter verdict, rather than a first- or second-degree murder
verdict, or a larceny verdict instead of a robbery verdict. However, each of these verdicts corresponds to a precise charge (e.g., whether premeditation and malice aforethought were involved)
and doubt about a particular charge can only be imperfectly expressed by returning a guilty
verdict on a lower count. These instruments only offer a limited and, in fact, improper, wayof reflecting residual doubt. Furthermore, the less-included-offense rule is not a constitutionalright of the defendant; its application is therefore to some extent arbitrary and depends on the
http://www.capitalpunishmentincontext.org/issues/juryinstruct.
31

Capital sentences are unique in their irreversibility, which creates an additional reason for avoid this sentence

in case of lingering doubt: exonerating evidence may appear after the execution of the defendant, preventing any
release and compensation. In practice, however, this fundamental difference is attenuated by the fact that deathrow defendants spend many years in jail before their execution until all recourses have been exhausted, while
non-capital defendants serving long sentences may die in jail, which also prevents any release or compensation.

23

inclination of the jury (see Mascolo (1986)).
Even when the lesser-included-offense rule does not apply, residual doubt may be reflected byreturning a guilty verdict only on a subset of the charges brought against the defendant. Thereis anecdotal evidence that such compromise is sometimes used by the jurors to reflect doubt. In
the aforementioned State v. May, for instance, Nelson (2013) notes that “it seems likely that thedefendant molested either all of the children or none of them. So why did the jury ultimately
reach a verdict of guilty on five counts and not guilty on two? The answer is that the jurors
compromised.” Dropping some charges is, however, a very coarse instrument to incorporate
residual doubt: for example, this approach cannot be used to reduce the sentence of a defendantfacing a single but severe count, while it may be used for a defendant facing several counts, thesum of which adds to the same aggregate maximal sentence as in the single-count case.32 Even
when it is feasible, the approach exposes the defendant to another idiosyncratic component of thejury—whether it is sophisticated or willing enough to use this compromise strategy—introducing
a source of jury heterogeneity in trial outcomes even for otherwise identical cases.33
The U.S. justice system incorporates residual doubt about a defendant’s guilt in two otherways. First, a defendant found not guilty in a criminal trial may still be found guilty in a
civil suit, which uses the less demanding preponderance-of-evidence standard of proof. However,civil suit sentences carry no jail time and thus may be more limited in preventing recidivism.
Furthermore, the connection between criminal and civil trials is generally limited, preventing
any coordination and coherent decision across these trials. Second, residual doubt variations
also imply different likelihoods of post-trial events such as successful appeals and exonerations,which affect the defendant’s ultimate punishment. These events are largely beyond the controlof the first court and are not a close substitute for the additional verdicts introduced here.
In summary, the current criminal justice system includes various ways of reflecting residual
doubt in outcomes and it appears that these ways are used purposefully by some actors of thesystem. However, these ways are largely arbitrary, inconvenient, and uncoordinated. This paper
32

The set of charges leveled at the defendant may also be affected by the strategic decisions of the prosecutor,

which increases the prosecutor’s power and adds to the complexity of this problem.
33

It should also be noted that under the current law, such compromise is actually illegal if it results from abargaining between pro-acquittal and pro-conviction jurors. Such an arrangement currently violates the rightsof the defendant if the pro-acquittal jurors still believe that the defendant should be found not guilty (Mascolo(1986)).

24

proposes a structured, systematic approach for the consideration of residual doubt in criminal
justice decisions and explicit designs which are shown to improve welfare in many settings.

8

Implementation and jurors’ reactions to additional verdicts
Implementation: verdicts vs. sentences

Formalizing the intermediate sentence introduced in this paper as an intermediate verdict isconsistent with the not-proven verdict, discussed in Section 5, used by some criminal justicesystems. In this formulation, the jury must decide, according to some collective rule, among thethree verdicts.
An alternative “two-step” implementation maintains the current separation between the factfinding and sentencing stages. The verdict outcome is still binary (“guilty” or “not guilty”), andresidual doubt is expressed in the form of intermediate sentences decided in the sentencing stage.
The second implementation presents a significant advantage: in principle, the jury can begiven exactly the same instructions as in the current system, which allows to cleanly split the setof cases which would receive a “guilty” verdict under the current system into multiple sentencelevels reflecting the strength of evidence, and thus leaves unchanged the probability of acquitting
the defendant.
Intermediate sentences can be decided in a variety of ways, which may involve a sentencing
judge, sentencing guidelines (e.g., automatically rule out the death penalty if the evidence issolely based on a confession), or a jury.
Regardless of the implementation, a potential concern is how the jury may react to additionalverdicts. The remainder of our discussion focuses on this issue.
Jurors’ reaction to additional verdicts
Jury decisions involve collective and psychological considerations: jurors may have limited
and uneven ability to understand jury instructions or interpret the evidence, have varied tolerance for erroneous convictions and acquittals, and are subject to individual biases and to
persuasion and group-think dynamics, to cite only a few issues. Even abstracting from these25issues, jury decisions are difficult to analyze.34
The literature on criminal trial design varies from fully rational to completely reduced-form
models of jury behavior. At the most “rational” extreme, Lee (2015) considers jurors who perfectly take into account how prosecutors select the pool of defendants who go to trial. Prosecutors
can influence this pool by choosing the plea sentence that they propose to defendants before thetrial.35 Other papers on trial design (Kaplow (2011), Daughety and Reinganum (2015a,b), Da
Silveira (2015), Silva (2018)) abstract from any jury decision, focusing on reduced-form thresholds or on a mechanism design approach without jurors.
A key observation is that our Propositions 1 and 2 continue to hold under the two-step
implementation mentioned above, provided that jurors are given the same instructions as in
the current system to decide between the guilty and not-guilty verdicts, and react to these
instructions in the same way, no matter how imperfect, as they currently do. No matter how“tough on crime” or otherwise biased each juror is, and what voting, persuasion or other collective
processes are at play, all these components would play out in exactly the same way at the factfinding stage, under a standard binary verdict, as in the first step of the two-step approach,guaranteeing that no more defendants are found guilty in the three-verdict system than in thecurrent one.
The main question, therefore, is to what extent jurors would know and incorporate in thefact-finding stage the fact that residual doubt may play a significant role in the sentencing stage.
In practice, there is little evidence that jurors incorporate sentencing considerations into theirverdict decisions. On the contrary, in recent history judicial practice has been to keep the jury
uninformed about the punishment faced by the defendant (Sauer (1995)). In United States v.
Patrick (D.C. Circuit, 1974), the court affirmed that the jury’s role is limited to a determinationof guilt or innocence. Instructions entirely focus on describing the procedure for finding facts.
In many cases—such as People v. May above—jurors are unaware of the minimum-punishment
guidelines relevant for the case.
34

Austen-Smith and Banks (1996), Feddersen and Pesendorfer (1996, 1997), and Gerardi and Yariv (2007)

identify important informational effects, which may arise even when all jurors have identical preferences. A
central mechanism in this literature is that, conditional on being pivotal in a vote, a rational juror may put somuch weight on other jurors’ signals that he significantly discounts, and potentially discards, his own information.
35

The approach presumes that jurors are aware of the plea sentence offered to the defendant. In practice, thejury is often instructed to consider only the evidence produced at trial.

26

There is also empirical evidence that harsher sentences do not result in lower conviction
rates. In a study of non-homicide violent case-level data of North Carolina Superior Courts,
Da Silveira (2015) finds that the probability of conviction of defendants going to trial in factincreases with the sentence that they face.36 Such a correlation cannot be easily explained away
by prosecutor behavior: if, in particular, prosecutors attached more importance to obtaining
a conviction when the case is more severe, they would send to trial defendants who are more
likely to be found guilty and obtain a guilty plea from the other ones, and one would expect theprobability of plea settlements to increase with the severity of the trial sentence. This relation
seems contradicted by the data.37
More generally, there is strong evidence that jurors have a limited understanding of thesentences faced by defendants. For example, the aforementioned Capital Jury Project foundthat most jurors “grossly underestimated” the amount of time spent in jail entailed by a guilty
verdict. It is reasonable to believe that jurors would be as unaware of, say, maximum-sentencing
guidelines as they currently are of minimum-sentencing guidelines.
Finally, if contrary to expectations jurors incorporated the intermediate verdict into theirdecision, they might adopt a different standard of proof to convict defendants, knowing thatthe corresponding cases would result in a different sentence than in the current system. To theextent that jurors did this with the social welfare objective in mind, such a change would likely
be beneficial. Jurors may, however, have their own objective in mind. For example, they may
ignore, from the interim perspective in which they are placed, the deterrence value, ex ante, of
higher expected punishments—this issue arises even in a two-verdict system, and may explain
the fact, mentioned earlier, that jurors are specifically asked to focus on finding facts and left
relatively uninformed about the strength of the punishment implied by a guilty verdict. Jurors
may also worry about the length of deliberation, and be willing to continue deliberation only
if the social value of doing so is high. The analysis of Section 6 suggests that this value is not
lowered by the introduction of an intermediate verdict, and may in fact be higher, for a wide
range of beliefs.
36

Da Silveira’s analysis excludes the most and least severe cases to focus on a relatively homogeneous pool ofcases.
37

Elder (1989) finds evidence that circumstances that may aggravate punishment reduce the probability ofsettlement. Similarly, Boylan (2012) finds that a 10-month increase in prison sentences raises trial rates by 1
percent.

27

A

Foundation of the Bayesian Conviction Model

We now study whether actual court proceedings can be translated into a Bayesian updating process and athreshold. We address this by considering an evidence-based trial technology. There is a set X of evidence
elements, and “evidence collection” refers to a subset of X. The court technology is a mapping D : 2X → {G, N },which for every evidence collection decides whether the defendant is guilty or not guilty.38 Distributions Pθ on
2X , for θ ∈ {g, i}, describe the probability that different evidence collections arise conditional on the defendant
being actually guilty or innocent. We assume that both distributions have full support. Letting πθk denote theprobability that a defendant of type θ receive verdict k, we have πθk = Pθ D−1 (k) for each type θ and verdict
k in {G, N }. Recall that πiG < πgG , i.e., Pi D−1 (G) < Pg D−1 (G) , and that λ is the prior that the defendantis guilty. We ask several questions.
1. Given D, Pi , Pg , and λ, can D be rationalized as the result of Bayesian updating with a threshold on
the posterior for determining guilt? At a minimum, this would require D to respect “incriminating” and“exculpatory” evidence sets, which are determined by whether they indicate that the defendant is more
likely to be guilty than innocent.
2. Given D and λ, can Pi and Pg be chosen to rationalize D as the result of Bayesian updating with athreshold on the posterior for determining guilt?
3. Given λ, can D, Pi , and Pg be chosen to rationalize D as the result of Bayesian updating with a thresholdon the posterior for determining guilt?
To answer these questions, we formally order defendant types i and g so that i < g, and we order verdicts as
N < G. Then, we say that D can be rationalized as the result of Bayesian updating with a threshold on theposterior if for every E, E 0 ⊆ X we have D (E) < D (E 0 ) if and only if the posterior that the defendant is guilty
is higher under E 0 than under E, i.e.,λPg (E)λPg (E 0 )<.
λPg (E) + (1 − λ) Pi (E)λPg (E 0 ) + (1 − λ) Pi (E 0 )This condition is equivalent to λPg (E) (λPg (E 0 ) + (1 − λ) Pi (E 0 )) < λPg (E 0 ) (λPg (E) + (1 − λ) Pi (E)) and,after rearranging, toPg (E)Pg (E 0 )<.
Pi (E)Pi (E 0 )The likelihood ratios are thus ordered independently of λ. For every evidence set E ⊆ X, denote by r (E) =Pg (E) /Pi (E) its likelihood ratio. This shows the following proposition.
Proposition 6 D can be rationalized if and only if for every E, E 0 ⊆ X the following holds:r(E) ≤ r(E 0 ) ⇒ D (E) ≤ D (E 0 ) .
While we started with a Bayesian definition of rationalizability, this concept is in fact non-Bayesian: it ispurely based on the likelihood ratio of guilty given the observed evidence and, in particular, is independent ofany prior.
Equipped with this result, we can answer the questions above. For 1, the answer is “yes” if and only if
max {r (E) : D (E) = N } < max {r (E) : D (E) = G} .
38

The analysis can be generalized to stochastic decisions.

28

(22)

For 2, the answer is “yes:” choose Pg and Pi so that (22) holds. Since 2 implies 3, that answer to 3 is also “yes.”
Incriminating and exculpatory evidence: definitions and properties
When D can be rationalized, we say that evidence e ∈ X is D-incriminating if for every E ⊆ X with e ∈/ E,
D (E) = g implies that D (E ∪ {e}) = g. We say that evidence e ∈ X is P -incriminating if for every E ⊆ X withe∈
/ E we have that r (E) ≤ r (E ∪ {e}). Decision- and belief-based notions of exculpatory evidence are definedsimilarly. The following result establishes the logical connection between these concepts.
Proposition 7 If D is rationalized by P , any P -incriminating evidence is also D -incriminating.
The reverse need not hold: one can easily construct examples in which some evidence collection E suffices toconvict the defendant (i.e., D(E) = g) and the additional piece of evidence e reduces the ‘guilt’ ratio (r(E ∪{e}) <r(E)), but not enough the change the decision (D(E ∪ {e}) = g).
Our definition and characterization of rationalization extend without change to probabilistic functions D, inwhich the image of D is the probability that the defendant is found guilty.

A.1

Ordering posterior distributions with the MLRPIn the Bayesian conviction model, the posterior belief is formed by combining a prior with the signals observed
about the defendant. One may view each evidence collection E as a signal, and signals may be ordered according
to the likelihood ratio r(E). The distributions Pi and Pg over evidence collections can then be mapped into
distributions over likelihood ratios r. In a Bayesian conviction model, only the likelihood ratio matters for thedecision, and one can thus without loss identify any signal with r. Thus, without loss, signals may be ranked
according to this likelihood ratio. Let Rg and Ri denote the distributions of r, conditional on being guilty andinnocent, respectively. When the signal distributions, conditional on being guilty or innocent, are continuous,let ρg and ρi denote their densities. By construction, we have ρg (r)/ρi (r) = r. In statistical terms, this meansthat Rg and Ri are ranked according the MLRP: the ratio of their density is increasing in the signal. Moreover,because the posterior p(r), given a signal r, is equal to the conditional probability of θ = g given r, it inherits
the MLRP.39 Let Fg and Fi denote the distributions of p, conditional on being guilty and innocent, respectively,
and let fg and fi denote the densities of Fg and Fi (which exist as long as Rg and Ri are continuous), we havefg (p)/fi (p) is increasing in p.
Proposition 8 Suppose that both signal distributions, conditional on being guilty and innocent, are continuous.
Then both distributions of the posterior p are continuous, and their density functions satisfy the MLRP.
This property, which holds without loss (except for the continuity assumption, of a technical nature), plays
a key role for subsequent results.

B

Modeling continuous evidence gathering

As long as evidence is gathered, the belief pt that the defendant is guilty evolves as a martingale as in Boltonand Harris (1999):
dpt = Dpt (1 − pt )dBt ,39

This fact is well-known and straightforward to establish.: if θ is the state of the world, r is a signal, and theconditional distributions ρ(r|θ) are ranked according to MLRP, then the posterior distributions ρ(θ|r) are alsoranked according to the MLRP.

29

where B is the standard Brownian motion and D is a measure of the quality of the signal: the higher D is, thefaster p evolves toward the true probability that the defendant is guilty (0 or 1). At some time T , the evidence
formation process is stopped and the verdict is chosen based on the posterior pT , which results in social welfare
w(pT ).
Adapting the arguments of Bolton and Harris (1999) to our environment, the value function v (·) must satisfy
the Bellman equation
1
0 = max{w(p) − v(p); −rv(p) − c + D2 p2 (1 − p)2 v 00 (p)},(23)
2where r is a discount rate that captures the idea that longer judicial processes are penalizing for all parties.
The first part of the equation implies that v(p) ≥ w(p), which means that the value function always exceeds thewelfare obtained by stopping immediately. This is natural, since the option of stopping is available at any time.
The second part of the equation describes the evolution of the value function while evidence is accumulated:1
0 = −rv(p) − c + D2 p2 (1 − p)2 v 00 (p).
2
All solutions to this equation are in closed form when D2 /r = 3/2:




1
3
c
1
−2
v(p) = − + A1 + A2 p −(1 − p)p− 2 (1 − p) 2 ,
r
2

(24)

where A1 and A2 are free integration constants. For simplicity, in what follows we set r = 1 and D2 = 3/2 andvary the cost c.
The region in which evidence is gathered and value functions are determined by the conditions that v
is continuous, weakly above w, and when it hits w, it satisfies the smooth pasting property whenever w iscontinuously differentiable at the hitting point.
Starting with the two-verdict case, one should expect v to coincide with w when p is either close to 0 or close
to 1: in this case, there is a high degree of confidence in the defendant’s guilt and the value of further evidence
gathering is low. Near w’s kink (i.e., the threshold p∗ at which the sentence switches), however, the value ofadditional evidence is high, so v should be strictly above w. Thus, it suffices to connect v and w on both sidesof p∗ . At the connection points, p̂1 and p̂2 such that p̂1 < p∗ < p̂2 , v must be equal to w (this is the “value
matching” condition) and the derivatives must also coincide (this is the smooth pasting condition).
This imposes four conditions (two value matching and two smooth pasting), and there also four free parameters: the cutoffs p̂1 and p̂2 , and the constants A1 and A2 arising in equation (24). The result is depicted in
Figure 3.
Now consider the three-verdict case. Around the kink p2 , we still have a two-way smooth connection between
w and v, as in the two-verdict case. Around p1 = p∗ , however, w is discontinuous, jumping upward from w = −1/3¯
to w̄ = −2/9 as p passes p1 . In this case, if v(p1 ) > w̄ (the cost is low), then the situation is exactly as in the
two-verdict case. Intuitively, the cost is low enough that the intermediate verdict doesn’t matter: evidence isgathered until either the not guilty or the guilty verdict is reached. This a situation in which the trial technology
is quite accurate, so a two-verdict system suffices.
For larger costs, however, v hits w exactly at p1 = p∗ , due to the upward jump. The smooth pasting conditionis violated, because the left derivative of v is higher than its right derivative at p1 , and v is equal to w on a rightneighborhood of p1 . Intuitively, this kink in the value function reflects the fact that p1 = p∗ was not chosen
optimally for the three-verdict system, but rather inherited from the two-verdict system.
The evidence-gathering region now has two parts. When p is below p1 , there is a large incentive to gather
evidence, because such evidence can change the sentence from 0 to s1 , and s1 was tailored to provide a fairer

30

sentence around p1 than both 0 and s2 . This also implies that not gathering evidence in a right-neighborhood of
p1 is optimal. The second evidence-gathering region is around p2 , as before.40
Because the first region violates the smooth pasting condition at p1 , its determination is slightly different.
We must determine the threshold p̃0 at which the region begins, and we know that the region ends at the cutoff
p1 . At p̃0 , we have two conditions: the value matching and the smooth pasting conditions. At p1 , however, weonly have the value matching condition v(p1 ) = w̄, since the smooth pasting condition is violated. This gives
three conditions. There are also three free parameters: the cutoff p̃0 and the constants Â1 and Â2 in (24) forthat region. The result is depicted in Figure 4.
Because the welfare w3 is always higher than the welfare w2 , it is is straightforward to establish that the valuefunction v3 in the three-verdict case is (weakly) higher than the two-verdict value function v2 . This matters forhigh enough cost, i.e., when v(p1 ) = w̄. In that case, v3 is strictly above v2 around p1 , and it is also strictly above
v2 in the second evidence-gathering region, closer to p2 . This implies that the cutoff p̃0 is lower than the cutoffp̂1 of the two-verdict case, and the right cutoff p̃2 of the second evidence-gathering region in the three-verdict
case is greater than p̂2 .

40

As the search cost decreases, the two search regions become connected when v(p1 ) ≥ w̄.

31

C

Parameters for the welfare functions of Section 6

We set to 1 the ideal sentence s̄ for the guilty and use quadratic loss functions: W (s, g) = −(1−s)2 , W (s, i) = −s2 .
We also assume that the prior is equal to 1/2: the defendant is equally likely to be guilty or innocent ex ante.
To obtain simple expressions for the optimal cutoffs and sentences, we reverse-engineer the signal structure. The
optimal cutoff p∗ is given by the indifference conditionp∗ W (s∗ , g) + (1 − p∗ )W (s∗ , i) = p∗ W (0, g),or p∗ (1 − (s∗ )2 ) + (1 − p∗ )(−(s∗ )2 ) = p∗ . The optimal sentence s∗ solves
11s∗ ∈ arg max P r(p ≥ p∗ |g)W (s|g) + P r(p ≥ p∗ |i)W (s|i),s 22which yields the first-order condition, (1−F (p∗ |g))(1−s∗ ) = (1−F (p∗ |i))s∗ , where F (·|θ) denotes the probability
distribution of the posterior p conditional on the defendant’s type θ.. By choosing F (·, g) and F (·, i) so that the1−F (p|i)∗∗ratio q = 1−F(p|g) is equal to 1/2 when evaluated at p = 1/3, we verify that p = 1/3 and s = 2/3 solve themaximization problem. Note that q must be less than 1, from MLRP.
With three verdicts, we impose the restrictions p1 = 1/3 and s2 = 2/3, so that we are indeed splitting theguilty verdict, without increasing the guilty sentence, and optimize over the remaining two parameters, p2 ands1 . These parameters are again characterized by the indifference equation for p2 , given the sentences s1 and s2that are given above and below p2 ,p2 W (s1 , g) + (1 − p2 )W (s1 , i) = p2 W (s2 , g) + (1 − p2 )W (s2 , i),and by the optimality condition for s1 , which is11
s1 ∈ arg max P r(p ∈ [p1 , p2 ]|g)W (s|g) + P r(p ∈ [p1 , p2 ]|i)W (s|i),s 22which yields the first-order condition
F ([p1 , p2 ]|g)(1 − s1 ) = F ([p1 , p2 ]|i)s1 .
([p1 ,p2 ]|i)evaluated atUsing reverse engineering again, we choose F (·|g) and F (·, i) so that the ratio q 0 = FF ([p1 ,p2 ]|g)p1 = 1/3 and p2 = 1/2 be equal to 2. This inequality guarantees that s1 = 1/3 and p2 = 1/2 satisfy the
optimality conditions. Note that the ratio q 0 must be greater than q, by MLRP.
This yields the welfare functions w2 (p) = w3 (p) = −p for p < 1/3, w2 (p) = −p/9 − (1 − p) × 4/9 for p ≥ 1/3,
and w3 (p) = −p/9 − (1 − p) × 4/9 for p ≥ 1/2, and w3 (p) = −p 94 − (1 − p) 91 for p ∈ [1/3, 1/2).

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