The Utility of Text: The Case of Amicus Briefs and the Supreme Court

Yanchuan SimLanguage Technologies Institute

Carnegie Mellon UniversityPittsburgh, PA 15213, USAysim@cs.cmu.edu

Bryan R. RoutledgeTepper School of BusinessCarnegie Mellon UniversityPittsburgh, PA 15213, USAroutledge@cmu.edu

Noah A. SmithLanguage Technologies Institute

Carnegie Mellon UniversityPittsburgh, PA 15213, USAnasmith@cs.cmu.edu

AbstractWe explore the idea that authoring a piece of text is anact of maximizing one’s expected utility. To make thisidea concrete, we consider the societally important deci-sions of the Supreme Court of the United States. Exten-sive past work in quantitative political science providesa framework for empirically modeling the decisions ofjustices and how they relate to text. We incorporate intosuch a model texts authored by amici curiae (“friends ofthe court” separate from the litigants) who seek to weighin on the decision, then explicitly model their goals ina random utility model. We demonstrate the benefits ofthis approach in improved vote prediction and the abil-ity to perform counterfactual analysis.

1 IntroductionSome pieces of text are written with clear goals in mind.Economists and game theorists use the word utility for theconcept of satisfying a need or desire, and a huge array oftheories and models are available for analyzing how utility-seeking agents behave. This paper takes the first steps in in-corporating text into these models.

The Supreme Court of the United States (SCOTUS) is thehighest court in the American judicial system; its decisionshave far-reaching effects. While the ideological tendenciesof SCOTUS’ nine justices are widely discussed by press andpublic, there is a formal mechanism by which organized in-terest groups can lobby the court on a given case. Thesegroups are known as amici curiae (Latin for “friends of thecourt,” hereafter “amici,” singular “amicus”), and the textualartifacts they author—known as amicus briefs—reveal ex-plicit attempts to sway justices one way or the other. Takenalongside voting records and other textual artifacts that char-acterize a case, amicus briefs provide a fascinating settingfor empirical study of influence through language.

We build on well-established methodology from politicalscience known as ideal points for analyzing votes. Specif-ically, Lauderdale and Clark (2014) combined descriptivetext and ideal points in a probabilistic topic model. Althoughthe influence of amici has been studied extensively by legalscholars (Collins 2008), we are the first to incorporate theminto ideal points analysis (§2). Drawing on decision theory,

Copyright c© 2015, Association for the Advancement of ArtificialIntelligence (www.aaai.org). All rights reserved.

we then posit amici as rational agents seeking to maximizetheir expected utility by framing their arguments to influencejustices toward a favorable outcome (§3). We derive appro-priate inference and parameter estimation procedures (§4).

Our experiments (§5) show that the new approach offerssubstantial gains in vote prediction accuracy. More impor-tantly, we show how the model can be used to answer ques-tions such as: How effective were amici on each side ofa case? What would have happened if some or all amicusbriefs were not filed? How might an amicus have changedher brief to obtain a better outcome? Since our approachcharacterizes the amicus brief as a (probabilistic) functionof the case parameters, our approach could also be used toask how the amici would have altered their briefs given dif-ferent merits facts or a different panel of justices. Althoughwe focus on SCOTUS, our model is applicable to any set-ting where textual evidence for competing goals is availablealongside behavioral response.

SCOTUS Terminology. SCOTUS reviews the decisionsof lower courts and (less commonly) resolves disputes be-tween states.1 In a typical case, the petitioner writes abrief putting forward her legal argument; the respondent(the other party) then files a brief. These, together with around of responses to each other’s initial briefs, are collec-tively known as merits briefs. Amicus briefs—further ar-guments and recommendations on either side—may be filedby groups with an interest (but not a direct stake) in theoutcome, with the Court’s permission. After oral arguments(not necessarily allotted for every case) conclude, the jus-tices vote and author one or more opinions. In this paper, werelate the votes of justices to merits and amicus briefs.

2 Ideal Point ModelsIdeal point (IP) models are a mainstay in quantitative polit-ical science, often applied to voting records to place voters(lawmakers, justices, etc.) in a continuous space. A justice’s“ideal point” is a latent variable positioning her in this space.

Unidimensional Ideal Points The simplest model for ju-dicial votes is a unidimensional IP model (Martin and Quinn

1Details about the procedures and rules of the SCOTUS can befound at http://www.uscourts.gov.

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2002), which posits an IP ψj ∈ R for each justice j.2Often the ψj values are interpreted as positions along aliberal-conservative ideological spectrum. Each case i is rep-resented by popularity (ai) and polarity (bi) parameters.3 Aprobabilistic view of the unidimensional IP model is that jus-tice j votes in favor of case i’s petitioner with probability

p(vi,j = petitioner | ψj , ai, bi) = σ (ai + ψjbi)

where σ(x) = exp(x)1+exp(x) is the logistic function. When the

popularity parameter ai is high enough, every justice is morelikely to favor the petitioner. The polarity bi captures the im-portance of the justice’s ideology ψj : more polarizing cases(i.e., |bi| � 0) push justice j more strongly to the side of thepetitioner (if bi has the same sign as ψj) or the respondent(otherwise). While they recover dimensions that maximizestatistical fit, IP models conflate many substantive dimen-sions of opinion and policy, making it difficult to interpretadditional dimensions.4 Indeed, such embeddings are igno-rant of the issues at stake, or any content of the case, andthey cannot generalize to new cases.

Issues and Ideal Points Lauderdale and Clark (2014) in-corporate text as evidence and infer dimensions of IP that aregrounded in “topical” space. They build on latent Dirichletallocation (LDA; see Blei, Ng, and Jordan 2003), a popu-lar model of latent topics or themes in text corpora. In theirmodel, each case i is embedded as θi in a D-dimensionalsimplex; the dth dimension θi,d corresponds to the propor-tion of case i that is about issue (or, in LDA terminology,topic) d. The probability of justice j’s vote is given by

p(vi,j = petitioner | ψj ,θi, ai, bi) = σ(ai +ψ>j (biθi)

)where ψj,d is an issue-specific position for justice j. There-fore, the relative degree that each dimension predicts thevote outcome is determined by the text’s mixture propor-tions, resulting in the issue-specific IP ψ>j θi. In their work,they inferred the mixture proportions from justices’ opin-ions, although one can similarly use merits briefs, appealscourt opinions, or any other texts that serve as evidence forinferring the issues of a case.

Lauderdale and Clark (2014) found that incorporating tex-tual data in this manner5 addresses the labeling problem formultidimensional models, and is especially useful for smallvoting bodies (e.g., SCOTUS), where estimating multidi-mensional models is difficult due to few observations andvariation of preferences across issues.

2Martin and Quinn (2002) describe a dynamic unidimensionalIP model where justice IPs vary over time. In this work, we assumeeach justice’s IP is fixed over time, for simplicity.

3This model is also known as a two parameter logistic model initem response theory literature (Fox 2010), where ai is “difficulty”and bi is “discrimination.”

4A seminal finding of Poole and Rosenthal (1985) is that twodimensions, corresponding to left-right ideology and geographicallatitude, explain most of the variance in U.S. Congressional votes.

5Of course, LDA is not the only way to “embed” a case in asimplex. One can take advantage of expert categorization of caseissues. For example, Gerrish and Blei (2012) used bill labels assupervision to infer the proportions of issues.

Amici and Ideal Points The merits briefs describe the is-sues and facts of the case. It is our hypothesis that amicusbriefs serve to “frame” the facts and, potentially, influencethe case outcome. Collins (2008) argued that these organizedinterest groups play a significant role in shaping justices’choices. Public interest groups, such as the ACLU and Citi-zens United, frequently advocate their positions on any casethat impinges on their goals. These briefs can provide valu-able assistance to the Court in its deliberation; for example,they can present an argument not found in the merits.6

When filing amicus briefs, amici are required to identifythe side they are supporting (or if neither). However, it isnot trivial to automatically tell which side the amici are onas these intentions are not expressed consistently. We solvethis by training a classifier on hand-labeled data (§4).

We propose that amici represent an attempt to shift the po-sition of the case by emphasizing some issues more stronglyor framing the case distinctly from the perspectives given inthe merits briefs. The effective position of a case, previouslybiθi, is in our model biθi+c

pi∆

pi+c

ri∆

ri, where cp

i and cri are

the amicus polarities for briefs on the side of the petitionerand respondent. ∆p

i and ∆ri are the mean issue proportions

of the amicus briefs on the side of the petitioner and respon-dent, respectively. Our amici-augmented IP model is:

p(vi,j = petitioner | ψj ,θi,∆i, ai, bi, ci)

= σ(ai +ψ>j

(biθi + cp

i∆pi + cr

i∆ri

))(1)

In this model, the vote-specific IP is influenced by two formsof text: legal arguments put forth by the parties involved(merits briefs, embedded in θi), and by the amici curiae (am-icus briefs, embedded in ∆

{p,r}i ), both of which are rescaled

independently by the case discrimination parameters to gen-erate the vote probability. When either |cp

i | or |cri| is large

(relative to ai and bi), the vote is determined by the contentsof the amicus briefs. Hereafter, we let κi = 〈ai, bi, cp

i , cri〉.

By letting ∆si be the average mixture proportions inferred

from text of briefs supporting side s, we implicitly assumethat briefs supporting the same side share a single parameter,and individual briefs on one side influence the vote-specificIP equally. While Lynch (2004) and others have argued thatsome amici are more effective (i.e., influence on justices’votes varies across amicus authors), our model captures thecollective effect of amicus briefs and is simple.

3 Amici as AgentsIn the previous section, the IP models focus on justices’ po-sitions embedded in a continuous space. However, we wantto account for the fact that amici are purposeful decisionmakers who write briefs hoping to sway votes on a case.Suppose we have an amicus curiae supporting side s (e.g.,petitioner), which is presided by a set of justices, J. The am-icus is interested in getting votes in favor of her side, that is,vj = s. Thus, we assume that she has a simple evaluation

6On occasion, SCOTUS may adopt a position not advanced byeither side, but instead urged solely by an amicus brief. Some no-table cases are: Mapp v. Ohio, 367 U.S. 643, 646 (1961) and morerecently, Turner v. Rogers, 131 S. Ct. 2507 (2011).

function over the outcome of votes v1, . . . , v9,

u(v1, v2, . . . , v9) =∑j∈J I(vj = s), (2)

where I is the indicator function. This is her utility.

Cost of writing. In addition to the policy objectives of anamicus, we need to characterize her “technology” (or “bud-get”) set. We do this by specifying a cost function, C, that isincreasing in difference between ∆ and the “facts” in θ:

C(∆,θ) = ξ2‖∆− θ‖

22

where ξ > 0 is a hyperparameter controlling the cost (rela-tive to the vote evaluation). The function captures the notionthat amicus briefs cannot be arbitrary text; there is disutilityor effort required to carefully frame a case, or the mone-tary cost of hiring legal counsel. The key assumption hereis that framing is costly, while simply matching the merits iseasy (and presumably unnecessary). Note the role of the costfunction is analogous to regularization in other contexts.

Expected utility. The outcome of the case is uncertain, sothe amicus’ objective will consider her expected utility:7

max∆

E∆[u(v1, . . . , v9)]− ξ2‖∆− θ‖

22

When an amicus writes her brief, we assume that she hasknowledge of the justices’ IPs, case parameters, and contentsof the merits briefs, but ignores other amici.8 As such, tak-ing linearity of expectations, we can compute the expectedutility for an amicus on side s using Eq. 1:9

max∆

∑j∈J σ

(a+ψ>j (bθ + cs∆)

)− ξ

2‖∆− θ‖22

Random utility models. There are several conceivableways to incorporate amici’s optimization into our estima-tion of justices’ IP. We could maximize the likelihood andimpose a constraint that ∆ solve our expected utility opti-mization (either directly or by checking the first order condi-tions).10 Or, we can view such (soft) constraints as imposinga prior on ∆:

putil(∆) ∝ E∆[u(v1, . . . )] + ξ(1− 12‖∆− θ‖

22) (3)

7We use an evaluation function that is linear in votes for sim-plicity. The scale of the function is unimportant (expected utility isinvariant to affine transformations). However, we leave for futurework other specifications of the evaluation function; for example afunction that places more emphasis on the majority vote outcome.

8Capturing strategic amici agents (a petitioner amicus choosingbrief topics considering a respondent amicus’ brief) would requirea complicated game theoretical model and, we conjecture, wouldrequire a much richer representation of policy and goals.

9The first-order conditions for amicus’ purposeful maximiza-tion with respect to ∆ lead to interesting brief writing trade offs,which can be found in supplementary §A.

10This is reminiscent of learning frameworks where constraintsare placed on the posterior distributions (Chang, Ratinov, and Roth2007; Ganchev et al. 2010; McCallum, Mann, and Druck 2007).However, the nonlinear nature of our expectations makes it difficultto optimize and characterize the constrained distribution.

vi,j

ψjλ ρ U(0, 1)

κi σ

θi α

∆k

sk

α

Ai

J

C

vi,j

ψjλ ρ U(0, 1)

κi σθi

α

∆ksk

Ai

J

C

Figure 1: Plate diagrams for the IP models (left) and randomutility model (right). J, C and Ai are the sets of justices,cases, and amicus briefs (for case i), respectively. ψj is theIP for justice j; κi is the set of case parameters ai, bi, c

pi and

cri for case i; and α,σ, λ, and ρ are hyperparameters. The

mixture proportion nodes (dashed) are fixed in our estima-tion procedure. On the left, black nodes comprise the basicIP model, blue nodes are found in both the issues IP andamici IP models, while magenta nodes are found only in theamici IP model.

where the constant is added so that putil is non-negative.Note, were the utility negative, the amici would have chosennot to write a the brief. This approach is known as a randomutility model in the econometrics discrete-choice literature(McFadden 1974). Random utility models relax the preci-sion of the optimization by assuming that agent preferencesalso contain an idiosyncratic random component. Hence, thebehavior we observe (i.e., the amicus’ topic mixture propor-tions) has a likelihood that is proportional to expected utility.Considering all the amici, we estimate the full likelihood

L(w,v,ψ,θ,∆,κ)×[∏

k∈A putil(∆k)]η

(4)

where L(·) is the likelihood of our amici IP model (Eq. 1),and η is a hyperparameter that controls influence of utilityon parameter estimation.

Eq. 4 resembles product of experts model (Hinton 2002).For the likelihood of votes in a case to be maximized, it isnecessary that no individual component—generative storyfor votes, amicus briefs—assigns a low probability. Accord-ingly, this results in a principled manner for us to incorpo-rate our assumptions about amici as rational decision mak-ers, each of whom is an “expert” with the goal of nudginglatent variables to maximize her own expected utility.

4 Learning and InferenceModel priors. The models we described above combineideal points, topic models, and random utility; they can beestimated within a Bayesian framework. Following Laud-erdale and Clark (2014), we place Gaussian priors on the jus-tice and case parameters: ρ ∼ U(0, 1), ψj ∼ N(0, λI + ρ1)for each justice j, and κi ∼ N(0,σ) for each case i. Thepositive off-diagonal elements of the covariance matrix forjustice IPs orient the issue-specific dimensions in the samedirection (i.e., with conservatives at the same end) and pro-vide shrinkage of IP in each dimension to their commonmean across dimensions. Fig. 1 presents the plate diagramfor the IP models (§2) on the left and the random utilitymodel (§3) on the right.

For the non-utility IP models involving text (§2), LDA isused to infer the latent topic mixtures of text associated withthe case and amicus briefs, thus θi and ∆k are both drawnfrom a symmetric Dirichlet distribution with hyperparame-ter α and tokens in both sets of texts are drawn from sharedtopic-word distributions.11

Our random utility model can be described through a sim-ilar generative story. Instead of drawing amicus briefs (∆)from a Dirichlet, they are drawn from the expected util-ity distribution (Eq. 3). The right side of Fig. 1 shows thecorresponding plate diagram. Importantly, note that ∆ hereserves as direct evidence for the justice and case parameters,rather than influencing them through v-structures.

Parameter estimation. We decoupled the estimation ofthe topic mixture parameters as a stage separate from theIP parameters. This approach follows Lauderdale and Clark(2014), who argued for its conceptual simplicity: the textdata define the rotation of a multidimensional preferencespace, while the second stage estimates the locations in thatspace. We found in preliminary experiments that similar is-sue dimensions result from joint vs. stage-wise inference,but that the latter is much more computationally efficient.

Using LDA,12 θ (and, where relevant, ∆) are estimated,then fixed to their posterior means while solving for jus-tice parameters ψ and case parameters κ. For the secondstage, we used Metropolis within Gibbs (Tierney 1994), ahybrid MCMC algorithm, to sample the latent parametersfrom their posterior distributions. We sampled κi for eachcase and ψj for each justice blockwise from a multivariateGaussian proposal distribution, tuning the diagonal covari-ance matrix to a target acceptance rate of 15–45%. Likewise,ρ is sampled from a univariate Gaussian proposal, with itsvariance tuned similarly. For our random utility model, weused the same MCMC approach in sampling the latent IRTvariables, but include the expected utility term for each briefin the likelihood function (eq. 4). Details of our sampler andhyperparameter settings can be found in the supplementarymaterials (§B), while topics and justices’ IPs estimated byour model are found in §D and §E, respectively.

Data. We focused on 23 terms of the Court from 1990–2012 (Spaeth et al. 2013),13 using texts from LexisNexis.14

We concatenate each of the 2,074 cases’ merits briefs fromboth parties to form a single document, where the text isused to infer the representation of the case in topical space(θ; i.e., merits briefs are treated as “facts of the case”). Wedid not make use of case opinions as did Lauderdale andClark (2014) because opinions are written after votes are

11We omit details of LDA, as it is widely known.12We used the C++ implementation of LDA by Liu et al. (2011).13The unit of analysis is the case citation, and we select cases

where the type of decision equals 1 (orally argued cases with signedopinions), 5 (cases with equally divided vote), 6 (orally arguedper curiam cases), or 7 (judgements of the Court). In addition, wedropped cases where the winning side was not clear (i.e., coded as“favorable disposition for petitioning party unclear”).

14http://www.lexisnexis.com

Model AccuracyLogistic regression w/ topics 0.715 ± 0.008Unanimous 0.714 ± 0.003Unidimensional IP 0.583 ± 0.037Issues IP 0.671 ± 0.008Amici IP 0.690 ± 0.021Random utility IP 0.742 ± 0.006

Table 1: Average pairwise vote partition accuracy (five-foldcross-validation).

cast, tainting the data for predictive modeling. Each amicusbrief is treated as a single document.

As the amicus briefs in our dataset were not explicitly la-beled with the side that they support, and manually labelingeach brief would be a tedious endeavor, we built a classifierto automatically label the briefs with the side the amici aresupporting, taking advantage of cues in the brief content thatstrongly signal the side that the amici is supporting (e.g., “insupport of petitioner” and “affirm the judgement”).

Additionally, we find that using only phrases (instead ofstandard bag-of-words) gave us more interpretable topics(Sim et al. 2013). Details of our phrase extraction, data pre-processing steps, and brief “side” classification are in thesupplementary materials (§C).

5 Experiments and Analysis5.1 Vote PredictionWe evaluate each model’s ability to predict how justiceswould vote on a case out of the training sample. To com-pute the probability of justices’ votes, we first infer the topicmixture proportions for the case’s merits briefs (θ), and am-icus briefs (∆). Given all the justice’s IPs ψj , we find themost likely vote outcome for the case by integrating overthe case parameters κ:

arg maxv

∫κp(κ | σ)

∏j∈J p(vj | ψj ,θ,∆,κ)

×∏k∈A putil(∆k | ψ,θ,∆,κ, sk)

where sk is the side brief k supports, and the multiplier isthe expected utility term (Eq. 3) which is ignored for thenon-utility based models.

Due to the specification of IP models, the probability ofa vote is a logistic function of the vote-specific IP, whichis a symmetric function implying that justice j’s probabilityof voting towards the petitioner will be the same as if shevoted for the respondent when we negate the vote-specificIP. Thus, we would not be able to distinguish the actual sidethat the justice will favor, but we can identify the most likelypartitioning of the justices into two groups.15 We can thenevaluate, for each case, an average pairwise accuracy score,(

92

)−1∑j,j′∈J:j 6=j′ I[I[vj = vj′ ] = I[v∗j = v∗j′ ]]

where v (v∗) are predicted (actual) votes.We performed 5-fold cross validation, and present the

vote partition accuracy in Table 1. We have two naıve base-lines, (i) where all justices vote unanimously, and (ii) where

15Given the partitioning of justices, domain experts should beable to identify the side each group of justices would favor.

we trained an `1-regularized logistic regression classifier foreach justice using the concatenated topic proportions of θand ∆ as features for each case. The baselines exhibit sim-ilar accuracies, and perform better than when adjusting forissues and/or amici. This suggests that justices’ votes do notalways align with their IPs, and that topic models alone maybe inadequate for representing IPs. Furthermore, we believethere may be insufficient information to learn the amicus po-larity case parameters (cs) in the amici IP model (which isslightly better than the issues IP model). However, in the ran-dom utility model, amici-agents-experts weigh in, providingadditional signals for estimating these parameters, achiev-ing significantly (paired samples t-test, p < 0.001) betterpredictive accuracy than the baseline.

As an additional qualitative validation of our approach,we compared the log-likelihoods between models that con-sider vs. ignore amicus briefs. We found correlations withanecdotal evidence of how justices view the influence of am-icus briefs (see §F for details).

5.2 Post Hoc Analysis of VotesOn a case level, we can tease apart the relative contributioneach textual component to a justice’s decision by analyzingthe case parameters learnt by our random utility model. Byzeroing out various case parameters, and plotting them, wecan visualize the different impact that each type of text hason a justice’s vote-specific IP. For example, Fig. 2 shows thevote-specific IP estimates of justices for the 2011 term deathpenalty case Maples v. Thomas (132 S. Ct. 912). The issues-

−15−10−50510Maples v. Thomas

GinsburgBreyer

SotomayorKagan

RobertsAlito

KennedyScalia

Thomas

Issues IPAmici (Maples)Amici (Thomas)Amici + Issue IP

Figure 2: Vote-specific IP estimates decomposed into differ-ent influences on each justice’s vote on Maples v. Thomas.An IP towards the left (right) indicates higher log-odds ofvote that is favorable to Maples (Thomas).

only IPs are computed by zeroing out both the amicus polar-ity parameters (cp and cr). On the other hand, the IP due toamicus briefs supporting Maples (Thomas) is computed byzeroing out only cr (cp). We observe that the issues-only IPsare aligned with each justice’s (widely known) ideologicalstance on the issue of capital punishment. For instance, theissues-only IPs of Thomas, Scalia, Alito, and Roberts, thestrong conservative bloc, favor the respondents (that Maplesshould not be awarded relief); so did Kennedy, who is widelyrecognized as the swing justice. When effects of all amicusbriefs are taken into account, the justices’ IPs shift towardMaples with varying magnitudes, with the result reflectingthe actual ruling (7–2 with Thomas and Scalia dissenting).

5.3 Counterfactual AnalysisFollowing Pearl (2000), we can query the model and per-form counterfactual analyses using the vote prediction algo-rithm (§5.1). As an illustration, we consider National Fed-eration of Independent Business (NFIB) v. Sebelius (HHS)(132 S. Ct. 2566), a landmark 2011 case in which the Courtupheld Congress’s power to enact most provisions of the Af-fordable Care Act (ACA; “Obamacare”).16

In the merits briefs, the topics discussed revolve aroundinterstate commerce and the individual mandate, while thereis an interesting disparity in topics between briefs support-ing NFIB and HHS.17 Notably, amici supporting NFIB arefound, on average, to use language concerning individualmandate, while amici supporting HHS tend to focus moreon topics related to interstate commerce. This is commensu-rate with the main arguments put forth by the litigants, whereNFIB was concerned about the overreach of the governmentin imposing an individual mandate, while HHS argued thathealthcare regulation by Congress falls under the CommerceClause. Our model was most uncertain about Roberts andKennedy, and wrong about both (Fig. 3 top).

Choosing sides. The first type of counterfactual analysisthat we introduce is, “What if no (or only one side’s) amicusbriefs were submitted in the ACA case?” To answer it, wehold the case out of the training set and attempt to predict thevotes under the hypothetical circumstances with the randomutility model. Fig. 3 (top) shows the resulting IP of hypothet-ical situations where no amicus briefs were filed, or whenonly briefs supporting one side are filed. If no amici filedbriefs, the model expects that all but Kagan and Sotomayorwould favor NFIB, but with uncertainty. With the inclusionof the amicus briefs supporting NFIB, the model becomesmore confident that the conservative bloc of the court wouldvote in favor of NFIB (except for Alito). Interestingly, themodel anticipates that the same briefs will turn the liberalsaway. In contrast, the briefs on HHS’ side have more suc-cess in swaying the case in their favor, especially the crucialswing vote of Kennedy (although it turned out that Kennedysided with the conservative bloc, and Roberts emerged as thedeciding vote in HHS favor). Consequently, the model canprovide insights about judicial decisions, while postulatingdifferent hypothetical situations.

Choosing what to write. Another counterfactual analysiswe can perform, more useful from the viewpoint of the am-icus, is, “how should an amicus frame arguments to bestachieve her goals?” In the context of our model, such anamicus would like to choose the topic mixture ∆ to maxi-mize her expected utility (Eq. 3). Ideally, one would com-

16The case attracted much attention, including a record 136 am-icus briefs, of which 76 of these briefs are used in our dataset. 58(of the 76) were automatically classified as supporting NFIB.

17The merits briefs were estimated at 41% and 20% on the in-terstate commerce and the individual mandate topics, respectively.NFIB amicus briefs were 15% on interstate commerce and 41% onindividual mandate; these figures switch to 36% and 22% for HHSamicus briefs.

−2.0−1.5−1.0−0.50.00.51.01.5

GinsburgBreyer

SotomayorKagan

RobertsAlito

KennedyScalia

Thomas

Both amiciNo amicusNFIB amiciHHS amici

−0.8−0.6−0.4−0.20.00.20.40.60.8

GinsburgBreyer

SotomayorKagan

RobertsAlito

KennedyScalia

Thomas No amicusNFIB amicusHHS amicus

Figure 3: Counterfactual analyses for NFIB v. Sebelius(HHS). Top: What if amicus briefs for one side were notfiled? Bottom: What if a single amicus files an “optimally”written brief? An IP towards the left (right) indicates higherlog-odds of vote favorable to NFIB (HHS). Hollow markersdenote that prediction differed from the actual outcome.

pute such a topic mixture by maximizing over both ∆ andvote outcome v, while integrating over the case parameters.We resort to a cheaper approximation: analyzing the filer’sexpected utility curve over two particular topic dimensions:the individual mandate and interstate commerce topics. Thatis, we compute the expected utility curve (see supplementary§G) faced by a single amicus as we vary the topic propor-tions of individual mandate and interstate commerce topicsover multiples of 0.1.

Consequently, the amicus who supports NFIB can expectto maximize their expected utility (5.2 votes at a cost of0.21) by “spending” about 70% of their text on individualmandate. On the other hand, the best that an amicus sup-porting HHS can do is to write a brief that is 80% aboutinterstate commerce, and garner 4.7 votes at a cost of 0.31.We plot the justices’ predicted IPs in Fig. 3 (bottom) usingthese “best” proportions. The “best” proportions IPs are dif-ferent (sometimes worse) from that in Fig. 3 (top) becausein the latter, there are multiple amici influencing the case pa-rameters (through their utility functions) and other topics arepresent which will sway the justices. From the perspectiveof an amicus supporting HHS, the two closest swing votesin the case are Roberts and Kennedy; we know a posteriorithat Roberts sided with HHS.

5.4 DiscussionOur model makes several simplifying assumptions: (i) it ig-nores the effects of other amici on a single amicus’ writ-ing; (ii) amici are treated modularly, with a multiplicativeeffect and no consideration of diminishing returns, tempo-ral ordering, or reputation; (iii) the cost function does notcapture the intricacies of legal writing style (i.e., choice ofcitations, artful language, etc.); (iv) the utility function does

not fully capture the agenda of each individual amicus; (v)each amicus brief is treated independently (i.e., no sharingacross briefs with the same author), as we do not have ac-cess to clean author metadata. Despite these simplifications,the model is a useful tool for quantitative analysis and hy-pothesis generation in support of substantive research on thejudiciary.

6 Related WorkPoole and Rosenthal (1985) introduced the IP model, us-ing roll call data to infer latent positions of lawmakers.Since then, many varieties of IP models have been pro-posed for different voting scenarios: IP models for SCO-TUS (Martin and Quinn 2002), multidimensional IP mod-els for Congressional voting (Heckman and Snyder 1996;Clinton, Jackman, and Rivers 2004), grounding multidimen-sional IP models using topics learned from text of Congres-sional bills (Gerrish and Blei 2012) and SCOTUS opinions(Lauderdale and Clark 2014).

Amici have been studied extensively, especially their in-fluence on SCOTUS (Caldeira and Wright 1988; Kearneyand Merrill 2000; Collins 2008; Corley, Collins, and Ham-ner 2013). Collins (2007) found that justices can be influ-enced by persuasive argumentation presented by organizedinterests. These studies focus on ideology metadata (lib-eral/conservative slant of amici, justices, decisions, etc.),disregarding the rich signals encoded in the text of thesebriefs, whereas we use text as evidence of utility maximizingbehavior to study the influence of amicus curiae.

Our model is also related to Gentzkow and Shapiro (2010)who model the purposeful “slant” of profit-maximizingnewspapers looking to gain circulation from consumers witha preference for such slant. More generally, extensive litera-ture in econometrics estimates structural utility-based deci-sions (Berry, Levinsohn, and Pakes 1995, inter alia).

In addition to work on IP models, researchers have usedcourt opinions for authorship (Li et al. 2013) and historicalanalysis, while oral argument transcripts have been used tostudy power relationships (Danescu-Niculescu-Mizil et al.2012; Prabhakaran, John, and Seligmann 2013) and prag-matics (Goldwasser and Daume III 2014).

7 ConclusionWe have introduced a random utility model for persuasivetext; it is similar to a classical generative model and can beestimated using familiar algorithms. The key distinction isthat persuasive text is modeled as a function of the addresseeand the particulars of the matter about which she is beingconvinced; authors are agents seeking to maximize their ex-pected utility in a given scenario. In the domain of SCO-TUS, this leads to improved vote prediction performance, asthe model captures the structure of amicus briefs better thansimpler treatments of the text. Secondly, and more impor-tantly, our model is able to address interesting counterfactualquestions. Were some amicus briefs not filed, or had theybeen written differently, or had the facts of the case beenpresented differently, or had different justices presided, ourapproach can estimate the resulting outcomes.

AcknowledgementsThis research was supported in part by an A*STAR fellow-ship to Y. Sim, NSF grant IIS-1211277, a Google researchaward, and by computing resources from the Pittsburgh Su-percomputing Center.

AppendicesA Brief writing trade offs

Consider the first-order (or KKT) condition for the purpose-ful amicus’ maximization w.r.t. the dth issue:18∑

j∈J σ′ψj,dc

s − ξ(∆d − θd)=∑j∈J σ

′ψj,1cs − ξ(∆1 − θ1) if 0 < ∆d < 1

where σ′ is the first order derivative of the vote probabil-ity sigmoid function. Optimality drives all topics (with pos-itive weight) to have equal marginal values.19 The marginalvalue highlights the tradeoffs an amicus faces, in four com-ponents: (i) the cost of deviating from the merits, i.e., a largedifference between ∆d and θd, is ξ; (ii) a justice whose σ′is large, i.e., whose vote is uncertain, receives greater atten-tion, in particular (iii) on issues she cares about, i.e., ψj,d islarge, with (iv) cs controlling the strength of correspondenceto issues justices care about.

B Likelihood of Random Utility Ideal PointModel

The likelihood of our amici model with random utility isgiven below,

p(v, ρ,ψ,a, b, cp, r | λ,θ,∆, s)

∝∏j∈J

p(ψj | λ, ρ)∏i∈C

p(ai, bi, cp, ri | σ)

×∏i∈C

∏j∈J

Vi,j∏k∈Ai

E[Ui,k]η

where

Vi,j = p(vi,j | ψj ,θi,∆i, ai, bi, cp, ri )

= σ [(1− vi,j)ai+(1− vi,j)ψ>j (biθi + cp

i∆pi + cr

i∆ri)],

σ(x) = exp(x)1+exp(x) is the logistic function, and

E[Ui,k] =∑j∈J

σ(ai +ψ>j (biθi + c

si,ki ∆i,k)

)+ ξ

(1− 1

2‖∆i,k − θi‖22

)18We consider inner points for clarity; if ∆d = 1, equality is

replaced with ≥ (respectively, 0 and ≤).19Comparing each topic’s proportion to topic 1 is arbitrary. In a

D-topic model, the amicus has only D − 1 choices.

We add a constant to the expected utility term so that itwill always be ≥ 0. During each iteration of Gibbs sam-pling, we sampled each latent variable ρ,ψj and [ai, bi, c

p, ri ]

blockwise from the likelihood in turn using the Metropolis-Hastings algorithm. In each Metropolis-Hastings randomwalk, we took 500 steps, ignoring the first 250 for burn-inand keeping every 10th step to compute the mean, which weuse as our Gibbs update. In total, we performed 2,000 Gibbsiterations over the training data.

Hyperparameters. We fixed the number of topics in ourmodel to 30. For LDA, the symmetric Dirichlet parame-ter for document-topic and topic-word distributions are 0.1and 0.001, respectively. For priors on the IP latent variables,we follow the same settings used by Lauderdale and Clark(2014), setting priors on case parameters, σ, to 4.0, and jus-tice IPs component-wise variance, λ to 1.0. In the randomutility model, we set both hyperparameters η and ξ to 1.

C DataWe tokenized all the texts and tagged the tokens with theStanford part of speech tagger (Toutanova et al. 2003).We extract n-grams with tags that follow the simple (buteffective) pattern (Adjective|Cardinal|Noun)+ Noun(Justeson and Katz 1995), representing each document as a“bag of phrases”, and filtering phrases that appear in lessthan 25 or more than 3,000 documents obtaining a vocabu-lary of 55,113 phrase types. Table 2 summarizes details ofour corpus.

Cases 2,074w/amicus briefs 1,531

Max. briefs/case 76Word tokens 110.5MPhrase tokens 8.9MAve. words/brief 3,094Ave. phrases/brief 339

Table 2: Corpus statistics ignoring amicus briefs whosesupporting side could not be automatically classified con-fidently. In the last row, briefs include merits and amicusbriefs.

We manually labeled 1,241 randomly selected ami-cus briefs with its side (petitioner, respondent, neither),and trained a logistic regression classifier20 using lex-ical and formatting features. We identified 5 sectionswhich are common across almost all briefs and usedthem as features. The feature templates for our clas-sifier are: 〈w〉, 〈title,w〉, 〈counsel,w〉, 〈introduction,w〉,〈statement,w〉, 〈conclusion,w〉, where w can be any uni-gram, bigram, or trigram.

We evaluated the performance of our classifier using 5random splits, with 50% of our data for training, 30% fortesting, and 20% for the development set. We tuned the`1-regularization weights on our dev set over the range of

20We used a C++ implementation of logistic regression classifieravailable at https://github.com/redpony/creg.

coefficients {0.5, 1, 2, 4, 8, 16}. The average accuracy ofour classifier is 79.1%. Limiting our evaluation to instanceswhose posterior probability after classification is greaterthan 0.8, we obtain 90.0% accuracy and recall of 52.1%.Thus, we used 7,258 (out of 13,162) briefs that were classi-fied as supporting neither side or whose posterior probabilityis ≤ 0.8 (higher precision at the expense of recall).

D Topic DistributionTable 3 lists the topics and top phrases estimated from ourdataset using LDA.

E Justices’ Ideal PointsThe ideal points of justices vary depending on the issues. Wepresent the justices’ ideal points for each of the 30 topics inFig. 4.

F Amici Influence on JusticesVarious justices have expressed opinions on the value ofamicus briefs. Some justices, such as Scalia, are known tobe dubious of amicus briefs, preferring to leave the task ofreading these briefs to their law clerks, who will pick outany notable briefs for them.21 In contrast, recent surveys ofamicus citation rates in justice opinions have often rankedSotomayor and Ginsburg among justices who most oftencite amici in their opinions (Franze and Anderson 2011;Walsh 2013).

For each justice, we compute the difference in vote log-likelihood between the issues IP and random utility IP mod-els, as a (noisy) measure of amici influence. A larger dif-ference in suggests that more of a justice’s decision-makingcan be explained by the presence of amicus briefs. Table 4presents these differences, and the rankings are consistentwith extant hypotheses noted above. We take this consis-tency as encouraging, in the spirit of the “preregistered” hy-potheses of Sim et al. (2013), but caution that it does notimply a conclusion about causation.

G Expected utility curve for hypotheticalamicus in Obamacare case

0.0/1.0 0.2/0.8 0.4/0.6 0.6/0.4 0.8/0.2 1.0/0.0Proportions of individual mandate / interstate commerce

0.520.540.560.580.60

Exp

ecte

dut

ility

Support NFIBSupport HHS

Figure 5: Expected utility when varying between propor-tions of individual mandate and interstate commerce topics.

21See Roper v. Simmons, 543 U.S. 551, 617-18 (2005) (ScaliaJ., dissenting); also, “Dont re-plow the ground that you expect theparties to plow unless you expect the parties to plow with a partic-ularly dull plow” (attributed to Scalia on the California AppellateLaw Blog, http://bit.ly/1wOmlRE).

Justice Diff. (×.01)John Paul Stevens 31.5Clarence Thomas∗ 30.8Thurgood Marshall 29.4Elena Kagan∗ 28.9Sonia Sotomayor∗ 24.7John Roberts∗ 24.0Ruth Bader Ginsburg∗ 23.9William Rehnquist 23.8Stephen Breyer∗ 22.2David Souter 22.0Samuel Alito∗ 21.8Harry Blackmun 18.0Byron White 16.9Antonin Scalia∗ 16.6Anthony Kennedy∗ 15.3Sandra Day O’Connor 14.7

Table 4: Justices ordered by how much amicus briefs canexplain their votes, estimated as quadratic mean differencein vote log-likelihood between the issues IP model and therandom utility IP model. ∗ denotes current sitting justices.

Fig. 5 illustrates the expected utility curve faced by a sin-gle amicus as we vary the topic proportions of the individ-ual mandate and interstate commerce topics. The model ex-pects an amicus on NFIB’s side to get more votes and hence,higher utility, as the model expects justices to be in favor ofNFIB prior to amici influence.

We note that interior solutions for the expected utility op-timization only exist when proportions are ∈ (0, 1). Duringour experiment, we set the proportions of inactive topics to10−8 instead of 0.

ReferencesBerry, S.; Levinsohn, J.; and Pakes, A. 1995. Automobile pricesin market equilibrium. Econometrica: Journal of the Economet-ric Society 841–890.Blei, D. M.; Ng, A. Y.; and Jordan, M. I. 2003. Latent Dirichletallocation. Journal of Machine Learning Research 3:993–1022.Caldeira, G. A., and Wright, J. R. 1988. Organized interests andagenda setting in the U.S. Supreme Court. American PoliticalScience Review 82:1109–1127.Chang, M.-W.; Ratinov, L.; and Roth, D. 2007. Guiding semi-supervision with constraint-driven learning. In Proceedings ofthe 45th Annual Meeting of the Association for ComputationalLinguistics, 280–287.Clinton, J.; Jackman, S.; and Rivers, D. 2004. The statisticalanalysis of roll call data. American Political Science Review98:355–370.Collins, P. M. 2007. Lobbyists before the U.S. Supreme Court:Investigating the influence of amicus curiae briefs. PoliticalResearch Quarterly 60(1):55–70.Collins, P. M. 2008. Friends of the Supreme Court: Inter-est Groups and Judicial Decision Making. Oxford UniversityPress.Corley, P.; Collins, P. M.; and Hamner, J. 2013. The influenceof amicus curiae briefs on U.S. Supreme Court opinion content.In APSA 2013 Annual Meeting Paper.Danescu-Niculescu-Mizil, C.; Lee, L.; Pang, B.; and Kleinberg,J. 2012. Echoes of power: Language effects and power differ-

# Topic Top phrases1 Criminal procedure

(1)reasonable doubt, supervised release, grand jury, prior conviction, plea agreement, controlled substance,guilty plea, double jeopardy clause, sixth amendment, jury trial

2 Employment erisa plan, plan administrator, employee benefit plan, insurance company, pension plan, health care, planparticipant, individual mandate, fiduciary duty, health insurance

3 Due process due process clause, equal protection clause, fundamental right, domestic violence, equal protection, stateinterest, d e, assisted suicide, controlled substance, rational basis

4 Indians m r, m s, indian tribe, tribal court, indian country, fifth amendment, miranda warning, indian affair, vicepresident, tribal member

5 Economic activity attorney fee, limitation period, hobbs act, security law, rule 10b, actual damage, racketeering activity, fidu-ciary duty, loss causation, security exchange act

6 Bankruptcy law bankruptcy court, bankruptcy code, 1996 act, state commission, telecommunication service, network ele-ment, eighth circuit, new entrant, pole attachment, communication act

7 Voting rights voting right, minority voter, j app, voting right act, covered jurisdiction, fifteenth amendment, redistrictingplan, political process, political subdivision, minority group

8 First amendment first amendment right, commercial speech, strict scrutiny, cable operator, free speech, first amendment pro-tection, protected speech, child pornography, government interest, public forum

9 Taxation interstate commerce, commerce clause, state tax, tax court, gross income, internal revenue code, income tax,dormant commerce clause, state taxation, sale tax

10 Amicus briefs national association, amicus brief, vast majority, brief amicus curia, large number, wide range, recent year,public policy, wide variety, washington dc

11 Labor management north carolina, collective bargaining agreement, confrontation clause, sta t, north platte river, collective bar-gaining, inland lake, laramie river, labor organization, re v

12 Civil action class action, class member, injunctive relief, final judgment, federal claim, civil action, preliminary injunc-tion, class certification, civil procedure, subject matter jurisdiction

13 Civil rights title vii, title vi, civil right act, age discrimination, sexual harassment, old worker, major life activity, reason-able accommodation, prima facie case, disparate impact

14 State sovereign sovereign immunity, eleventh amendment, state official, absolute immunity, false claim, private party, 42 usc1983, state sovereign immunity, eleventh amendment immunity, federal employee

15 Federal administra-tions

federal agency, statutory construction, plain meaning, other provision, statutory text, dc circuit, sub (a), fiscalyear, senate report, agency action

16 Interstate relations special master, new mexico, prejudgment interest, arkansas river, rt vol, comp act, new jersey, elli island,john martin reservoir, video game

17 Court of Appeals eleventh circuit, sixth circuit, circuit court, fourth circuit, oral argument, tenth circuit, further proceeding,appeal decision, instant case, defendant motion

18 Fourth amendment fourth amendment, probable cause, arbitration agreement, police officer, national bank, search warrant, ex-clusionary rule, arbitration clause, reasonable suspicion, law enforcement officer

19 Eighth amendment eighth amendment, sex offender, prison official, facto clause, copyright act, copyright owner, unusual pun-ishment, public domain, liberty interest, public safety

20 International law international law, foreign state, vienna convention, human right, foreign country, foreign government, jonesact, united kingdom, native hawaiian, foreign nation

21 Equal protectionclause

peremptory challenge, law school, equal protection clause, strict scrutiny, high education, racial discrimina-tion, prima facie case, school district, consent decree, compelling interest

22 Commerce clause(2)

interstate commerce, commerce clause, local government, political subdivision, state regulation, supremacyclause, federal regulation, tobacco product, tenth amendment, federal fund

23 Immigration law judicial review, immigration law, final order, removal proceeding, immigration judge, due process clause,deportation proceeding, administrative remedy, compliance order, time limit

24 Death penalty death penalty, habeas corpus, reasonable doubt, trial judge, death sentence, ineffective assistance, directappeal, defense counsel, new rule, mitigating evidence

25 Environmentalissues

navigable water, clean water act, colorado river, special master, project act, public land, fill material, waterright, point source, lake mead

26 Establishmentclause

establishment clause, school district, public school, private school, religious school, ten commandment, boyscout, religious belief, religious organization, free exercise clause

27 Patent law federal circuit, patent law, prior art, subject matter, expert testimony, lanham act, hazardous substance, patentsystem, patent act, new drug

28 Antitrust law antitrust law, sherman act, contr act, market power, postal service, joint venture, natural gas, high price,public utility, interstate commerce

29 Election law political party, taking clause, private property, property owner, fifth amendment, independent expenditure,federal election, property right, contribution limit, general election

30 Criminal procedure(2)

punitive damage, habeas corpus, second amendment, punitive damage award, enemy combatant, militarycommission, compensatory damage, state farm, new trial, due process clause

Table 3: Topics and top-10 phrases estimated from briefs using LDA. We manually annotated each topic with the topic labels.

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