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Abstract:
Prospect theory scholars have identified important human decision-making biases, but they have been conspicuously silent on the question of the origin of these biases. Here we create a model that shows preferences consistent with prospect theory may have an origin in evolutionary psychology. Specifically, we derive a model from risk-sensitive optimal foraging theory to generate an explanation for the origin and function of context-dependent risk aversion and risk seeking behavior. Although this model suggests that human cognitive architecture evolved to solve particular adaptive problems related to finding sufficient food resources to survive, we argue that this same architecture persists and is utilized in other survival-related decisions that are critical to understanding political outcomes. In particular, we identify important departures from standard results when we incorporate prospect theory into theories of spatial voting and legislator behavior, international bargaining and conflict, and economic development and reform.

Abstract:
We construct the complete network of 28,951 majority opinions written by the U.S. Supreme Court and the cases they cite from 1792 to 2005. We illustrate some basic properties of this network and then describe a method for creating importance scores using the data to identify the most important Court precedents at any point in time. This method yields dynamic rankings that can be used to predict the future citation behavior of state courts, the U.S. Courts of Appeals, and the U.S. Supreme Court, and these rankings outperform several commonly used alternative measures of case importance.

Supreme Court, precedent, law, citation analysis, case importance

Abstract:
We construct the complete network of 28,951 majority opinions written by the U.S. Supreme Court and the cases they cite from 1792 to 2005. We illustrate some basic properties of this network and then describe a method for creating importance scores using the data to identify the most important Court precedents at any point in time. This method yields dynamic rankings that can be used to predict the future citation behavior of state courts, the U.S. Courts of Appeals, and the U.S. Supreme Court, and these rankings outperform several commonly used alternative measures of case importance.

Supreme Court, precedent, law, citation analysis, case importance

Abstract:
The Supreme Court's reasoning in a decision, including the precedent it cites in support of that reasoning, can be as significant as the outcome in determining the long-term impact of a case. As a result, the content of opinions can be used to provide important new insights into existing debates regarding judicial politics. In this article we present a strategic content model of the judicial process, which demonstrates how opinion content results from the strategic interaction between justices during the Court's bargaining process. This is the first article to show on a large scale that the extent to which a majority opinion writer cites authoritative precedent is systematically influenced by the decisions and ideology of other justices. We find that the Court generates opinions that are better grounded in law when more justices write concurring opinions. This demonstrates that justices write concurring opinions based not just on a preference for making their opinions known, but also to influence the reasoning relied on by the majority opinion. We also show that diversity of opinion on the Court, a factor often overlooked in the political science literature, has a significant impact on the extent to which a Court opinion cites authoritative precedent. Finally, our results provide a novel test of the agenda-control and median-justice models. We find that the ideology of the median justice influences the citation of precedent in the majority opinion, whereas the majority opinion writer's ideology does not, suggesting that agenda-setting powers are not as strong as previously claimed.

Abstract:
We construct the complete network of 30,288 majority opinions written by the U.S. Supreme Court and the cases they cite from 1754 to 2002 in the United States Reports. Data from this network demonstrates quantitatively the evolution of the norm of stare decisis in the 19th Century and a significant deviation from this norm by the activist Warren court. We further describe a method for creating authority scores using the network data to identify the most important Court precedents. This method yields rankings that conform closely to evaluations by legal experts, and even predicts which cases they will identify as important in the future. An analysis of these scores over time allows us to test several hypotheses about the rise and fall of precedent. We show that reversed cases tend to be much more important than other decisions, and the cases that overrule them quickly become and remain even more important as the reversed decisions decline. We also show that the Court is careful to ground overruling decisions in past precedent, and the care it exercises is increasing in the importance of the decision that is overruled. Finally, authority scores corroborate qualitative assessments of which issues and cases the Court prioritizes and how these change over time.

Abstract:
The decision to vote has puzzled scholars for decades. Theoretical models predict little or no variation in participation in large population elections and empirical models have typically explained only a relatively small portion of individual-level variance in turnout behavior. However, these models have not considered the hypothesis that part of the variation in voting behavior can be attributed to genetic effects. Matching public voter turnout records in Los Angeles to a twin registry, we study the heritability of political behavior in monozygotic and dizygotic twins. The results show that genes account for a significant proportion of the variation in voter turnout. We also replicate these results with data from the National Longitudinal Study of Adolescent Health and show that they extend to a broad class of acts of political participation. These are the first findings to suggest that humans exhibit genetic variation in their tendency to participate in political activities.

Abstract:
How did human cooperation evolve? Recent evidence shows that many people are willing to engage in altruistic punishment, voluntarily paying a cost to punish noncooperators. While this behaviour helps to explain how cooperation can persist, it creates an important puzzle. If altruistic punishment provides benefits to nonpunishers and is costly to punishers, then how could it evolve? Drawing on recent insights from voluntary public goods games, I present a simple evolutionary model in which altruistic punishers can enter and will always come to dominate a population of contributors, defectors, and nonparticipants. The model suggests that the cycle of strategies in voluntary public goods games does not persist in the presence of punishment strategies. It also suggests that punishment can only enforce payoff-improving strategies, contrary to a widely-cited "folk theorem" result that suggests punishment can allow the evolution of any strategy.

Abstract:
Scholars have recently extended the traditional calculus of participation model by adding a term for benefits to others. We advance this work by distinguishing theoretically a concern for others in general (altruism) from a concern for others in certain groups (social identification). We posit that both concerns generate increased benefits from participation. To test these theories, we use allocations in dictator games towards an unidentified anonymous recipient and two recipients identified only as a registered Democrat or a registered Republican. These allocations permit a distinction between altruism and social identification. The results show that both altruism and social identification significantly increase political participation. The results also demonstrate the usefulness of incorporating benefits that stem from sources beyond material self-interest into rational choice models of participation.

Abstract:
Fowler, Baker, and Dawes (2008) recently showed in two independent studies of twins that voter turnout has very high heritability. Here we investigate two specific genes that may contribute to this heritability via their impact on neurochemical processes that influence social behavior. Using data from the National Longitudinal Study of Adolescent Health, we show that a polymorphism of the MAOA gene significantly increases the likelihood of voting. We also find evidence of a gene-environment interaction between religious attendance and a polymorphism of the 5HTT gene that significantly increases voter turnout. These are the first results to ever link specific genes to political behavior and they suggest that political scientists should take seriously the claim that at least some variation in political behavior is due to innate predispositions.

genes, turnout, political participation, MAOA, 5HTT, seratonin

Abstract:
Drawing on recent methodological advances, we examine the social network of political science department placements. This network permits us to estimate simultaneously 1) how well departments place their own students and 2) how effective they are in hiring students from other institutions. Using data collected by Masuoka, Grofman and Feld (2006a, b) on U.S. Ph.D. granting institutions, we provide visualizations of the connectivity among 132 departments as a social network graph in which core and periphery departments can be identified. We also show how this network has changed over time. The new social network measures conform closely to qualitative expert rankings and show that a department's placement record contributes more to its prestige than a department's ability to hire and retain faculty from core institutions.

Abstract:
Previous studies have found that both political orientations (Alford, Funk & Hibbing 2005) and voting behavior (Fowler, Baker & Dawes 2007, Fowler & Dawes 2007) are significantly heritable. In this article we study genetic variation in another important political behavior: partisan attachment. Using the National Longitudinal Study of Adolescent Health, we show that individuals with the A1 allele of the D2 dopamine receptor gene are significantly less likely to identify as a partisan than those with the A2 allele. Further, we find that this gene's association with partisanship also mediates an indirect association between the A1 allele and voter abstention. These results are the first to identify a specific gene that may be responsible for the tendency to join political groups, and they may help to explain correlation in parent and child partisanship and the persistence of partisan behavior over time.

Abstract:
Economists have long realized the importance of credit markets and borrowing behavior for household finance and economics more generally. However, none of this previous work has explored the role of biological constraints. Here we present the first evidence of a specific gene that may influence borrowing behavior. Using data from the National Longitudinal Study of Adolescent Health, we show that individuals with a polymorphism of the MAOA gene that has lower transcriptional efficiency are significantly more likely to report having credit card debt. Having one or both MAOA alleles of the low efficiency type raises the average likelihood of having credit card debt by 7.8% and 15.9% respectively. About half of our population has one or both MAOA alleles of the low type. The results suggest that economists should integrate innate propensities into economic models and consider the welfare consequences of possible discrimination by lenders on the basis of genotype.

Abstract:
Participants in laboratory games are often willing to alter others' incomes at a cost to themselves and this behaviour has the effect of promoting cooperation. What motivates this action is unclear: punishment and reward aimed at promoting cooperation cannot be distinguished from attempts to produce equality. To understand costly taking and costly giving, we create an experimental game that isolates egalitarian motives. The results show that subjects reduce and augment others' incomes, at a personal cost, even when there is no cooperative behaviour to be reinforced. Furthermore, the size and frequency of income alterations are strongly influenced by inequality. Emotions towards top earners become increasingly negative as inequality increases, and those who express these emotions spend more to reduce above-average earners' incomes and to increase below-average earners' incomes. The results suggest that egalitarian motives affect income altering behaviours, and may thus be an important factor underlying the evolution of strong reciprocity and, hence, cooperation in humans.

Abstract:
The discrepancy between an individual's perceived social isolation (ie., loneliness) and the number of connections in their social network is well documented. Yet, few details are known about the placement of loneliness within, or the spread of loneliness through, social networks. Here, we use network linkage data from the population-based Framingham Heart Study to trace the topography of loneliness in people's social networks, and the path through which loneliness spreads through these networks. The source of participants (N = 5,124) is the Offspring Cohort of the Framingham Heart Study, and individuals to whom these participants are linked are drawn from the entire set of cohorts in the Framingham Heart Study (N = 12,067 individuals in the social network). Results indicated that loneliness occurs in clusters within social networks, extends up to three degrees of separation, and is disproportionately represented at the periphery of social networks. In addition, loneliness appears to spread through a contagious process even though lonely individuals are moved closer to the edge of social networks over time. The spread of loneliness was found to be stronger than the spread of perceived social connections, stronger for friends than family members, and stronger for women than for men. The results advance our understanding of the broad social forces that drive loneliness and suggest that efforts to reduce loneliness in our society may benefit by aggressively targeting the people in the periphery to help repair their social networks and to create a protective barrier against loneliness that can keep the whole network from unraveling.

Abstract:
Why do individuals engage in personally costly, partisan activities that benefit others? If individuals act according to rational self-interest, then partisan activity occurs only when the benefits of that activity exceed its costs. However, laboratory experiments suggest that many people are willing to contribute to public goods and to punish those who do not contribute - even when these activities are personally costly and when members of the experimental group are completely anonymous. We hypothesize that these individuals, called strong reciprocators, underlie the capacity of political parties to organize competition for scarce resources and the production of public goods. To test this hypothesis, we conducted an experiment that includes a random income game with costly income alteration and a standard public goods game with costly punishment. These games allow us to gauge participants' willingness to contribute to public goods and to engage in the costly punishment of free-riders. The results show that partisans are more likely than nonpartisans to contribute to public goods and to engage in costly punishment. Thus, inherent tastes for cooperation and sanctioning help resolve the paradox of party participation.

Abstract:
We examine the social network structure of Congress from 1973-2004. We treat two Members of Congress as directly linked if they have cosponsored a bill together. We then construct explicit networks for each year using data from all forms of legislation, including resolutions, public and private bills, and amendments. We show that Congress exemplifies the characteristics of a "small world" network and that the varying small world properties during this time period are strongly related to the number of important bills passed.

Social Networks, Congress

Abstract:
Models of political participation have begun to incorporate actors who possess "social preferences". However, these models have failed to take into account the potentially incongruent political goals of different social preference types. These goals are likely to play an important role in shaping political behavior. To examine the effect of distinct social preferences on political activity we conducted an experiment in which participants played five rounds of a modified dictator game (Andreoni and Miller 2002). We used the decisions in these games to determine their preference type and mapped these types to reported political activity. Our results show that subjects who were most interested in increasing total welfare in the dictator game were more likely to participate in politics than subjects with selfish preferences, whereas subjects most interested in reducing the difference between their own well-being and the well-being of others were no more likely to.

Abstract:
Previous research has indicated that higher levels of social interactions, or greater social capital, tend to create higher levels of civic engagement and social trust. We used a large, nationally representative longitudinal survey of youth to examine the relationship between social capital, as measured by reported friendships in junior high and high school, and experiences in young adulthood with various dimensions of community engagement and trust in the federal, state, and local governments. After adjustment for a number of potential confounding variables, results showed that the total number of friends students had in middle and high school was positively associated with community engagement and trust in all levels of government in young adulthood. These findings suggest that social relationships, community engagement, and trust in government are linked, and that social experiences early in life may have an enduring effect on the capacity of communities and political institutions to govern themselves.

Abstract:
In 1987 South Korea initiated a successful transition to democracy, while previous attempts in 1979 and 1980 failed. This paper distinguishes two cycles of liberalization in South Korea and then develops a conceptual understanding that is used to test two common schools of thought. One school asserts that the United States had little impact on democratization in Korea and that domestic factors explain the delayed transition. The other school implies that the U.S. could have improved prospects for democratization by not approving Chun Doo Hwan's request to use Combined Forces Command troops to repress demonstrations in 1980. Finding both sets of explanations unsatisfactory, this paper draws on recently declassified documents and interviews with State Department officials to advance the hypothesis that it was U.S. public pressure which played a critical role in determining the timing of South Korea's transition to democracy. Finally, the use of public pressure is found to have been greatly affected by unrelated foreign policy crises in Iran and the Philippines, illuminating the process whereby conflicts in other countries that had no direct bearing on South Korea ultimately affected the outcome of its own domestic political process.

Abstract:
This article investigates between-voter interactions in a social network model of turnout. It shows that if 1) there is a small probability that voters imitate the behavior of one of their acquaintances, and 2) individuals are closely connected to others in a population (the "small-world" effect), then a single voting decision may affect dozens of other voters in a "turnout cascade." If people tend to be ideologically similar to other people they are connected to, then these turnout cascades will produce net favorable results for their favorite candidate. By changing more than one vote with one's own turnout decision, the turnout incentive is thus substantially larger than previously thought. We analyze conditions that are favorable to turnout cascades and show that the effect is consistent with real social network data from Huckfeldt and Sprague's South Bend and Indianapolis-St. Louis election surveys. We also suggest that turnout cascades may help explain over-reporting of turnout and the ubiquitous belief in a duty to vote.

Abstract:
In the past fifty years, biologists have learned a tremendous amount about human brain function and its genetic basis. At the same time political scientists have been intensively studying the effect of the social and institutional environment on mass political attitudes and behaviors. However, these separate fields of inquiry are subject to inherent limitations that may only be resolved through collaboration across disciplines. Here we describe recent advances in the emerging fields of genopolitics and neuropolitics and argue that biologists and political scientists must work together to advance a new science of human nature.

Abstract:
We recently showed that obesity can spread socially from person to person in adults (Christakis and Fowler 2007). A natural question to ask is whether or not these results generalize to a population of adolescents. Three separate teams of researchers have analyzed the National Longitudinal Study of Adolescent Health (Add Health) and shown evidence of person-to-peron spread of obesity, but they use different methods and disagree on the interpretation of their results. Here, we conduct our own analysis of the Add Health data, provide additional evidence from the Framingham Heart Study on the social spread of obesity, and use Monte Carlo simulations to test the econometric methods we use to model peer effects. The results show that the existence of peer effects in body mass is robust to several specifications in both adults and in adolescents.

Abstract:
One of the strongest regularities in the empirical political science literature is the well-known correlation in parent and child partisan behavior. Until recently this phenomenon was thought to result solely from parental socialization, but new evidence on genetic sources of behavior suggests it might also be due to heritability. In this article we hypothesize that genes contribute to variation in a general tendency toward strength of partisanship. Using data collected at the Twins Days Festival, we compare the similarity of partisan strength in identical twins who share all of their genes to the similarity of partisan strength in non-identical twins who share only half of their genes. The results show that heritability accounts for almost half of the variance in strength of partisan attachment, and they suggest that we should pay closer attention to the role of biology in the expression of important political behaviors.

Abstract:
Background: The prevalence of obesity has increased substantially over the past 30 years. We quantitatively explored the nature and extent of person-to-person spread of obesity as a possible contributing factor explaining this increase.

Methods: We developed a densely interconnected social network of 12,067 people assessed repeatedly from 1971 to 2003 as part of the Framingham Heart Study. Measured body mass index was available for all subjects. We used longitudinal statistical models to examine whether weight gain in one person was associated with weight gain in friends, siblings, spouses, and neighbors.

Results: Discernible clusters of obese persons were present in the network at all time points, and the clusters extended three people deep. These clusters were not solely due to selective formation of social ties among obese persons. A friend becoming obese in a given time interval increased a person's chances of becoming obese by 57% (95% CI: 6%-123%). Among pairs of adult siblings, one becoming obese increased the chance that the other became obese by 40% (21%-60%). Among spouses, one becoming obese increased the likelihood that the other became obese by 37% (7%-73%). Immediate neighbors did not exhibit these effects. In general, same-gender persons showed relatively greater influence on each other. The spread of smoking cessation did not account for the inter-personal spread of obesity.

Conclusions: Network phenomena appear relevant to the bio-behavioral trait of obesity. Obesity appears to spread across social ties, a finding with implications for clinical and public health interventions.

Abstract:
We analyze the influence of party and preferences on Senate agenda-setting. We find a significant majority party advantage in getting bills reported from committee, but otherwise little variation within parties based on preferences. In addition, our results suggest that Senate committees are more likely to report bills written by committee leaders and senior members, or bills with cosponsors. This suggests that Senate agenda-setters are sensitive to cues that bills are high-quality and relatively easy to pass.

Abstract:
Altruism refers to a willingness to pay a personal cost to make others better off. Past research has established a link between altruism and political participation, primarily among college students. We show that dictator game behavior predicts support for humanitarian norms and donations to Hurricane Katrina victims, suggesting that dictator game allocations are valid measures of altruism. Moreover, we show that this measure of altruism predicts participation in politics, suggesting that past results with students can be generalized to a broader population. Finally, consistent with the argument that altruists only participate when they think doing so will make everyone better off, we show that there is no relationship between altruism and voter turnout in an election where the outcome is distributive and where it is not clear that either political outcome will produce a net societal gain.

Abstract:
Social networks exhibit strikingly systematic patterns across a wide range of human contexts. While genetic variation accounts for a significant portion of the variation in many complex social behaviors, the heritability of egocentric social network attributes is unknown. Here we show that three of these attributes (in-degree, transitivity, and centrality) are heritable. We then develop a "mirror network" method to test extant network models and show that none account for observed genetic variation in human social networks. We propose an alternative "Attract and Introduce" model with two simple forms of heterogeneity that generates significant heritability as well as other important network features. We show that the model is well suited to real social networks in humans. These results suggest that natural selection may have played a role in the evolution of social networks. They also suggest that modeling intrinsic variation in network attributes may be important for understanding the way genes affect human behaviors and the way these behaviors spread from person to person.

Abstract:
We analyze a model of a dynamic political competition between two policy-motivated parties under uncertainty. The model suggests that electoral mandates matter: increasing the margin of victory in the previous election causes both parties to shift towards policies preferred by the winner, and the loser typically shifts more than the winner. The model also provides potential answers to a number of empirical puzzles in the field of electoral politics. In particular, we provide possible explanations for why close elections may lead to extreme platforms by both parties, why increased extremism in the platform of one party may lead to greater moderation in the platform of the other party, and why increasing polarization of the electorate causes winning candidates to become more sensitive to mandates. We also show that, contrary to previous findings, increasing uncertainty sometimes decreases platform divergence. Finally, we pay special attention to the proper methodology for doing numerical comparative statics analysis in computational models.

Abstract:
Acyclic digraphs arise in many natural and artificial processes. Among the broader set, dynamic citation networks represent a substantively important form of acyclic digraphs. For example, the study of such networks includes the spread of ideas through academic citations, the spread of innovation through patent citations, and the development of precedent in common law systems. The specific dynamics that produce such acyclic digraphs not only differentiate them from other classes of graphs, but also provide guidance for meaningful distance measures for these networks. We apply our sink based distance measure and the single-linkage hierarchical clustering algorithm to the first quarter century of decisions of the United States Supreme Court. Despite applying the simplest distance measure and a straight forward clustering algorithm, qualitative analysis reveals that accurate clusterings are produced by this scheme.

citation networks, acyclic digraphs, dynamic network analysis, judicial citations, patent citations, distance measures, community detection, clustering

Abstract:
Objectives To evaluate whether happiness can spread from person to person and whether niches of happiness form within social networks.

Design Longitudinal social network analysis.

Setting Framingham Heart Study social network.

Participants 4739 individuals followed from 1983 to 2003.

Main outcome measures Happiness measured with validated four item scale; broad array of attributes of social networks and diverse social ties.

Results Clusters of happy and unhappy people are visible in the network, and the relationship between people's happiness extends up to three degrees of separation (for example, to the friends of one's friends' friends). People who are surrounded by many happy people and those who are central in the network are more likely to become happy in the future. Longitudinal statistical models suggest that clusters of happiness result from the spread of happiness and not just a tendency for people to associate with similar individuals. A friend who lives within a mile (about 1.6 km) and who becomes happy increases the probability that a person is happy by 25% (95% confidence interval 1% to 57%). Similar effects are seen in coresident spouses (8%, 0.2% to 16%), siblings who live within a mile (14%, 1% to 28%), and next door neighbors (34%, 7% to 70%). Effects are not seen between coworkers. The effect decays with time and with geographical separation.

Conclusions People's happiness depends on the happiness of others with whom they are connected. This provides further justification for seeing happiness, like health, as a collective phenomenon.

Abstract:
Bendor, Diermeier, and Ting (2003) develop a behavioral alternative to rational choice models of turnout. However, the assumption they make about the way individuals adjust their probability of voting biases their model towards their main result of significant turnout in large populations. Moreover, the assumption causes individuals to engage in casual voting (sometimes people vote and sometimes they abstain). This result is at odds with a substantial literature that indicates most people engage in habitual voting (they either always vote or always abstain). I develop an alternative model to show how feedback in the probability adjustment mechanism affects the behavioral model. The version of this model without feedback yields both high turnout and habitual voting.

Abstract:
We conduct experiments in which subjects participate in both a game that measures preferences for income equality and a public goods game involving costly punishment. The results indicate that individuals who care about equality are those who are most willing to punish free-riders in public goods games.

Abstract:
Rational partisan theory's exclusive focus on electoral uncertainty ignores the importance of policy uncertainty for the economy. I develop a theory of policy risk to account for this uncertainty. Using an innovative measure of electoral probabilities based on Iowa Electronic Markets futures data for the U.S. from 1988-2000, I test both theories. As predicted by rational partisan theory, positive changes in the probability that the Left wins the Presidency or the Congress lead to increases in nominal interest rates, implying that expectations of inflation have increased. As predicted by the policy risk theory, positive changes in the electoral probability of incumbent governments and divided governments lead to significant declines in interest rates, implying that expectations of inflation risk have decreased. And as an extension to both theories, I find that electoral margins matter for the economy--partisan and policy risk effects depend not only on which party controls the government, but how large its margin of victory is.

Abstract:
Scholars in many fields have long noted the importance of social context in the development of political ideology. Recent work suggests that political ideology also has a heritable component, but no specific gene variant associated with political ideology has so far been identified. In this article we hypothesize that individuals with a genetic predisposition towards seeking out new experiences will tend to be more liberal, but only if they are embedded in a social context that provides them with multiple points of view. Using data from the National Longitudinal Study of Adolescent Health, we test this hypothesis by investigating an association between self-reported political ideology and the 7R variant of the dopamine receptor D4 gene (DRD4), which has previously been associated with novelty-seeking. We find that the number of friendships a person has in adolescence is significantly associated with liberal political ideology among those with DRD4-7R. Among those without the gene variant there is no association. This is the first study ever to elaborate a specific gene-environment interaction that contributes to ideological self-identification, and it highlights the importance of incorporating both nature and nurture into the study of politics.

Abstract:
Scholars have recently reworked the traditional calculus of voting model by adding a term for benefits to others. Although the probability that a single vote affects the outcome of an election is quite small, the number of people who enjoy the benefit when the preferred alternative wins is large. As a result, people who care about benefits to others and who think one of the alternatives makes others better off are more likely to vote. I test the altruism theory of voting in the laboratory by using allocations in a dictator game to reveal the degree to which each subject is concerned about the well-being of others. The main findings suggest that variation in concern for the well-being of others in conjunction with strength of party identification is a significant factor in individual turnout decisions in real world elections. Partisan altruists are much more likely to vote than their nonpartisan or egoist peers.

Abstract:
A recent model by Panchanathan and Boyd suggests that mutual aid among cooperators can promote large-scale human cooperation without succumbing to a second order free riding problem in which individuals receive aid without giving it. In a companion article, Fehr claims the model "solves [the] second order free riding problem." However, the model does not include second order free riders as one of the possible behavioural types. I present a simplified version of their model to demonstrate how cooperation unravels if second round defectors enter the population.

Abstract:
We develop an agent-based model of dynamic parties with social turnout built upon developments in different fields within social science. This model yields significant turnout, divergent platforms, and numerous results consistent with the rational calculus of voting model and the empirical literature on social turnout. In a simplified version of the model, the authors show how a local imitation structure inherently yields dynamics that encourage positive turnout. The model also generates new hypotheses about the importance of social networks and citizen-party interactions.

Abstract:
We use the network science concept of modularity to measure polarization in the United States Congress. As a measure of the relationship between intra-community and extra-community ties, modularity provides a conceptually-clear measure of polarization that directly reveals both the number of relevant groups and the strength of their divisions. Moreover, unlike measures based on spatial models, modularity does not require predefined assumptions about the number of coalitions or parties, the shape of legislator utilities, or the structure of the party system. Importantly, modularity can be used to measure polarization across all Congresses, including those without a clear party divide, thereby permitting the investigation of partisan polarization across a broader range of historical contexts. Using this novel measure of polarization, we show that party influence on Congressional communities varies widely over time, especially in the Senate. We compare modularity to extant polarization measures, noting that existing methods underestimate polarization in periods in which party structures are weak, leading to artificial exaggerations of the extremeness of the recent rise in polarization. We show that modularity is a significant predictor of future majority party changes in the House and Senate and that turnover is more prevalent at medium levels of modularity. We utilize two individual-level variables, which we call 'divisiveness' and 'solidarity,' from modularity and show that they are significant predictors of reelection success for individual House members, helping to explain why partially-polarized Congresses are less stable. Our results suggest that modularity can serve as an early-warning signal of changing group dynamics, which are reflected only later by changes in formal party labels.

modularity, congress, networks, polarization, parties

Abstract:
Altruistic punishment is a behaviour in which individuals punish others at a cost to themselves in order to provide a public good. Fehr and Gächter1 present experimental evidence suggesting that negative emotions toward non-cooperators motivate punishment which, in turn, facilitates high levels of cooperation in humans. Using Fehr and Gächter's original data, we provide an alternative analysis of the experiment that suggests egalitarian motives are more important than motives to punish non-cooperative behaviour - a finding consistent with evidence that humans may have an evolutionary incentive to punish the highest earners in order to promote equality, not cooperation.

Abstract:
Using large-scale network analysis I map the cosponsorship networks of all 280,000 pieces of legislation proposed in the U.S. House and Senate from 1973 to 2004. In these networks a directional link can be drawn from each cosponsor of a piece of legislation to its sponsor. I use a number of statistics to describe these networks such as the quantity of legislation sponsored and cosponsored by each legislator, the number of legislators cosponsoring each piece of legislation, the total number of legislators who have cosponsored bills written by a given legislator, and network measures of closeness, betweenness, and eigenvector centrality. I then introduce a new measure I call 'connectedness' which uses information about the frequency of cosponsorship and the number of cosponsors on each bill to make inferences about the social distance between legislators. Connectedness predicts which members will pass more amendments on the floor, a measure which is commonly used as a proxy for legislative influence. It also predicts roll call vote choice even after controlling for ideology and partisanship.

Abstract:
Self-citations - those where authors cite their own works - account for a significant portion of all citations. These self-references may result from the cumulative nature of individual research, the need for personal gratification, or the value of self-citation as a rhetorical and tactical tool in the struggle for visibility and scientific authority. In this article we examine the incentives that underlie self-citation by studying how authors' references to their own works affect the citations they receive from others. We report the results of a macro study of more than half a million citations to articles by Norwegian scientists that appeared in the Science Citation Index. We show that the more one cites oneself the more one is cited by other scholars. Controlling for numerous sources of variation in cumulative citations from others, our models suggest that each additional self-citation increases the number of citations from others by about one after one year, and by about three after five years. Moreover, there is no significant penalty for the most frequent self-citers - the effect of self-citation remains positive even for very high rates of self-citation. These results carry important policy implications for the use of citations to evaluate performance and distribute resources in science and they represent new information on the role and impact of self-citations in scientific communication.

Abstract:
A number of scholars have demonstrated that voter turnout is influenced by the costs of processing information and going to the polls, and the policy benefits associated with the outcome of the election. However, no one has yet noted that the costs of voting are paid on and before Election Day while policy benefits may not materialize until several days, months, or even years later. Since the costs of voting must be borne before the benefits are realized, people who are more patient should be more willing to vote. We use a "choice game" from experimental economics to estimate individual discount factors which are used to measure patience. We then show that patience significantly increases voter turnout.

Abstract:
We study the United States Congress in terms of the network between Members of Congress and the legislation that they cosponsor. Using the concept of modularity, we identify the community structure of legislators, as connected via sponsorship/cosponsorship of the same legislation, to investigate the collaborative communities of legislators in both chambers of Congress. This analysis yields an explicit and conceptually clear measure of political polarization, demonstrating a sharp increase in partisan polarization which preceded and then culminated in the 104th Congress (1995-1996), when Republicans took control of both chambers. Although polarization has since waned in the U.S. Senate, it remains at historically high levels in the House of Representatives.

Abstract:
In the spirit of Axelrod's famous prisoners' dilemma tournaments published in the Journal of Conflict Resolution, we conducted a "tournament of party decision rules" in a dynamic agent-based spatial model of party competition. Entrants submitted rules for selecting party positions in a two-dimensional policy space with unknown voter locations. Each submitted rule was pitted against all others in a suite of very long-running simulations. The most successful rule 1) satisficed rather than maximized in the short run; 2) was "parasitic" on choices made by other successful rules; and 3) used a "secret handshake" to avoid attacking other agents using the same rule. In additional simulations, we show that the same rule wins when we alter the population of strategies and the method by which new parties are assigned strategies. The most successful strategies stayed away from the center of the voter distribution and they tended to make only small changes to their positions between elections.

Abstract:
The Southern California Twin Register was initiated in 1984 at the University of Southern California, and continues to grow. This paper provides an update of the register since it was described in the 2002 special issue of this journal. The register has expanded considerably in the past four years, primarily as a result of recent access to Los Angeles County birth records and voter registration databases. Currently, this register contains nearly 5000 twin pairs, the majority of whom are school age. The potential for further expansion in adult twins using voter registration records is also described. Using the Los Angeles County voter registration database, we can identify a large group of individuals with a high probability of having a twin who also resides in Los Angeles County. In addition to describing the expansion of register, this paper provides an overview of an ongoing investigation of 605 twin pairs who are participating in a longitudinal study of behavioral problems during childhood and adolescence. Characteristics of the twins and their families are presented, indicating baseline rates of conduct problems, depression and anxiety disorders, and ADHD diagnoses which are comparable to non-twins in this age range.

Abstract:
In the U.S. House and Senate, each piece of legislation is sponsored by a unique legislator. In addition, legislators can publicly express support for a piece of legislation by cosponsoring it. The network of sponsors and cosponsors provides information about the underlying social networks among legislators. I use a number of statistics to describe the cosponsorship network in order to show that it behaves much differently than other large social networks that have been recently studied. In particular, the cosponsorship network is much denser than other networks and aggregate features of the network appear to be influenced by institutional arrangements and strategic incentives. I also demonstrate that a weighted closeness centrality measure that I call 'connectedness' can be used to identify influential legislators.

Abstract:
I develop a theory of dynamic responsiveness that suggests parties react to past elections in the following way: winning parties choose candidates who are more extreme and losing parties choose candidates who are more moderate. Moreover, the size of past victories matters. Close elections yield little change, but landslides yield larger changes in the candidates offered by both parties. I test this theory by analyzing the relationship between Republican vote share in US Senate elections and the ideology of candidates offered in the subsequent election. The results show that Republican (Democratic) victories in past elections yield candidates who are more (less) conservative in subsequent elections, and the effect is proportional to the margin of victory. This suggests that parties or candidates pay attention to past election returns and change their behavior in a way that privileges winning party candidates who are more extreme and losing party candidates who are more moderate in the next election. One major implication is that parties may remain polarized in spite of their responsiveness to the median voter.

Abstract:
Divorce is the dissolution of a social tie, but it is also possible that attitudes about divorce flow across social ties. To explore how social networks influence divorce and vice versa, we utilize a longitudinal data set from the long-running Framingham Heart Study. We find that divorce can spread between friends, siblings, and coworkers, and there are clusters of divorcees that extend two degrees of separation in the network. We also find that popular people are less likely to get divorced, divorcees have denser social networks, and they are much more likely to remarry other divorcees. Interestingly, we do not find that the presence of children influences the likelihood of divorce, but we do find that each child reduces the susceptibility to being influenced by peers who get divorced. Overall, the results suggest that attending to the health of one’s friends’ marriages serves to support and enhance the durability of one’s own relationship, and that, from a policy perspective, divorce should be understood as a collective phenomenon that extends far beyond those directly affected.