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O altruísmo é um importante constructo comportamental envolvido nas relações sociais; mesmo que aparentemente não haja ganho imediato em ações altruístas, o objetivo de favorecer o progresso do grupo e da sociedade resulta no beneficiamento do próprio agente. Objetivos são mais facilmente alcançados com a ajuda de outros, ao mesmo tempo que há melhora na qualidade das relações sociais. Alguns fatores influenciam o comportamento altruísta, o estresse é um deles; a indução aguda de estresse pode ser eficiente em verificar comportamentos a curto prazo em laboratório, apesar de não haver consenso de que maneira este modifica as intenções de ajuda e o comportamento prósocial. O estresse também pode influenciar a maneira como avaliamos a distribuição de dinheiro ou bens feita por outrem: percepção de justiça. O Jogo do Ditador mostrase eficiente para analisar altruísmo e percepção de justiça, assim como o cortisol salivar se mostra capaz em auxiliar a análise do estresse agudo. Este trabalho avaliou em 94 estudantes universitários, de diversos cursos da Universidade Federal do Rio Grande do Sul, se a indução de estresse agudo via tarefa de estresse social (T rier Social Stress Task ) relacionase a comportamentos altruístas e maior avaliação negativa para divisões injustas de dinheiro em dois jogos do ditador computadorizados a interação em ambos foi com desconhecidos. Os grupos foram divididos em dois sexos e em duas condições: experimental estresse e controle placebo. O dinheiro fictício usado para jogar foi ganho em uma tarefa de planejamento, executada em seguida da indução de estresse. Este foi mensurado por duas medidas fisiológicas: batimentos cardíacos e coleta de saliva para análise de cortisol; e por resposta de autoavaliação de ansiedade; as sessões duraram 60 minutos. Não foram encontradas correlações entre medidas fisiológicas e tomada de decisão egoísta, entretanto, encontrouse diferenças estatísticas significativas comportamentais entre os grupos: mulheres do grupo de controle são mais altruístas, quando dividem montantes mais altos de dinheiro; o grupo experimental avaliou como mais injustas ofertas egoístas (menor parte para ele(a)) do montante mais baixo e também como menos injustas ofertas altruístas (maior parte para ele(a)) do montante mais alto; os grupos também diferiram significativamente na oferta de divisão feita a desconhecidos; ademais, o grupo experimental levou mais tempo para iniciar a tarefa de planejamento do que o controle para ambos sexos. Concluise que medidas fisiológicas não se relacionam aos resultados pósestresse, porém diferenças comportamentais podem ser analisadas. Inferese que mulheres tem comportamentos mais altruístas por influência cultural. Percepção de justiça aumentada, comportamento egoísta e aumento do tempo de preparação na tarefa de planejamento podem ser o resultado do aumento da carga cognitiva por efeito da indução de estresse. / Altruism is an important behavioral construct in social relations; even though it appears to be no immediate gain in selfless actions. The purpose of promoting the group's and society’s progress results in the agent's own benefit. Goals are more easily achieved with the help of others, while there is improvement in the quality of social relations. Some factors have influence in altruistic behavior, stress is one of them; inducing acute stress can be effective in checking behaviors in imediate reaction in the laboratory, although there is no consensus on how this modifies intentions to help others and prosocial behavior. Stress can also influence how we evaluate the distribution of cash or goods made by others: perception of justice. The Dictator Game proves efficient to analize altruism and sense of justice, and salivary cortisol has shown able to assist in the analysis of acute stress. This study evaluated the impact of acute stress induction on 94 university students of different graduation courses of the Federal University of Rio Grande do Sul. Acute stress was induced via social stress task (Trier Social Stress Task) to identify if it relates to altruistic behavior and the negative assessment to unfair money distribution in two computerized dictator games. The subjects in both games were unknown to each other. Subjects were divided by sex and than two groups: experimental stress condition and control placebo condition. The fictitious money used to play was won in a planning task, followed by the stress induction. The impact was measured by two physiological metrics: heart rate and saliva collection for cortisol analysis; and the selfassessment test of anxiety. Each individual sessions lasted 60 minutes. Correlations between physiological measurements and the making of selfish decision were not found. However, significant statistical behaviors differences were found between: the control group of women who were more altruistic when dividing larger amounts of money; t he experimental group evaluated selfish money offers as more unfair in the lower cash amount experiment (lower share for him/her). We also found as less unfair the altruistic offers made on the experiment’s larger cash amount (highest share for him/her). The groups also differ significantly in the division of money made to unknown individuals; in addition, the experimental group took longer to start the planning task than the control group for both sexes. We conclude that physiological measures are not related to poststress results, but behavioral differences can be analyzed. It appears that women tend to be more altruistic because of cultural influence. Increased perception of justice, selfish behavior and longer time taking to plan the planning task can be the result of increased cognitive overload as a result of induction of stress.

The primary objectives of the current research were to (1) test the effectiveness of conspicuous consumption as a status-enhancement tactic and (2) examine access to material resources as an interpersonal benefit that incentivizes status striving behavior. The studies that follow investigated the status striving motivations of both men and women; however, this research endeavor was primarily designed to address the paucity of research on female status. In Study 1, a nation-wide sample of participants perceived target women to be higher status when they were depicted conspicuously consuming than when not. Several individual difference variables that predict conspicuous consumption were also identified, many of which related to the attainment of high status. In Studies 2 and 3, conspicuous consumption was shown to increase perceptions of status in face-to-face interactions, further supporting the status signaling function of conspicuous consumption. Study 3 utilized a Dictator Game methodology to test the prediction that participants would share more of a monetary allotment with confederates who were conspicuously consuming than with those who were not. Results indicated that conspicuous consumption did not increase generosity except in male participants who shared more of a monetary allotment with conspicuous consumers, particularly those of the same sex. This sex-specific result is discussed in light of the possibility that conspicuous consumption signals a type of status that is particularly relevant to men (i.e., economic status). In conclusion, I consider the different pathways by which high status individuals receive increased access to resources. / text

O altruísmo é um importante constructo comportamental envolvido nas relações sociais; mesmo que aparentemente não haja ganho imediato em ações altruístas, o objetivo de favorecer o progresso do grupo e da sociedade resulta no beneficiamento do próprio agente. Objetivos são mais facilmente alcançados com a ajuda de outros, ao mesmo tempo que há melhora na qualidade das relações sociais. Alguns fatores influenciam o comportamento altruísta, o estresse é um deles; a indução aguda de estresse pode ser eficiente em verificar comportamentos a curto prazo em laboratório, apesar de não haver consenso de que maneira este modifica as intenções de ajuda e o comportamento prósocial. O estresse também pode influenciar a maneira como avaliamos a distribuição de dinheiro ou bens feita por outrem: percepção de justiça. O Jogo do Ditador mostrase eficiente para analisar altruísmo e percepção de justiça, assim como o cortisol salivar se mostra capaz em auxiliar a análise do estresse agudo. Este trabalho avaliou em 94 estudantes universitários, de diversos cursos da Universidade Federal do Rio Grande do Sul, se a indução de estresse agudo via tarefa de estresse social (T rier Social Stress Task ) relacionase a comportamentos altruístas e maior avaliação negativa para divisões injustas de dinheiro em dois jogos do ditador computadorizados a interação em ambos foi com desconhecidos. Os grupos foram divididos em dois sexos e em duas condições: experimental estresse e controle placebo. O dinheiro fictício usado para jogar foi ganho em uma tarefa de planejamento, executada em seguida da indução de estresse. Este foi mensurado por duas medidas fisiológicas: batimentos cardíacos e coleta de saliva para análise de cortisol; e por resposta de autoavaliação de ansiedade; as sessões duraram 60 minutos. Não foram encontradas correlações entre medidas fisiológicas e tomada de decisão egoísta, entretanto, encontrouse diferenças estatísticas significativas comportamentais entre os grupos: mulheres do grupo de controle são mais altruístas, quando dividem montantes mais altos de dinheiro; o grupo experimental avaliou como mais injustas ofertas egoístas (menor parte para ele(a)) do montante mais baixo e também como menos injustas ofertas altruístas (maior parte para ele(a)) do montante mais alto; os grupos também diferiram significativamente na oferta de divisão feita a desconhecidos; ademais, o grupo experimental levou mais tempo para iniciar a tarefa de planejamento do que o controle para ambos sexos. Concluise que medidas fisiológicas não se relacionam aos resultados pósestresse, porém diferenças comportamentais podem ser analisadas. Inferese que mulheres tem comportamentos mais altruístas por influência cultural. Percepção de justiça aumentada, comportamento egoísta e aumento do tempo de preparação na tarefa de planejamento podem ser o resultado do aumento da carga cognitiva por efeito da indução de estresse. / Altruism is an important behavioral construct in social relations; even though it appears to be no immediate gain in selfless actions. The purpose of promoting the group's and society’s progress results in the agent's own benefit. Goals are more easily achieved with the help of others, while there is improvement in the quality of social relations. Some factors have influence in altruistic behavior, stress is one of them; inducing acute stress can be effective in checking behaviors in imediate reaction in the laboratory, although there is no consensus on how this modifies intentions to help others and prosocial behavior. Stress can also influence how we evaluate the distribution of cash or goods made by others: perception of justice. The Dictator Game proves efficient to analize altruism and sense of justice, and salivary cortisol has shown able to assist in the analysis of acute stress. This study evaluated the impact of acute stress induction on 94 university students of different graduation courses of the Federal University of Rio Grande do Sul. Acute stress was induced via social stress task (Trier Social Stress Task) to identify if it relates to altruistic behavior and the negative assessment to unfair money distribution in two computerized dictator games. The subjects in both games were unknown to each other. Subjects were divided by sex and than two groups: experimental stress condition and control placebo condition. The fictitious money used to play was won in a planning task, followed by the stress induction. The impact was measured by two physiological metrics: heart rate and saliva collection for cortisol analysis; and the selfassessment test of anxiety. Each individual sessions lasted 60 minutes. Correlations between physiological measurements and the making of selfish decision were not found. However, significant statistical behaviors differences were found between: the control group of women who were more altruistic when dividing larger amounts of money; t he experimental group evaluated selfish money offers as more unfair in the lower cash amount experiment (lower share for him/her). We also found as less unfair the altruistic offers made on the experiment’s larger cash amount (highest share for him/her). The groups also differ significantly in the division of money made to unknown individuals; in addition, the experimental group took longer to start the planning task than the control group for both sexes. We conclude that physiological measures are not related to poststress results, but behavioral differences can be analyzed. It appears that women tend to be more altruistic because of cultural influence. Increased perception of justice, selfish behavior and longer time taking to plan the planning task can be the result of increased cognitive overload as a result of induction of stress.

O altruísmo é um importante constructo comportamental envolvido nas relações sociais; mesmo que aparentemente não haja ganho imediato em ações altruístas, o objetivo de favorecer o progresso do grupo e da sociedade resulta no beneficiamento do próprio agente. Objetivos são mais facilmente alcançados com a ajuda de outros, ao mesmo tempo que há melhora na qualidade das relações sociais. Alguns fatores influenciam o comportamento altruísta, o estresse é um deles; a indução aguda de estresse pode ser eficiente em verificar comportamentos a curto prazo em laboratório, apesar de não haver consenso de que maneira este modifica as intenções de ajuda e o comportamento prósocial. O estresse também pode influenciar a maneira como avaliamos a distribuição de dinheiro ou bens feita por outrem: percepção de justiça. O Jogo do Ditador mostrase eficiente para analisar altruísmo e percepção de justiça, assim como o cortisol salivar se mostra capaz em auxiliar a análise do estresse agudo. Este trabalho avaliou em 94 estudantes universitários, de diversos cursos da Universidade Federal do Rio Grande do Sul, se a indução de estresse agudo via tarefa de estresse social (T rier Social Stress Task ) relacionase a comportamentos altruístas e maior avaliação negativa para divisões injustas de dinheiro em dois jogos do ditador computadorizados a interação em ambos foi com desconhecidos. Os grupos foram divididos em dois sexos e em duas condições: experimental estresse e controle placebo. O dinheiro fictício usado para jogar foi ganho em uma tarefa de planejamento, executada em seguida da indução de estresse. Este foi mensurado por duas medidas fisiológicas: batimentos cardíacos e coleta de saliva para análise de cortisol; e por resposta de autoavaliação de ansiedade; as sessões duraram 60 minutos. Não foram encontradas correlações entre medidas fisiológicas e tomada de decisão egoísta, entretanto, encontrouse diferenças estatísticas significativas comportamentais entre os grupos: mulheres do grupo de controle são mais altruístas, quando dividem montantes mais altos de dinheiro; o grupo experimental avaliou como mais injustas ofertas egoístas (menor parte para ele(a)) do montante mais baixo e também como menos injustas ofertas altruístas (maior parte para ele(a)) do montante mais alto; os grupos também diferiram significativamente na oferta de divisão feita a desconhecidos; ademais, o grupo experimental levou mais tempo para iniciar a tarefa de planejamento do que o controle para ambos sexos. Concluise que medidas fisiológicas não se relacionam aos resultados pósestresse, porém diferenças comportamentais podem ser analisadas. Inferese que mulheres tem comportamentos mais altruístas por influência cultural. Percepção de justiça aumentada, comportamento egoísta e aumento do tempo de preparação na tarefa de planejamento podem ser o resultado do aumento da carga cognitiva por efeito da indução de estresse. / Altruism is an important behavioral construct in social relations; even though it appears to be no immediate gain in selfless actions. The purpose of promoting the group's and society’s progress results in the agent's own benefit. Goals are more easily achieved with the help of others, while there is improvement in the quality of social relations. Some factors have influence in altruistic behavior, stress is one of them; inducing acute stress can be effective in checking behaviors in imediate reaction in the laboratory, although there is no consensus on how this modifies intentions to help others and prosocial behavior. Stress can also influence how we evaluate the distribution of cash or goods made by others: perception of justice. The Dictator Game proves efficient to analize altruism and sense of justice, and salivary cortisol has shown able to assist in the analysis of acute stress. This study evaluated the impact of acute stress induction on 94 university students of different graduation courses of the Federal University of Rio Grande do Sul. Acute stress was induced via social stress task (Trier Social Stress Task) to identify if it relates to altruistic behavior and the negative assessment to unfair money distribution in two computerized dictator games. The subjects in both games were unknown to each other. Subjects were divided by sex and than two groups: experimental stress condition and control placebo condition. The fictitious money used to play was won in a planning task, followed by the stress induction. The impact was measured by two physiological metrics: heart rate and saliva collection for cortisol analysis; and the selfassessment test of anxiety. Each individual sessions lasted 60 minutes. Correlations between physiological measurements and the making of selfish decision were not found. However, significant statistical behaviors differences were found between: the control group of women who were more altruistic when dividing larger amounts of money; t he experimental group evaluated selfish money offers as more unfair in the lower cash amount experiment (lower share for him/her). We also found as less unfair the altruistic offers made on the experiment’s larger cash amount (highest share for him/her). The groups also differ significantly in the division of money made to unknown individuals; in addition, the experimental group took longer to start the planning task than the control group for both sexes. We conclude that physiological measures are not related to poststress results, but behavioral differences can be analyzed. It appears that women tend to be more altruistic because of cultural influence. Increased perception of justice, selfish behavior and longer time taking to plan the planning task can be the result of increased cognitive overload as a result of induction of stress.

This thesis deals with the differences in the degree of altruism between men and women, with varying degrees of information. In three rounds of the dictator game experiment, the dictators decided how to redistribute the money between themselves and the recipient. The differences in altruism among the sexes were already dealt with, for example in the works by Andreoni, Vesterlund (2001), Dufwenberg, Muren (2004) or Eckel, Grossman (1998), whose experiments served to inspire the creation of the experimentation in this work. However, the differences in behaviour of men and women in all the observed rounds proved to be statictically insignificant, in this work. The results of this work do not support the hypothesis of a higher degree of altruism in the situation when the dictator has the information about the sex of the recipient, compared to the situation where he does not have it. Male dictators, as well as female ones, behaved equally altruistically to female and male and anonymous recipients.

View of rational choice in coherence with ultimatum game. Rational choice theory adjusted by joining behavioral economics to better comprehend decision-making processes. In this thesis I focus on researching strong influences using multiple simple games. Namely: ultimatum game, dictator game and modifications of previous. They will be carried out by online questionnaire. Next step will be analysis of components to determine significant ones and impact of game's modifications. I assume that decision-maker's rationality will be disproven. This thesis will continue with assessing influence of risk and fear of loss. Goal is to gather enough of detailed data about motivations and participants in order to perform analysis and establish statistically significant influences.

This dissertation aims to contribute to the literature of cooperation and social preferences. We use experimental and computational methods to understand the role and extent of reciprocity on cooperation. The first paper is a methodological contribution to the large literature on conditional preferences of cooperation. Cooperation generated by this type of preferences is notoriously unstable, as individuals reduce their contributions to the public good in reaction to other subjects' free-riding. This has led to the widely-shared conclusion that cooperation observed in experiments (and its collapse) is mostly driven by imperfect reciprocity. In this study, we explore the possibility that reciprocally cooperative preferences may themselves be unstable. We do so by observing the evolution of subjects' preferences in an anonymously repeated social dilemma. Our unsettling result is that, in the course of the experiment, a significant fraction of reciprocally cooperative subjects become egoistic, while the reverse is rarely observed. The non-selfish preferences that appear to be more stable are those most easily attributed to confusion. We are thus driven to the conclusion that egoism is more resistant to exposure to social dilemmas than reciprocity. The second paper the evolutionary success of conditional preferences by using simulations. We use an agent-based model in which agents play a variation of the iterated Prisoner's Dilemma game. We estimate the likelihood of cooperation levels as well as the likelihood of the existence of conditional types for different continuation probabilities. We show that an all-or-none type of conditional cooperation strategy together with the perfect conditional cooperation strategy are most likely to emerge when the continuation probability is sufficiently high. Our most surprising finding is related to the so-called hump-shaped strategy, a conditional type that is commonly observed in experiments. Our simulations show that those types are likely to thrive for intermediate levels of the continuation probability due to their relative advantage when probability of interaction is not enough to sustain a full-cooperation, but instead merely sustains mid-level cooperation. The third paper aims to understand the underlying reciprocal motives in altruistic behavior. We argue that the altruism that is revealed in dictator games can be explained by what we call presumptive reciprocity. Subjects may display non-selfish preferences because they presume that the other subjects would have revealed similar, non-selfish preferences if the roles had been reversed. This kind of intuitive reasoning, although partially captured by indirect reciprocity, is overlooked in the literature on social preferences, especially when it comes to explaining the behavior that appears to be purely altruistic. The experimental evidence we provide shows that people's choices reveal mostly presumptive reciprocity, while purely altruistic preferences play a much smaller role.

1. Introduction This dissertation consists of six chapters that span a very diverse set of topics. Yet, it has two unifying themes, economics and biology, that tie it together. The first four chapters present the principal findings from a project that was initiated jointly with David Cesarini and Magnus Johannesson, and that applies the twin method from behavioral genetics to economics. The last two chapters instead use a simple regression framework and evidence from biological anthropology to investigate recent claims regarding the effects of child bearing and past slave trades. 2. Genes and economics There is a small, but rapidly growing, literature studying the genetic and environmental origins of economic behavior and outcomes (Bowles et al., 2005; Beauchamp et al., 2011). Until recently, this literature focused exclusively on outcomes, and in particular income. In chapters 1-4 we instead focus on economic behavior and decision-making. Previous behavioral genetic work outside the domains of economics has changed the way that we think about a number of behavioral traits. In this literature it is typically found that i) variation is heritable ii) genetic factors are more important than family environment iii) a large fraction of variation cannot be explained by neither genes nor family environment (Turkheimer, 2000; Plomin et al., 2009). However, compared to many other disciplines, and psychology in particular, economics is lagging behind. In fact, as recently as 2009 the leading text book in behavioral genetics described economics as "still essentially untouched by genetic research" (Plomin et al., 2009, p. 353). Hopefully, the chapters in this dissertation can help to improve on this somewhat unsatisfactory state of the art. Chapters 1 and 2 study economic decision-making in the laboratory using the twin method. More specifically, we study the ultimatum and dictator games alongside risky gambles, using same-sex twin pairs as our subject pool. Given a few additional assumptions, the fact that identical twins have, in expectation, a twice as high coefficient of genetic relatedness as fraternal twins implies that we can study the genetic and environmental contributions to variation in behavior by studying twin correlations in observed choices. Chapters 3 and 4 apply the same method to actual portfolio choices associated with a far-reaching pension reform, as well as to a set of standard behavioral anomalies. Taken together, these four chapters provide strong evidence in favor of the hypothesis that genes influence economic decision-making. Thus, economic behavior does not appear to be much different from other types of behavior. 3. Economics and history The last two chapters of the dissertation turn to the past, rather than genes, in an effort to evaluate recent findings regarding two important welfare outcomes. In chapter 5 we investigate Nunn’s (2008) claim that past slave trades had a negative impact on current economic performance in Africa. By extending the sample period back in time we demonstrate that this relationship was not significant in 1960. In addition, by applying Nunn’s method to an episode of large scale slave raiding in Italy, we demonstrate that there exists a similar negative relationship across Italian regions, although it becomes insignificant when geographical controls are included. Intriguingly, going back to 1960, the coefficient on slave raids for Italy also has a similar time trend to that for Africa. Taking these facts, and our reading of the historical and anthropological literature, which is much different from that of Nunn, into account we do not find much support for the hypothesis that the African slave trades had a negative impact on current economic performance. Finally, chapter 6 investigates the large and negative relationship between giving birth to a son, rather than a daughter, and maternal longevity that was documented in a Sami hunter-gatherer population from Finland (Helle et al., 2002). Using a substantially larger sample of pre-industrial Swedish Sami we find no evidence in favor of such a relationship. 4. Brasklapp Five of the chapters in this dissertation (Ch. 1-4 &amp; 6) are slightly altered versions of previously published papers (Wallace et al., 2007; Cesarini et al., 2009 a, b; 2010; 2011). Unfortunately, the fact that earlier versions of the chapters were prepared as separate articles for five different journals means that they can at times appear both repetitive, and in terms of notation and formatting, somewhat inconsistent. I apologize to the reader for these inconveniences. / <p>Diss. Stockholm :  Stockholm School of Economics, 2011. Introduction together with 6 papers</p>

This thesis consists of four independent papers. The first two papers use experimental methods to study pro-social behaviors. The other two use theoretical methods to investigate questions about belief formation. The first paper “Communication: Content or Relationship?” investigates the effect on communication on generosity in a dictator game. In the basic experiment (the control), subjects in one room are dictators and subjects in another room are recipients. The subjects are anonymous to each other throughout the whole experiment. Each dictator gets to allocate a sum of 100 SEK between herself and an unknown recipient in the other room. In the first treatment we allow each recipient to send a free-form message to his dictator counterpart, before the dictator makes her allocation decision. In order to separate the effect of the content of the communication, from the relationship-building effect of communication, we carry out a third treatment, where we take the messages from the previous treatment and give each of them to a dictator in this new treatment. The dictators are informed that the recipients who wrote the messages are not the recipients they will have the opportunity to send money to. We find that this still increases donation compared to the baseline but not as much as in the other treatment. This suggests that both the impersonal content of the communication and the relationship effect matters for donations. The second paper, “Limbic justice – Amygdala Drives Rejection in the Ultimatum Game”, is about the neurological basis for the tendency to punish norm violators in the Ultimatum Game. In the Ultimatum Game, a proposer proposes a way to divide a fixed sum of money. The responder accepts or rejects the proposal. If the proposal is accepted the proposed split is realized and if the proposal is rejected both subjects gets zero. Subjects were randomly allocated to receive either the benzodiazepine oxazepam or a placebo substance, and then played the Ultimatum Game in the responder role, while lying in and fMRI camera. Rejection rate is significantly lower in the treatment group than in the control group. Moreover a mygdala was relatively more activated in the placebo group than in the oxazepam group for unfair offers. This is mirrored by differences in activation in the medial prefrontal cortex (mPFC) and right ACC. Our findings suggest that the automatic and emotional response to unfairness, or norm violations, are driven by amygdala and that balancing of such automatic behavioral responses is associated with parts of the prefrontal cortex. The conflict of motives is monitored by the ACC. In order to decide what strategy to choose, a player needs to form beliefs about what other players will do. This requires the player to have a model of how other people form beliefs – what psychologists call a theory of mind. In the third paper “Evolution of Theories of Mind” I study the evolution of players’ models of how other players think. When people play a game for the first time, their behavior is often well predicted by the level-k, and related models. According to this model, people think in a limited number of steps, when they form beliefs about other peoples' behavior. Moreover, people differ with respect to how they form beliefs. The heterogeneity is represented by a set of cognitive types {0,1,2,...}, such that type 0 randomizes uniformly and type k&gt;0 plays a k times iterated best response to this. Empirically one finds that most experimental subjects behave as if they are of type 1 or 2, and individuals of type 3 and above are very rare. When people play the same game more than once, they may use their experience to predict how others will behave. Fictitious play is a prominent model of learning, according to which all individuals believe that the future will be like the past, and best respond to the average of past play. I define a model of heterogeneous fictitious play, according to which there is a hierarchy of types {1,2,...}, such that type k plays a k time iterated best response to the average of past play. The level-k and fictitious play models, implicitly assume that players lack specific information about the cognitive types of their opponents. I extend these models to allow for the possibility that types are partially observed. I study evolution of types in a number of games separately. In contrast to most of the literature on evolution and learning, I also study the evolution of types across different games. I show that an evolutionary process, based on payoffs earned in different games, both with and without partial observability, can lead to a polymorphic population where relatively unsophisticated types survive, often resulting in initial behavior that does not correspond to a Nash equilibrium. Two important mechanisms behind these results are the following: (i) There are games, such as the Hawk-Dove game, where there is an advantage of not thinking and behaving like others, since choosing the same action as the opponent yields an inefficient outcome. This mechanism is at work even if types are not observed. (ii) If types are partially observed then there are Social dilemmas where lower types may have a commitment advantage; lower types may be able to commit to strategies that result in more efficient payoffs. The importance of categorical reasoning in human cognition is well-established in psychology and cognitive science, and one of the most important functions of categorization is to facilitate prediction. Prediction on the basis of categorical reasoning is relevant when one has to predict the value of a variable on the basis of one's previous experience with similar situations, but where the past experience does not include any situation that was identical to the present situation in all relevant aspects. In such situations one can classify the situation as belonging to some category, and use the past experiences in that category to make a prediction about the current situation. In the fourth paper, “Optimal Categorization”, I provide a model of categorizations that are optimal in the sense that they minimize prediction error. From an evolutionary perspective we would expect humans to have developed categories that generate predictions which induce behavior that maximize fitness, and it seems reasonable to assume that fitness is generally increasing in how accurate the predictions are. In the model a subject starts out with a categorization that she has learnt or inherited early in life. The categorization divides the space of objects into categories. In the beginning of each period, the subject observes a two-dimensional object in one dimension, and wants to predict the object’s value in the other dimension. She has a data base of objects that were observed in both dimensions in the past. The subject determines what category the new object belongs to on the basis of observation of its first dimension. She predicts that its value in the second dimension will be equal to the average value among the past observations in the corresponding category. At the end of each period the second dimension is observed, and the observation is stored in the data base. The main result is that the optimal number of categories is determined by a trade-off between (a) decreasing the size of categories in order to enhance category homogeneity, and (b) increasing the size of categories in order to enhance category sample size. In other words, the advantage of fine grained categorizations is that objects in a category are similar to each other. The advantage of coarse categorizations is that a prediction about a category is based on a large number of observations, thereby reducing the risk of over-fitting. Comparative statics reveal how the optimal categorization depends on the number of observations as well as on the frequency of objects with different properties. The set-up does not presume the existence of an objectively true categorization “out there”. The optimal categorization is a framework we impose on our environment in order to predict it. / <p>Diss. Stockholm : Handelshögskolan, 2010. Sammanfattning jämte 4 uppsatser.</p>