Sijilmassi et al. argue that myths around shared ancestry and history exploit an evolved psychology of interdependence. In contrast, we argue that psychological exploitation is not required. Rather, such myths may be one method, among many, to create a shared understanding of group boundaries, which can be "self-enforcing." We summarize the game-theoretic basis for this account and some supportive evidence.

Almaatouq et al.'s prescription for more integrative experimental designs is welcome but does not address an equally important problem: Lack of adequate theories. We highlight two features theories ought to satisfy: "Well-specified" and "grounded." We discuss the importance of these features, some positive exemplars, and the complementarity between the target article's prescriptions and improved theorizing.

Recent theoretical work shows that the better-than-average effect, where a majority believes their ability to be better than average, can be perfectly consistent with Bayesian updating. However, later experiments that account for this theoretical advance still find behavior consistent with overconfidence. The literature notes that overoptimism can be caused by either overconfidence (optimism about performance), wishful thinking (optimism about outcomes), or both. To test whether the better-than-average effect might be explained by wishful thinking instead of overconfidence, we conduct an experiment that is similar to those used in the overconfidence literature, but removes performance as a potential channel. We find evidence that wishful thinking might explain overconfidence only among the most optimistic subjects and that conservatism is possibly more of a worry; if unaccounted for, overconfidence might be underestimated.

The last three centuries have witnessed a moral and political transformation. Groups previously denied equivalent moral standing—including propertyless men, women, ethnic and religious minorities, homosexuals, and slaves—became moral equals deserving of similar legal treatment. Here we argue that this process was driven by the reputational benefits of demonstrating commitment to individualist moral principles. These principles flourished in “fluid” social ecologies with high relational mobility and weak kinship institutions, both as individuals aimed to signal impersonal prosociality and as they strove to be governed under institutions that protected substitutable individuals unbound by formal obligations. As long as parties benefited from appearing committed to these principles, and denying rights to certain groups appeared inconsistent with these principles, then those parties were incentivized to grant those rights. Given the universalist nature of these principles, people signaling commitment were also incentivized to sanction rights-based violations in other countries, helping expand rights beyond their original context. We use this account to explain both expansions and contractions of the moral circle and reconcile the roles of ideas, markets, reasoning, reputation, the Catholic Church, argumentation, moral intuition, social organization, individual strategizing, and large-scale cultural evolution.

Abstract Why are norms unduly sensitive to categorical distinctions compared to continuous variation? For instance, the norm against the use of chemical weapons considers the type of weapon used, not how much suffering was caused; human rights are conditioned on membership in the species homo spaiens not on an animal's degree of sentience; norms promoting philanthropy emphasize donating, and are relatively insensitive to the impact a donation will have. Here we present a game theoretic model, which explains why it is easier for norms to be conditioned on categorical distinctions rather than on continuous variation. We explore the robustness of our results and present evolutionary simulations. Then, in a series of experiments, we demonstrate that participants’ moral intuitions and willingness to sanction norm violations are influenced by categorical distinctions rather than by continuous variation. We show that the reliance on categorical distinctions weakens when norm enforcement plays less of a role. We discuss various applications, including territoriality, human rights, inefficient altruism, institutionalized racism, and collusion.

We conduct an intercultural experiment in three locations on three different continents to elicit competitiveness and study whether individual differences in competitiveness are related to handedness. Being a "lefty" (i.e., having either a dominant left hand or a dominant left foot) is associated with neurological differences which are determined prenatally, and can therefore be seen as a proxy for innate differences. In large-scale data with incentivized choices from 3664 participants from India, Norway and Tanzania, we find a significant gender gap in competitiveness in all cultures. However, we find inconsistent results when comparing the competitiveness of lefties and righties. In north-east India we find that lefties of both genders are significantly more competitive than righties. In Norway we find that lefty men are more competitive than any other group, but women's competitiveness is not related to handedness. In Tanzania, we find no relationship between handedness and the competitiveness of either gender. The merged data show weak evidence of a positive correlation between being a lefty and competitiveness for men, but no such evidence for women. Thus, our data provide suggestive but not robust evidence that individual and gender differences in competitiveness are partially determined by innate factors, where innate factors are proxied by the complex, prenatally shaped trait of handedness.

Principled behavior seems to defy evolutionary logic. Principled people consistently abide by their principles, ignore tradeoffs or compromises, and pursue the principles for transcendental reasons, such as that they are “right”, decreed by God, or part of an eternal debt to the emperor. Here, we explain principled behavior as a combination of what we call “committed agents” and “impersonators”. Committed agents are individuals whose extreme psychology compels them to never deviate from a maxim and who are especially trustworthy for it. Imitators non-consciously masquerade as committed agents to garner trust. Given that observers can only determine whether a person is genuinely committed on the basis of their behavior, impersonators must appear to never deviate from the maxim, never think about deviating, pursue the maxim for the reason motivating committed agents, and justify ambiguous or compromising decision as conforming to the principle. We use this account to explain key features of principled behavior as well as seemingly unrelated phenomena, including cognitive dissonance, foot-in-the-door effects, moral licensing, sacred values, the expanding moral circle, and beliefs in supernatural punishment. Principled behavior consists of the behavior of rare extreme individuals and strategic attempts by others to pass as them.

We conduct a large-scale intercultural experiment to elicit competitiveness and ask whether individual and gender differences in competitiveness are partially determined by nature. We use being a “lefty” (i.e., having either a dominant left hand or a dominant left foot) as a proxy for nature, as it is associated with neurological differences which are determined prenatally and reflects a masculinized neurology. That way we use handedness and footedness as a proxy for innate differences. In large-scale data with incentivized choices from 3683 participants from India, Norway and Tanzania, we find a significant gender gap in competitiveness in all cultures. However, we find inconsistent results when comparing the competitiveness of lefties and righties. In northeast India we find that lefties of both genders are significantly more competitive than righties. In Norway we find that lefty men are more competitive than any other group, but women’s competitiveness is not related to handedness or footedness. In Tanzania, we find no effect of handedness or footedness on the competitiveness of either gender. The merged data show weak evidence of a positive correlation between being a lefty and competitiveness for men, but no such evidence for women. Thus, our data do not provide robust evidence that gender differences in competitiveness are partially determined by nature, where nature is represented by the complex, physiologically-rooted phenomenon of handedness.

We donate billions to charities each year, yet much of our giving is ineffective. Why are we motivated to give but not to give effectively? Building on evolutionary game theory, we argue that donors evolved (genetically or culturally) to be insensitive to efficacy because people tend not to reward efficacy, as social rewards tend to depend on well-defined and highly observable behaviours. We present five experiments testing key predictions of this account that are difficult to reconcile with alternative accounts based on cognitive or emotional limitations. Namely, we show that donors are more sensitive to efficacy when helping (1) themselves or (2) their families. Moreover, (3) social rewarders don’t condition on efficacy or other difficult-to-observe behaviours (4, 5), such as the amount donated.

When coordination is required, we often attend closely to categorical distinctions while overlooking continuous variation. For instance, the norm against chemical weapons is based on the type of weapon used and is applied irrespective of the number of civilians killed. Building off the game theory literature on Global Games, and using a standard Nash Equilibrium analysis, we show that it is harder to coordinate on the basis of a continuous variable than a discrete variable whenever the variable is observed with idiosyncratic noise. We demonstrate our result using several stylized signal structures, and also present a general theorem specifying which signal structures permit coordination on the basis of one's signal. We also present dynamic models that do not rely on any rationality assumptions. We end by mapping the model to possible applications, including territoriality, human rights, inefficient altruism, institutionalized racism, territorial disputes, revolutions, and collusion.

People sometimes make their admirable deeds and accomplishments hard to spot, such as by giving anonymously or avoiding bragging. Such 'buried' signals are hard to reconcile with standard models of signalling or indirect reciprocity, which motivate costly pro-social behaviour by reputational gains. To explain these phenomena, we design a simple game theory model, which we call the signal-burying game. This game has the feature that senders can bury their signal by deliberately reducing the probability of the signal being observed. If the signal is observed, however, it is identified as having been buried. We show under which conditions buried signals can be maintained, using static equilibrium concepts and calculations of the evolutionary dynamics. We apply our analysis to shed light on a number of otherwise puzzling social phenomena, including modesty, anonymous donations, subtlety in art and fashion, and overeagerness.

Why do we think it's wrong to treat people merely as a means to end? Why do we consider lies of omission less immoral than lies of commission? Why do we consider it good to give, regardless of whether the gift is effective? We use four simple game theoretic models—the Coordination Game, the Hawk–Dove game, Repeated Prisoner's Dilemma, and the Envelope Game—to shed light on these and other puzzling aspects of human morality. We also justify the use of game theory for the study of morality and explore implications for group selection and moral realism.

Humans frequently cooperate without carefully weighing the costs and benefits. As a result, people may wind up cooperating when it is not worthwhile to do so. Why risk making costly mistakes? Here, we present experimental evidence that reputation concerns provide an answer: people cooperate in an uncalculating way to signal their trustworthiness to observers. We present two economic game experiments in which uncalculating versus calculating decision-making is operationalized by either a subject's choice of whether to reveal the precise costs of cooperating (Exp. 1) or the time a subject spends considering these costs (Exp. 2). In both experiments, we find that participants are more likely to engage in uncalculating cooperation when their decision-making process is observable to others. Furthermore, we confirm that people who engage in uncalculating cooperation are perceived as, and actually are, more trustworthy than people who cooperate in a calculating way. Taken together, these data provide the first empirical evidence, to our knowledge, that uncalculating cooperation is used to signal trustworthiness, and is not merely an efficient decision-making strategy that reduces cognitive costs. Our results thus help to explain a range of puzzling behaviors, such as extreme altruism, the use of ethical principles, and romantic love.

Humans frequently cooperate without carefully weighing the costs and benefits. As a result, people may wind up cooperating when it is not worthwhile to do so. Why risk making costly mistakes? Here, we provide the first experimental demonstration that reputation concerns provide an answer: people cooperate in an uncalculating way in order to signal their trustworthiness to observers. We present two economic game experiments in which uncalculating versus calculating decision-making is operationalized by (i) a subject’s choice of whether to reveal the precise costs of cooperating (Experiment 1) and (ii) the time a subject spends considering these costs (Experiment 2). In both, we find that subjects are more likely to engage in uncalculating cooperation when their decision-making process is observable to others. Furthermore, we confirm that people who engage in uncalculating cooperation are perceived as, and actually are, more trustworthy than people who cooperate in a calculating way. Together, these data provide the first evidence that uncalculating cooperation is used to signal trustworthiness, and is not merely an efficient decision-making strategy that reduces cognitive costs. Our results thus help to explain a range of puzzling behaviors such as extreme altruism, the use of ethical principles, and romantic love.

This paper reviews and addresses a variety of issues relating to inclusive fitness. The main question is: are there limits to the generality of inclusive fitness, and if so, what are the perimeters of the domain within which inclusive fitness works? This question is addressed using two well-known tools from evolutionary theory: the replicator dynamics, and adaptive dynamics. Both are combined with population structure. How generally Hamilton's rule applies depends on how costs and benefits are defined. We therefore consider costs and benefits following from Karlin and Matessi's (1983) "counterfactual method", and costs and benefits as defined by the "regression method" (Gardner et al., 2011). With the latter definition of costs and benefits, Hamilton's rule always indicates the direction of selection correctly, and with the former it does not. How these two definitions can meaningfully be interpreted is also discussed. We also consider cases where the qualitative claim that relatedness fosters cooperation holds, even if Hamilton's rule as a quantitative prediction does not.We furthermore find out what the relation is between Hamilton's rule and Fisher's Fundamental Theorem of Natural Selection. We also consider cancellation effects - which is the most important deepening of our understanding of when altruism is selected for. Finally we also explore the remarkable (im)possibilities for empirical testing with either definition of costs and benefits in Hamilton's rule.

In human societies, individuals who violate social norms may be punished by third-party observers who have not been harmed by the violator; this study suggests that a reason why the observers are willing to punish is to be seen as more trustworthy by the community.

Game theory describes social behaviors in humans and other biological organisms. By far, the most powerful tool available to game theorists is the concept of a Nash Equilibrium (NE), which is motivated by perfect rationality. NE specifies a strategy for everyone, such that no one would benefit by deviating unilaterally from his/her strategy. Another powerful tool available to game theorists are evolutionary dynamics (ED). Motivated by evolutionary and learning processes, ED specify changes in strategies over time in a population, such that more successful strategies typically become more frequent. A simple game that illustrates interesting ED is the generalized Rock-Paper-Scissors (RPS) game. The RPS game extends the children's game to situations where winning or losing can matter more or less relative to tying. Here we investigate experimentally three RPS games, where the NE is always to randomize with equal probability, but the evolutionary stability of this strategy changes. Consistent with the prediction of ED we find that aggregate behavior is far away from NE when it is evolutionarily unstable. Our findings add to the growing literature that demonstrates the predictive validity of ED in large-scale incentivized laboratory experiments with human subjects.

Evolutionary game theory typically focuses on actions but ignores motives. Here, we introduce a model that takes into account the motive behind the action. A crucial question is why do we trust people more who cooperate without calculating the costs? We propose a game theory model to explain this phenomenon. One player has the option to "look" at the costs of cooperation, and the other player chooses whether to continue the interaction. If it is occasionally very costly for player 1 to cooperate, but defection is harmful for player 2, then cooperation without looking is a subgame perfect equilibrium. This behavior also emerges in population-based processes of learning or evolution. Our theory illuminates a number of key phenomena of human interactions: authentic altruism, why people cooperate intuitively, one-shot cooperation, why friends do not keep track of favors, why we admire principled people, Kant's second formulation of the Categorical Imperative, taboos, and love.

We propose a simple model for why we have more trust in people who cooperate without calculating the associated costs. Intuitively, by not looking at the payoffs, people indicate that they will not be swayed by high temptations to defect, which makes them more attractive as interaction partners. We capture this intuition using a simple four-stage game. In the first stage, nature draws the costs and benefits of cooperation according to a commonly-known distribution. In the second stage, Player 1 chooses whether or not to look at the realized payoffs. In the third stage, Player 2 decides whether to exit or let Player 1 choose whether or not to cooperate in the fourth stage. Using backward induction, we provide a complete characterization for when we expect Player 1 to cooperate without looking. Moreover, we show with numerical simulations how cooperating without looking can emerge through simple evolutionary processes.