Rejection of unfair offers in the ultimatum game is no evidence of strong reciprocity.

The strong reciprocity model of the evolution of human cooperation has gained some acceptance, partly on the basis of support from experimental findings. The observation that unfair offers in the ultimatum game are frequently rejected constitutes an important piece of the experimental evidence for strong reciprocity. In the present study, we have challenged the idea that the rejection response in the ultimatum game provides evidence of the assumption held by strong reciprocity theorists that negative reciprocity observed in the ultimatum game is inseparably related to positive reciprocity as the two sides of a preference for fairness. The prediction of an inseparable relationship between positive and negative reciprocity was rejected on the basis of the results of a series of experiments that we conducted using the ultimatum game, the dictator game, the trust game, and the prisoner's dilemma game. We did not find any correlation between the participants' tendencies to reject unfair offers in the ultimatum game and their tendencies to exhibit various prosocial behaviors in the other games, including their inclinations to positively reciprocate in the trust game. The participants' responses to postexperimental questions add support to the view that the rejection of unfair offers in the ultimatum game is a tacit strategy for avoiding the imposition of an inferior status.

The private rejection of unfair offers and emotional commitment.

In a series of experiments, we demonstrate that certain players of an economic game reject unfair offers even when this behavior increases rather than decreases inequity. A substantial proportion (30-40%, compared with 60-70% in the standard ultimatum game) of those who responded rejected unfair offers even when rejection reduced only their own earnings to 0, while not affecting the earnings of the person who proposed the unfair split (in an impunity game). Furthermore, even when the responders were not able to communicate their anger to the proposers by rejecting unfair offers in a private impunity game, a similar rate of rejection was observed. The rejection of unfair offers that increases inequity cannot be explained by the social preference for inequity aversion or reciprocity; however, it does provide support for the model of emotion as a commitment device. In this view, emotions such as anger or moral disgust lead people to disregard the immediate consequences of their behavior, committing them to behave consistently to preserve integrity and maintain a reputation over time as someone who is reliably committed to this behavior.

Salivary testosterone levels and autism-spectrum quotient in adults.

OBJECTIVES: The purpose of this study is to examine the relationship between salivary testosterone levels and autistic traits in adults. METHODS: A total of 92 male and female adults participated in the present study. Their salivary testosterone level (T) and score of Japanese version of Autism-spectrum Quotient (AQ) were assessed to examine the relationship between salivary testosterone level and autistic traits in adults. RESULTS: We observed a positive correlation between T and AQ in a group of both sexes. The correlation disappeared when we conducted correlation analysis by sex. However, although there was no sex difference in the score of the subscale of attention switching, attention switching was related to T. CONCLUSIONS: Although the relationship between T and AQ may mainly result from sex differences, the subscale of attention switching may be modulated by testosterone.

Salivary alpha-amylase levels and big five personality factors in adults.

OBJECTIVES: Salivary alpha-amylase (sAA) has been utilized as a non-invasive measure of sympathoadrenal medullary (SAM) activation. Little is known regarding the relationship between personality inventories and baseline sAA. This study was designed to examine the relationships between the scores of big five inventory (BFI) factors, age, and sAA in adults (aged twenty to seventy years old). METHODS: We assessed 97 participants' BFI scores and sAA. The correlations between the BFI factor scores and sAA were examined. RESULTS: We observed (1) a positive correlation between Neuroticism and sAA, and a negative correlation between Agreeableness and sAA and (2) a positive correlation between age and sAA. These correlations between BFI scores and sAA remained significant after controlling for age. After controlling for age, all BFI factors except Conscientiousness were related to sAA. CONCLUSIONS: Subjects with high Neuroticism and low Extraversion, Agreeableness and Openness may have high sAA. sAA has been demonstrated to be useful for examining the relationship between adrenergic activity and personality, in a non-invasive manner.

Stress hormones predict hyperbolic time-discount rates six months later in adults.

OBJECTIVES: Stress hormones have been associated with temporal discounting. Although time-discount rate is shown to be stable over a long term, no study to date examines whether individual differences in stress hormones could predict individuals' time-discount rates in the relatively distant future (e.g., six month later), which is of interest in neuroeconomics of stress-addiction association. METHODS: We assessed 87 participants' salivary stress hormone (cortisol, cortisone, and alpha-amylase) levels and hyperbolic discounting of delayed rewards consisting of three magnitudes, at the time-interval of six months. For salivary steroid assays, we employed a liquid chromatography/ mass spectroscopy (LC/MS) method. The correlations between the stress hormone levels and time-discount rates were examined. RESULTS: We observed that salivary alpha-amylase (sAA) levels were negatively associated with time-discount rates in never-smokers. Notably, salivary levels of stress steroids (i.e., cortisol and cortisone) negatively and positively related to time-discount rates in men and women, respectively, in never-smokers. Ever-smokers' discount rates were not predicted from these stress hormone levels. CONCLUSIONS: Individual differences in stress hormone levels predict impulsivity in temporal discounting in the future. There are sex differences in the effect of stress steroids on temporal discounting; while there was no sex defference in the relationship between sAA and temporal discounting.

[Accuracy of judgment about others' cooperative behavior: effects of attractiveness and facial expressiveness].

Cooperation in interdependent relationships is based on reciprocity in repeated interactions. However, cooperation in one-shot relationships cannot be explained by reciprocity. Frank, Gilovich, & Regan (1993) argued that cooperative behavior in one-shot interactions can be adaptive if cooperators displayed particular signals and people were able to distinguish cooperators from non-cooperators by decoding these signals. We argue that attractiveness and facial expressiveness are signals of cooperators. We conducted an experiment to examine if these signals influence the detection accuracy of cooperative behavior. Our participants (blind to the target's behavior in a Trust Game) viewed 30-seconds video-clips. Each video-clip was comprised of a cooperator and a non-cooperator in a Trust Game. The participants judged which one of the pair gave more money to the other participant. We found that participants were able to detect cooperators with a higher accuracy than chance. Furthermore, participants rated male non-cooperators as more attractive than male cooperators, and rated cooperators more expressive than non-cooperators. Further analyses showed that attractiveness inhibited detection accuracy while facial expressiveness fostered it.

[Linking behavior in a social dilemma to outcome in another exchange: an experimental study of social dilemma resolution].

Recent studies have shown that a social dilemma (SD) may be resolved when people base their behavior in it on the outcome in another exchange domain (e.g., Milinski, Semmann, & Krambeck, 2002). Since people often engage simultaneously in different social exchanges with the same partner, such behavioral linkage, making behavior contingent upon outcome, across different exchange domains may provide an ecologically feasible solution to a SD. In this study, a further analysis of the theoretical statement concerning the positive effect of behavioral linkage was made, and it was argued that such linkage would resolve a SD, since another exchange relationship could provide a means to administer selective incentives. An experiment with 120 participants was conducted to test the hypotheses that people would adjust their behavior in an iterated prisoner's dilemma game depending on their partner's behavior and outcome in another SD, and such behavioral linkage would promote cooperation in the SD. Results of the experiment supported the hypotheses.

[Emergence of frequency-dependent cooperative strategies in iterated social dilemma: an experimental study].

Previous formal work on iterated social dilemma has shown that the strategy prescribing one to cooperate if m or more others cooperated in the previous round and defect otherwise, loses their effectiveness as the group size increases. The only reciprocal strategy formally proved to be effective in an N-person situation is "trigger strategy" (Friedman, 1986). This strategy presumes that people set the threshold for cooperation, m, at its maximum, namely N-1: all members other than self. This study investigated whether people's choice behavior in iterated social dilemma approximated such a strategy. In groups of seven, participants played a social dilemma game repeatedly. They were provided with complete information regarding others' choices in the three preceding rounds, when making a choice in the current round. Results indicated that participants became more reciprocal as the game progressed, responding more sensitively to how many others cooperated in the previous round. Furthermore, the participants used a strategy in the game that were more lenient for reciprocation, with lower threshold for cooperation, than trigger strategy. Implications of current findings and some future directions were discussed.

[Social learning as an uncertainty-reduction strategy: an adaptationist approach].

Social learning is an effective mechanism to reduce uncertainty about environmental knowledge, helping individuals adopt an adaptive behavior in the environment at small cost. Although this is evident for learning about temporally stable targets (e.g., acquiring avoidance of toxic foods culturally), the functional value of social learning in a temporally unstable environment is less clear; knowledge acquired by social learning may be outdated. This paper addressed adaptive values of social learning in a non-stationary environment empirically. When individual learning about the non-stationary environment is costly, a hawk-dove-game-like equilibrium is expected to emerge in the population, where members who engage in costly individual learning and members who skip the information search and free-ride on other members' search efforts coexist at a stable ratio. Such a "producer-scrounger" structure should qualify effectiveness of social/cultural learning severely, especially "conformity bias" when using social information (Boyd & Richerson, 1985). We tested these predictions by an experiment implementing a non-stationary uncertain environment in a laboratory. The results supported our thesis. Implications of these findings and some future directions were discussed.