Testosterone is a steroid hormone associated with aggression and dominance both in animals and humans. While the mechanisms through which testosterone functions in animals have been well documented1, the effects of testosterone on human social behaviour are not fully understood. What is commonly agreed among behavioural endocrinologists is that testosterone is related to sensitivity to status-related cues2,3,4,5. Eisenegger, Haushofer, and Fehr6 contend that testosterone promotes status- and dominance-seeking motives in human social interaction, and thus plays an important role in modulating behaviour in status hierarchies. A meta-analysis of testosterone studies in humans by Archer2 also reveals a consistent association between testosterone levels and various measures of dominance, especially when the actor’s status is challenged.

The aim of the present study is to examine how the link between testosterone and status-seeking motives affects the economic decision making of the members of a pre-existing and hierarchically-organised group. This question has implications for neuroendocrinology and social neuroeconomics, since recent studies suggest that testosterone is related to strategic social behaviour, and that the conception of androgens as drivers of socially dominant and aggressive behaviour is too simplistic7, 8. This recent theory on the role of testosterone predicts that, among persons ranked lower in a social hierarchy, testosterone will foster non-aggressive, socially obedient behaviour; in contrast, among those ranked more highly, testosterone will induce socially dominant and aggressive behaviour. To test these predictions, we employed the paradigm of economic game experiments, which is known to be useful in disentangling motivations during social interactions9. The ultimatum game (UG10) is particularly suitable for this purpose. The UG is typically considered a measure of fairness-seeking behaviour, but it is also proposed to measure dominance-seeking behaviour in response to a challenge11. The UG pits two players against one another: a “proposer” offers a proportion of a monetary endowment to the “responder”. The responder decides whether to accept or reject the offer. If the responder accepts the offer, the responder receives the money offered by the proposer while the proposer receives the balance. If the responder rejects the offer, neither player receives any money. A rational and self-interested proposer should make a small offer, and a rational and self-interested responder should accept it12. Nevertheless, a substantial proportion of proposers make equitable offers and responders frequently reject small offers, at least in Western or market-integrated societies13, 14.

Burnham11 showed that lower offers in the UG were more often rejected by men with high levels of salivary testosterone, and suggested that the effect could be mediated by testosterone’s impact on status-asserting behaviour: low offers are interpreted by responders as a challenge to their status, and this challenge is more salient to responders high in testosterone. This finding was replicated by Mehta and Beer15 both in men and women, but other studies have reported no statistically significant effect of exogenous testosterone on the rejection of inequitable offers16,17,18,19.

The direction of effects of testosterone on offer behaviour is also inconsistent. Zak et al.18 report that men who are administered testosterone rather than a placebo offer significantly less money. Conversely, Eisenegger et al.17 has shown that women administered testosterone offer significantly more than those who receive a placebo. Burnham11 showed that male proposers constrained to offer their male opponent either $25 or $5 out of an endowment of $40, and who choose to offer $25, have higher average salivary testosterone levels than those who choose to offer $5, though the difference was not statistically significant. Eisenegger et al.17 attributed their finding to the status-preserving motives of proposers administered high levels of testosterone. Because a rejected offer may lower the proposer’s status, high levels of testosterone may motivate proposers to avoid circumstances likely to lead to rejection.

The inconsistent effects of testosterone on offer and rejection behaviour may be due to several other factors, including sex (i.e., male sample11, 15, 16, 18 vs. female sample15, 17, 19), and whether testosterone is endogenous or exogenous (i.e., measured salivary testosterone11, 15 vs. administered testosterone16,17,18,19). Another possibility is that subtle cues of interdependency influence the responder’s rejection behaviour. For example, Declerck, Kiyonari, and Boone20 showed that small offers were rejected more frequently when responders were informed that they would be matched with proposers after deciding to accept or reject. The authors suggest that their responders may have wished to signal to potential opponents their intention to reject small offers (for an effect of decision timing in a different economic game, see ref. 21).

More generally, sensitivity to environmental cues is known to play a crucial role in human decision making, such that the same economic game may be perceived to represent different challenges depending on the players’ sensitivity to particular environmental cues22, 23. We propose that the inconsistent findings with respect to testosterone and aggression may be due to varying cues that enhance or reduce players’ perception of challenges to status. These cues may moderate the relationship between testosterone and proposal/rejection behaviours in a typical laboratory experiment in which there are no a priori status differences between anonymously matched participants who do not know each other. In such experimental situations, the effects of individual differences in sensitivity to status cues unrelated to testosterone levels may obscure the independent effect of testosterone.

Boksem et al.24 suggest that testosterone affects game behaviour differently depending on context, and argue that the effects of testosterone will depend on whether players face an imminent threat to their status. When threat is imminent, high testosterone may enhance competitive and aggressive behaviour. When threat is not imminent—when an individual’s status is not challenged—competitive behaviour may damage the social status of the individual. In such a situation, testosterone may not stimulate aggressive competitive behaviour but rather prosocial behaviour as a means of enhancing reputation.

We consider it likely that the presence or absence of a threat to status explains at least some inconsistencies between the effects of testosterone on behaviour in economic games. Here, we investigate the effect of testosterone on UG behaviour among individuals who are known to one another and who are embedded in a pre-existing status hierarchy. This approach will allow us to determine the effects of testosterone when a player’s status is challenged (i.e., when a high-status responder is faced with a small offer) and when the same player does not face such a challenge (i.e., when making an offer as a proposer). We hypothesise that responders with higher levels of testosterone will be less tolerant of small offers than will responders with lower levels of testosterone. We also test whether testosterone is related to offer behaviour, but make no specific predictions. Testosterone may promote offers that are small18 or large17.

We also follow Zak et al.’s18, 25 strategy of calculating the difference between offers made by players in the role of proposer and the offers those same players tolerate as receiver. If proposers make offers that are above the minimum value at which they themselves would reject (their minimum acceptable offer, or MAO), the difference between the offer and MAO may be termed “generosity”18, 25. This term is appropriate when players are (or are assumed to be) equals. However, we are interested in the behaviour of participants who differ in status, where lower ranked players may feel obliged to submit to the wishes of their seniors while higher ranked players may feel entitled to dominate their juniors. We contend that, in such a situation, accepting a lower offer than one would make reflects a willingness to yield to the implicit coercion of the other player. Thus, we refer to the difference between a player’s offer and MAO as “acquiescence” rather than “generosity”.

We hypothesise that acquiescence will parallel the status differences between proposers and responders: lower ranked players will acquiescence more than higher ranked players. The relationship between testosterone and acquiescence is difficult to predict. Zak et al.18 has shown that higher testosterone is associated with lower acquiescence. However, Beksem et al.24 argue that testosterone’s effect will depend on the presence or absence of threats, which suggests that this relationship may not hold in the case of the lower ranked players.