A longitudinal study involving members of a Japanese university rugby team revealed that individuals with higher testosterone levels exhibited increased dominance correlating with their rising hierarchical status. While all participants became more dominant as their social status improved, this effect was significantly stronger in those with higher testosterone levels. The paper was published in Scientific Reports.
Testosterone is a steroid hormone that plays a crucial role in the development and maintenance of male physical characteristics, such as muscle mass, bone density, and body hair. It is primarily produced in the testes in males and in smaller amounts in the ovaries in females. Small quantities of testosterone are also produced by the adrenal glands.
Testosterone is involved in various bodily functions, including libido, sperm production, fat distribution, and red blood cell production. It influences mood and energy levels. Testosterone imbalances can lead to health issues such as hypogonadism in men and polycystic ovary syndrome in women.
Research suggests a link between testosterone and dominance behaviors across both humans and animals, with higher levels of this hormone associated with increased male-male competition and territorial aggression. Moreover, facing competition can stimulate testosterone secretion in various species. This association tends to be weaker in humans than in animals, but it still exists.
Study author Yukako Inoue and her colleagues wanted to examine whether shifts in social status within real-world social groups lead to differences in the relationship between testosterone levels and dominance behaviors.
Two years before the current study, the same researchers investigated how social status influenced the relationship between testosterone levels and dominance, employing economic games with monetary incentives among members of a Japanese university rugby team. This group was selected for its strict hierarchical structure based on members’ years of study. Their findings indicated that higher testosterone levels correlated with more dominant behavior exclusively among senior team members, suggesting that these individuals perceived their status as being under threat from unfair offers.
For their new study, the researchers revisited the same rugby team, inviting former participants to engage once more. Since the initial study, approximately half had graduated, with the remaining first- and second-year students now in their third and fourth years.
In total, 71 male students participated. They were between 18 and 23 years of age. All belonged to the Aoyama Gakuin University’s rugby team. Of these, 30 had participated in the previous study and had since advanced in seniority.
The study was conducted in a large conference room. To measure testosterone and cortisol levels, participants provided saliva samples in the morning and after playing an Ultimatum Game with teammates. Cortisol is a steroid hormone produced by the adrenal glands that plays a vital role in the body’s stress response and helps regulate metabolism, immune response, and blood pressure. The researchers thought that cortisol might also happen to be associated with behavior in the economic games participants would play.
The Ultimatum Game involved participants deciding how to divide 1000 JPY (approximately $7) between themselves and another player. The challenge was to make an offer that the other party would accept, knowing that rejection meant neither player received any money. As responders, participants had to decide how they would react to various offers without knowing the specific amount proposed. Each participant played against opponents from all four years of study (i.e., from all seniority levels), including their own year.
Researchers compared the offers that participants made to others with what the participants themselves stated they would accept when acting as responders. They termed the difference between these two as “acquiescence,” interpreting it as a measure of the willingness to yield to the other party and accept implicit coercion. Lower levels of acquiescence suggest dominance behaviors, as these participants demand more in the role of responder than they offer when making the proposal.
Results showed that first-year students’ cortisol levels were initially lower than those of other students, while testosterone levels were similar across the board.
When participants were playing the game against junior opponents (i.e., those hierarchically lower than them), individuals with higher testosterone levels displayed lower levels of acquiescence (greater dominance). The researchers observed that the negative relationship between testosterone and acquiescence became stronger as individuals advanced in status. This longitudinal analysis indicates that within the same individuals, higher baseline testosterone leads to increased dominance behavior as their social status within the group rises.
In cases where participants were of equal or junior status compared to their opponents, the relationship between testosterone and acquiescence was not significant and even trended negatively in the second wave. This suggests that testosterone may promote strategic behaviors aimed at achieving high status rather than outright dominance, indicating that in certain contexts, individuals might adopt submissive behaviors if it benefits their status ascension.
Unlike testosterone, which showed a clear link with dominance behaviors and social status, cortisol’s relationship with these factors did not appear to be significant. The researchers did not find evidence to support the dual-hormone hypothesis, which proposes an interaction between testosterone and cortisol in modulating social behaviors such as dominance.
“In summary, our study demonstrated the context-dependency of the relationship between testosterone and dominance behavior. Those with higher baseline testosterone display more dominance as their status increases,” the study authors concluded.
The study sheds light on the complexities of the relationship between testosterone levels and complex social behaviors. However, it also has limitations that need to be taken into account. Notably, study participants were exclusively Japanese students who also played rugby. Studies on other demographic groups and outside the strict hierarchies of a sports team might not produce equal results.
The paper, “Testosterone promotes dominance behaviors in the Ultimatum Game after players’ status increases,” was authored by Yukako Inoue, Robert P. Burriss, Toshikazu Hasegawa, and Toko Kiyonari.