By Emma Young

Having more testosterone makes men more aggressive – right? It’s a popular lay belief that’s supported by animal studies, but there’s been very little relevant research in humans. Now a study, published in Psychological Science, reveals a more nuanced picture: some men are more affected by raised levels of testosterone than others.

Shawn Geniole at Nipissing University in Canada and colleagues studied 308 men aged 18 to 40. After completing a personality questionnaire, the participants gave a saliva sample, so their baseline testosterone level could be measured, and also a mouthwash sample, for DNA extraction.

The participants were told they would soon be paired with a partner for an online “decision making game”. But first they received a nasal gel, which was either a placebo or contained 11 mg of testosterone, and then they were videoed answering general personal questions, which they were told would be shown to their “partner” who was currently located in another room. To add even more realism to this cover story, the men then watched a video of their partner being interviewed in the same way (actually a researcher who provided scripted answers to the questions).

Next came the “decision-making game”, which the men started half an hour after receiving the gel, and which was actually the well-validated “point-subtraction aggression paradigm”, which measures aggressive behaviour in response to provocations. (Points can be stolen from the other player, with no benefit to the participant themselves other than the satisfaction of penalising their partner; the number of points stolen is taken as a measure of behavioural aggression.)

After this, the participants completed a questionnaire that asked about their impressions of the game and the other player. It asked, for example, “Did it make you feel good when you stole points from your game partner?” And “To what extent did you become angry when your game partner stole points from you?”

Earlier studies have suggested that testosterone has bigger effects on aggression among men who rank high on a personality measure of dominance. In this study, the researchers considered men who scored higher for dominance, higher for “independent self-construal” (which involves defining yourself in terms of internal attributes, such as traits, rather than in terms of relationships with others), and lower for self-control, as having high “personality risk” for the dose of testosterone making them more aggressive – and this is what they found

However, this personality effect was even stronger for the more than 200 men in the sample whose DNA analysis showed that they have a variation of the androgen receptor gene (that codes for the receptor that testosterone binds to) that makes the receptor more efficient. This “provides the clearest, albeit correlational, evidence to date that testosterone’s effects on human aggression are likely AR [androgen receptor] dependent,” the researchers write. They added that this is the first time that such fast-acting effects of testosterone on human social behaviour have been observed.

Further analysis revealed that for the men with the more efficient androgen receptor, testosterone didn’t enhance the anger that they felt in response to being provoked — but it did boost the pleasure they derived from being aggressive.

This suggests that the dopaminergic system is involved in testosterone-promoted aggression. The high-risk personality traits noted in this study have also previously been linked to increased reward-related dopaminergic function, the researchers note. “If dopamine-mediated, testosterone may more strongly promote aggression among individuals with this personality profile because the profile may be indicative of an underlying hypersensitive dopaminergic system,” Geniole and his colleagues write.

There are some caveats in relation to the work. One, of course, is that it shows correlations between personality and the androgen receptor gene on the one hand and testosterone-linked increases in aggression on the other, rather than demonstrating causal relationships between these factors. It also doesn’t look at the effects of testosterone on aggression levels in individual men, but rather across the experimental groups. As the researchers note, further work will be needed to rule out other explanations for their findings. It will also need to look at groups other than white men, who were used in this study.

However, potentially there could be significant real-world implications. For example: “It will also be important to determine whether these interactive personality-risk effects extends to more chronic doses of testosterone, such as those used in the treatment of hypogonadism [testosterone deficiency],” the researchers note.

—Using a Psychopharmacogenetic Approach To Identify the Pathways Through Which—and the People for Whom—Testosterone Promotes Aggression

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest