Testosterone makes men more impulsive

TESTOSTERONE is a hormone with a reputation. Though both sexes generate the stuff, that reputation is macho. Numerous experiments on non-human animals show that boosting testosterone levels boosts levels of aggression. And in most species—humans included—males are the more aggressive sex.

Doing experiments specifically designed to increase aggression in people is ethically problematic. Aggression is not, however, the only behavioural trait that seems to differ between the sexes. Generally speaking, males are also more impulsive than females. And that, too, may be linked with testosterone levels—a link that Gideon Nave at the University of Pennsylvania and Amos Nadler at Western University in Ontario have recently been exploring.

Impulsiveness can be measured in many ways. That chosen by Dr Nave and Dr Nadler was mathematical. In the largest experiment yet conducted on the effect of testosterone on human behaviour, which they have just reported in Psychological Science, they tested the hormone’s influence on volunteers’ capacity to do mental arithmetic. They conclude that, in this sphere at least, testosterone encourages men to jump to the wrong conclusions.

The researchers arranged for 243 male college students to come into their laboratories. These volunteers were asked to remove their shirts and smother some gel onto their chests and shoulders. The gel samples they were given all looked and smelled the same, but in only 125 cases did they contain testosterone; the other 118 were hormone-free controls. As is usual in such experiments, those handing out the samples did not know which was which. That knowledge was restricted to the people who had labelled the samples, who had had no contact with the volunteers at any time.

Four hours after each volunteer had anointed himself, which was the point when any testosterone he might have absorbed would be at peak concentration in his bloodstream, he was asked a series of questions, for which small cash prizes were awarded for the correct answers. Three of the questions were designed in a way that might encourage an impulsive but incorrect reply. (For example, a bat and a ball cost $1.10 in total. The bat costs $1.00 more than the ball. How much does the ball cost?*) The others, which involved adding up as many sets of five two-digit numbers as possible within five minutes, simply required mental arithmetical skills to be applied accurately.

The researchers suspected that those whose gel-rub had included testosterone would do worse than the controls on the trickier questions, but that both groups would do equally well on the adding up. And so it proved. Participants who had not received the testosterone rub answered an average of 2.1 of the tricky questions correctly. Those who had been dosed with the hormone managed only 1.7. The probability of this difference happening by chance is less than one in 500. In contrast, the two groups’ scores for the straightforward questions (10.8 successful additions in the time available versus 10.9) were statistically identical.

Impulsive answers to mathematical questions are, admittedly, quite a specialised form of impulsiveness. But, having established the principle, Dr Nave and Dr Nadler hope to extend the scope of their research. They will use bigger groups and different tests. And they will also try the same experiments on women, to see if their powers of self-control, too, succumb to testosterone’s impulsive influence.

^{*The answer is five cents. But a surprising number of people answer ten.}