If you’re looking to haggle, steer clear of big, beefy salesmen. The same hormone responsible for their brawn may also reduce their generosity, new research suggests.

“Our broad conclusion is that testosterone causes men essentially to be stingy,” says Karen Redwine, a neuro-economist at Whittier College in California, who presented the work at the Society for Neuroscience’s annual meeting in Chicago last week.

A previous study of 17 City of London traders found that morning testosterone levels correlated with each day’s gains and losses, with more of the hormone associated with a profit. But that study didn’t establish a cause-effect relationship between testosterone and shrewdness.

To make this case, Redwine and her colleague Paul Zak, at the Claremont Graduate University in California, gave a testosterone-containing gel to 25 male university students, and then tested their generosity. All the participants also got a placebo cream with no testosterone, either a few days before or after the testosterone boost. Neither the researchers nor the participants knew which was which until the end of the study.


Power gel

The testosterone cream worked. The next day, twice as much of the potent sex hormone coursed through the veins of volunteers, on average.

The students then played a simple economic game with another participant via a computer. One volunteer is tasked with splitting $10 with another volunteer in any way he likes. The other volunteer either accepts the offer or rejects it as unfair, in which case no one gets any money. Each volunteer played this game in both roles, on and off the testosterone gel.

Overall, the testosterone cream caused a 27 per cent reduction in the generosity of the offers, from averages of $2.15 to $1.57, Redwine and Zak found.

A more potent variant of testosterone, dihydrotestosterone (DHT), exerted an even stronger influence on behaviour. Men with the most DHT in their bloodstream offered their partners a paltry $0.55 of the $10, while men with the least amount of DHT tendered $3.65, on average.

DHT was also associated with a propensity to punish unfair offers. Men with the highest levels rejected offers below $4, while men with low levels of the androgen only punished if the offer was below $2.15, the researchers found.

Selfish hormones

There are two ways of looking at the findings, Redwine says. On one hand, testosterone pushed men to demand a larger split of the money, whether they were making an offer or deciding to accept or reject one.

Yet by rejecting unfair offers, testosterone-fuelled volunteers are actually enforcing a social order that calls for a 50-50 split. “People are selfish, but they’re selfless as well, and it’s not understood why the behaviour shifts,” she says.

One biological factor could be the dynamics between testosterone and another hormone called oxytocin. Sometimes called the cuddle chemical, oxytocin also influences generosity. In a 2007 study, Zak’s team found that oxytocin administration boosted generosity in the same game by 80 per cent.

Redwine notes that testosterone blocks the action of oxytocin in the brain. “It’s possible that by creating these alpha males we actually inhibited oxytocin,” she says.