Testosterone makes tough muscular men much less generous than weedier peers

Spurts of exercise increases testosterone levels. A testosterone cream caused a 27 per cent reduction in the generosity of cash offers during a money sharing game

Women who want lavish presents on their birthday and wedding anniversary should not opt for big, beefy men.

A new study has found those with high testosterone levels are less generous than men with lower levels of the hormone.

The researchers tested the generosity of 25 male students at Claremont Graduate University.

The volunteers were given a testosterone-containing gel, which doubled the amount of potent sex hormone coursing through their veins, as well as a placebo cream 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.

The students then played a simple economic game with another participant via a computer. One volunteer was tasked with splitting $10 (£6) with another volunteer in any way he likes. The other volunteer either accepted the offer or rejected it as unfair, in which case no one received 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.

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.

Lead researcher Karen Redwine, a neuro-economist at Whittier College in California, said: 'Our broad conclusion is that testosterone causes men essentially to be stingy.

'People are selfish, but they're selfless as well, and it's not understood why the behaviour shifts.'

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.

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, a team 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.

The research was presented at the Society for Neuroscience's annual meeting in Chicago last week and was reported in New Scientist.