In the advertising world, “sex sells” is an accepted truism. In a new study of the male brain, Stanford University researchers have determined why.

In the current issue of the journal NeuroReport, Brian Knutson, an assistant professor of psychology at Stanford University, reports that in heterosexual males, erotic imagery stimulates an area of the brain associated with anticipation of reward. That process makes men more likely to take bigger financial risks than they otherwise would.

This explains why cocktail waitresses in casinos are inevitably dressed in skimpy outfits. The sex appeal they exude “might activate brain regions that encourage risk-taking and therefore get people to gamble more,” Knutson said.

Knutson and his colleagues took a group of male heterosexual college undergraduates and had them view images of an erotic encounter (to elicit a positive emotional response), snakes and spiders (negative) or office supplies (neutral). After viewing each image, participates had to quickly decide whether to gamble either a dollar or a dime on a 50-50 proposition -- a significant decision, since they only had $10 to play with.

“After people had seen these erotic pictures, they tended to pick the high-risk gamble more often, especially if they had been picking the low-risk gamble before,” Knutson told the Stanford News Service.

While all this was occurring, the students’ brains were being monitored with functional MRI technology. After viewing the erotic images, an area of the brain called the nucleus accumbens -- which the researchers associated with risk-taking in previous studies -- showed an unusually high level of activation. That level of brain activity remained high when they were choosing the high-risk gamble.

Having known this intuitively for many years, automakers tend to have beautiful models in car advertisements, sometimes looking lasciviously at the leather interiors. Knutson isn’t certain that imagery is effective. His test measured very quick responses, and he doesn’t know how long it takes for the effects of the erotic stimulation to wear off.

That will be the subject of future research, as will the responses of women to similar stimuli. In the meantime, guys, if you feel the urge to take a big financial risk, it might be wise to close your checkbook and take a cold shower.