Women don't just like men with muscles — they go for them.

Men who are more muscular than average are much more likely to have short-term affairs and multiple sex partners than their scrawnier peers, according to new UCLA research published in the August issue of the Personality and Social Psychology Bulletin.

"If you're trying to figure out why men — especially young men — spend so much time at the gym, here's your answer," said David Frederick, lead author and a UCLA doctoral candidate in psychology. "The stereotype is that men work out to compete with each other, but our research suggests that pumping iron is a way for men to enhance their attractiveness to women."

The series of studies, conducted by Frederick and co-author Martie Haselton, a UCLA associate professor of communication studies and psychology, is the first published research to quantify an association between men's muscularity and their success in the sack. The four-year project also scientifically quantified for the first time women's perceptions of the importance of muscularity in selecting short- and long-term partners.

"A lot of mate-selection research focuses on what men find attractive," Haselton said. "This shows women are putting a premium on attractiveness. Women care about muscularity when they choose sex partners."

Frederick and Haselton lead a team that photographed 99 male undergraduates. A panel of independent judges rated the young men on a nine-point scale, with "1" being much less muscular than average and "9" being much more muscular than average. The researchers then asked the men about their sexual histories.

When compared with their less-muscular peers, young men who were more muscular than average were twice as likely to have had more than three sex partners in their lives.

In another study, Frederick and Haselton asked 120 undergraduate males to rate their own physiques on the same scale and then asked them about their sexual histories.

The self-identified muscular men had not only had more sexual partners than their less burly peers, but they were twice as likely to have had brief flings or one-night stands with women. The difference in the number of sexual partners reported by the men who were more muscular than average was also notable: They reported having had an average of four partners, compared with an average of 1.5 partners for men who reported average or below-average muscularity.

In a similar study, Frederick and Haselton asked 60 undergraduate males an additional question: How many affairs had they had with women who already had a boyfriend at the time of the affair? Muscularity mattered here as well. The more muscular individuals were twice as likely as their less well-built peers to have hooked up with someone else's sweetheart.

The researchers, who are associated with UCLA's Center for Behavior, Evolution and Culture, say that their findings on muscularity are consistent with research findings on the secondary sexual characteristics of other animals, such as the attention-getting tail feathers of male peacocks.

"Everybody knows that testosterone is a hormone that promotes strength in men, but less well-known is the fact that the hormone is also associated with poorer immune system functioning," Frederick said. "Secondary sexual characteristics are thought to have evolved as indicators of mate quality because they demonstrate an ability to flourish in the face of what's really a drag on the system. Males in good enough shape to withstand the deleterious effects of immunosuppression have to be especially fit and are therefore more likely to transmit fitness to their offspring than less well-endowed males."

"Evolutionary scientists have long maintained that exaggerated secondary sexual characteristics — such as large muscles in men — are cues to genes that increase the viability of offspring or their reproductive success," Haselton said. "In an age when medical advancements play such a large role in the survival and health of children and so many people use contraceptives, it's not clear whether these genes continue to offer reproductive benefits. But women today are still attracted to muscular men, just as their ancestors would have been, because that's how we've evolved."

Interestingly, women in the study seemed to be on to muscular men. When presented with six standardized silhouettes of men ranging from brawny to slender, 141 undergraduate women consistently identified the most muscular ones as not only less likely to commit but also more volatile and domineering. In the study, the women rated "toned" guys — the physical type two notches down from "brawny" — as the most sexually attractive.

"Moderate muscularity demonstrates that men are in good condition, but they're not so overloaded with testosterone that they are volatile, aggressive and dominant," Frederick said. "Just based on their experiences, women seem to be able to weigh good and bad male traits."

Still, in a study by Frederick and Haselton of 82 college coeds, most women reported that their short-term partners were more muscular than their long-term ones. They characterized their long-term — and presumably less muscular — partners as more trustworthy and romantic than their one-night stands or brief affairs.

"This suggests that the sweet-guy approach works better for less muscular men," Frederick said. "The muscular men don't need to put in this kind of effort, especially for a short-term relationship."

Source: UCLA