A woman prefers a more masculine man when she is fertile and looking for a fling rather than a mate for life, according to a new study.
The finding suggests the value that women place on masculinity changes with context and with women's reproductive cycles and immediate goals. A woman's preference for manly men also was found to vary based on how attractive she rated herself.
And some of a woman's sex drive might involve tricks in the brain over which she has no control.
Previous research has shown that women view facial masculinity--square jaws and well-defined brow ridges--as good characteristics for short-term partners, while more feminine traits are perceived as better for long-term mates. Another study found that women smell better to men at certain points in their menstrual cycles.
A new look
In the new study, researchers asked women who were at different points in their menstrual cycles (and who were not on the pill) to rate their own attractiveness. Then researchers presented them with image pairs representing "feminized" and "masculinized" versions of the same male body. The women were asked to choose the body they thought was most attractive for a short-term relationship and then again for a long-term relationship.
Some of the women performed the experiment again at the opposite point in their cycle.
Fertile women chose the masculine version of each image 15 percent more often, on average, than women who were not fertile, said lead researcher Anthony Little, a psychologist at the University of Stirling in Scotland. The effect was strongest if they were looking for a short-term partner rather than a long-term one, and if they considered themselves attractive.
These findings, to be detailed in an upcoming issue of the journal Hormones and Behavior, reveal that "preferences are not absolute," Little said.
Temptation by testosterone
Women might be drawn to Brad Pitt types at the peak of their cycles for reasons related to evolutionary biology. Masculine bodies tend to have higher circulating levels of the hormone testosterone. While testosterone isn't necessarily a good thing in itself--research suggests that it can inhibit immune function, and eunuchs, who have low levels of testosterone, tend to live longer than normal males--its presence can be indicative of quality.
Masculinity could be "sexy for the same reason that the peacock's tail is sexy," said UCLA researcher Dave Frederick, who was not involved in the study. "Only males in good condition can generate these traits," because they require a great deal of energy and can only be sustained in healthy individuals, Frederick explained.
Men with hard bodies and sculpted jaws may therefore have better genes for producing better babies, so it would make sense for fertile women to be attracted to them.
A hormonal trade-off
But masculinity isn't always the bee's knees. As this study illustrates, women tend to prefer more feminine body types when they are less fertile, probably because high levels of testosterone makes men less likely to stay around.
Research suggests that men who are less masculine tend to invest more in relationships, making them more appealing to women who are at the least fertile points in their menstrual cycle--a hormonal profile that mimics that of pregnancy, when mate investment in the relationship is crucial.
This could also explain why women tend to prefer masculine men when they only want a fling.
As for why women perceiving themselves as attractive tend to prefer masculine bodies, it could be as simple as a brain trick, Little speculates. The less attractive a woman feels she is, the less likely she is to score a high-quality mate. A woman's brain might, therefore, trick her into not wanting what she can't have.
This ensures that "she's partnering up with someone who's most likely to reciprocate her own interest," Little told LiveScience. That would be good news, because it means we're not all competing for the same mates.
"Brad Pitt might be the most attractive man in the world, but he's not everybody's cup of tea," he said.