(PhysOrg.com) -- When their romantic partners are not quintessentially masculine, women in their fertile phase are more likely to fantasize about masculine-looking men than are women paired with George Clooney types.
But women with masculine-looking partners do not necessarily become more attracted to their partners, a recent study co-authored by a University of Colorado at Boulder researcher concludes.
Meanwhile, a man's intelligence has no effect on the extent to which fertile, female partners fantasize about others, the researchers found. They say the lack of an observed "fertility effect" related to intelligence is puzzling.
The findings augment the emerging understanding of how human sexual selection evolved over time, and how the vestiges of that evolution are evident today.
The findings come from a study published recently in the journal Evolution and Human Behavior. The study was conducted by Steven Gangestad and Randy Thornhill of the University of New Mexico and Christine Garver-Apgar, a postdoctoral fellow at CU's Institute for Behavioral Genetics.
A "masculine face" has a relatively pronounced chin, strong jaw, narrow eyes and well-defined brow. George Clooney fits this bill, Gangestad suggests. A less-masculine face, on the other hand, would include a less-pronounced jaw and wider eyes, a la Pee-wee Herman.
But this does not mean that pretty boys are less attractive as life partners.
"When they rate men's sexiness, in a sense, that's when (women) show the shift," Gangestad. "If they rate men's attractiveness as a long-term partner, then they don't show it."
The team interviewed 66 heterosexual couples in which women's ages ranged from 18 to 44. Their relationships ranged from one month to 20 years in length. Nine couples were married.
A host of studies has shown that women's interest in men with masculine features peaks during ovulation. But this study is the first to confirm that the effect occurs in real couples.
"The effects of facial masculinity and attractiveness fit in a larger picture that has emerged," says Garver-Apgar.
The prevailing wisdom during much of the last half-century was that women did not experience estrus, the period in which other primates signal their fertility with swollen genitals. But newer research suggests that women may not have lost all remnants of estrus.
Evolutionary biologists have documented that women are choosy when fertile, and their freedom to choose mates is increased because their fertile phase is not advertised as it is in other primates. A growing body of evidence suggests that, when most fertile, women gravitate toward males who show signs of good genetic quality.
Masculine facial features suggest that a man is of good genetic quality, because he had the resources during development not only to survive but also to expend energy on a macho visage. Rugged-looking jaws and eyebrows are signals of testosterone.
Instead of using his energy on other features or to maintain his immune system, the masculine-looking male may have had a "surplus energy budget," Garver-Apgar says.
During development, individuals make trade-offs. They can build big brains, large muscles or stronger immune systems. Brains, brawn and immunity may all compete for the same resources.
While it is not surprising that women's gazes would fall on masculine-looking men when they are most fertile, Garver-Apgar says the lack of a similar effect with intelligence is perplexing.
"That we didn't find any effect of men's intelligence on their partners' sexual interests across the cycle is important because some evidence suggests that intelligence associates with genetic quality."
But the data on the intelligence-attraction equation are mixed. If intelligence correlates with good genetic quality, Garver-Apgar wonders, why is it that intelligence is not among those traits that women prefer mid-cycle? "Why don't you see a fertility effect?"
Further research should help answer those questions, she and her co-authors suggest.
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