Hormonal women are more responsive to manly men, and Kinsey Institute researchers have the brain scans to prove it.

Women participating in the Kinsey study were shown 224 photos of men's faces, some of which had been "masculinized" or "feminized" using photo-morphing software. MRI scans revealed higher levels of brain response to the masculinized photos, particularly in women who were in the phase of their menstrual cycle immediately preceding ovulation and higher fertility.

The study, published this month in the journal Evolution and Human Behavior, is the first to show differences in neural activation to masculinized and feminized faces. One of the areas of the brain most activated by the images, the anterior cingulated cortex, is involved in decision-making and assessment, which indicates that women are calculating the risks and rewards related to a man with high levels of testosterone.

"Because male traits generally thought to predict good condition and even genetic quality often coincide with less desirable characteristics, women must balance potentially disparate mating priorities," writes Heather Rupp, who headed up the study. "For example, although men characterized by more masculine testosterone-linked traits may be socially dominant and physically healthy, they are also less likely to invest in offspring and to enter into a partnered relationship."

In the battle of nurture vs. nature, however, ovulating women ultimately will choose in favor of genetically strong children. At other points in the menstrual cycle, the feminized faces are preferred, indicating a dip in certain hormones and a preference for men who are willing to stick around and play catch with Junior.

"As is true for most social behaviors, both biology and social influences impact the output of behavior, and likely interact," Rupp told LiveScience.

In addition, Rupp and her colleagues asked the women — none of whom were on hormonal contraceptives — to subjectively rate the images, and discovered that scan results didn't always match up with stated preference.

The finding is a nod to the complexity of sex, and the phenomenon of making up your mind without using your brain.

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Sally Law has written about health and sexuality for the Cleveland Clinic, and has appeared regularly as a guest host on Sirius Radio. Her column, The Science of Sex, appears weekly on LiveScience.