Women will gaze at pictures of heterosexual sex longer than men will, a new study finds, and what they're looking at depends on whether they're taking birth-control pills .

Also surprising was the finding that men are more likely to look at a female's face before other areas when looking at pictures of naked women, according to the study by Emory University researchers.

Both findings, published in the journal Hormones and Behavior, shed light on sexual attitudes that really aren't all that mysterious when considered in a scientific light, said Emory psychologist Kim Wallen.

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Wallen and his former graduate student, Heather Rupp, showed still photos of heterosexual couples having sex to 30 women and 15 men between the ages of 23 and 28.

Each subject was rigged with a high-tech eye-tracking device that measured where his or her gaze went first — and how long it stayed there.

Men went straight to the faces of the women in the photos and lingered awhile, but most of the women were more interested in the sexual activity.

How much so depended on whether they were taking hormone-filled birth-control pills.

Those who were taking the pills, Wallen said, were interested in the overall view of the photos and "background" items like jewelry.

Women not on the pill were more interested in areas normally covered by clothing.

"Eye-tracking data suggested that what women paid most attention to was dependent on their hormonal state," said Rupp, now at Indiana University's Kinsey Institute for Research in Sex, Gender and Reproduction.

Wallen and Rupp said their study suggests that men's increased attention to faces may be related to higher activation in the amygdala, a portion of the brain that processes emotional information and excitement.

Women can tell by looking at naked men whether they are aroused, Wallen said, but female bodies don't reveal much.

"It's cryptic, but facial expression is one way of showing an indication of interest in and enjoyment of sex," Wallen said.