"This effect is not apparent when a woman is judging another female's orientation," says Professor Nicholas Rule of the Department of Psychology at the University of Toronto, lead author of a new study published in Psychological Science. "This suggests that fertility influences a heterosexual woman's attention to potential mates rather than merely increasing sensitivity to sexual orientation or nonverbal cues more generally."

In the first of three experiments, 40 undergraduate women judged the sexual orientation of 80 images of men's faces. Forty of the photos were of self-identified gay males while the other 40 were of straight men. The men did not differ in emotional expression or attractiveness, and the female participants were encouraged to use their intuition in making judgments. In addition, the women reported the length of time since their last menstrual cycle and its average duration; none were using any systemic contraceptive medications.

The researchers correlated the participants' accuracy in judging sexual orientation with the point at which the women were in their fertility cycle, and found that the nearer women were to peak ovulation, the more accurate they were at judging each male's sexual orientation.

The second experiment featured 34 women who viewed a similar series of female faces, 100 of whom were self-identified lesbians while another 100 were straight. The researchers found no relationship between fertility and accurate judgments of the women's sexual orientation.

"Together, these findings suggest that women's accuracy may vary across the fertility cycle because men's sexual orientation is relevant to conception and thus of greater importance as women are nearer to ovulation."

The researchers tested this hypothesis further with a third experiment in which female participants were primed with a mating goal in order to manipulate reproductive relevance. Half of the 40 participants were asked to read a story which described a romantic encounter while the other half did not, before performing the same tasks in the two previous studies. Rule and his colleagues found that the women primed with a mating goal were significantly more accurate in their judgments than the women who were not, implying that inducing romantic or mating-related thoughts improved accuracy in identifying men's sexual orientations.

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• Gender Difference
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