MONDAY, June 27 (HealthDay News) -- Uncovering some science behind the type of intuition known as "gaydar," a team of American and Canadian researchers reports that a woman can tell whether a man is straight or gay by looking at his face when she is ovulating.

Women also appear to have a heightened sensitivity towards a man's sexual orientation whenever they start to harbor romantic notions or mating aspirations, the new study revealed.

"This effect is not apparent when a woman is judging another female's orientation," lead study author Nicholas Rule, from the department of psychology at the University of Toronto, said in a university news release. "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."

Rule and his colleagues report their findings in the June 27 issue of Psychological Science.

To explore the subject, the authors conducted three investigations, the first of which involved 40 undergraduate women who were asked to guess the sexual orientation of 80 men based on photos of their face.

Half of the men were gay, the other half straight. All held similar expressions in the photos or were deemed to be equally attractive. None of the women were using any contraceptive drugs at the time of the test.

The result: the closer a woman was to her peak ovulation the more accurate her guess.

The second trial involved 34 women who were shown 200 similarly composed photos of 200 women, half of whom were lesbian. However, there was no correlation between predictive accuracy and peak ovulation, according to the researchers.

"Together," said Rule, "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."

To test whether or not a romantic disposition might aid the sexual orientation-guessing process, the research team lastly asked 20 of the female participants to read an outline of a romantic encounter before all 40 women repeated the first two experiments.

The investigators found that those women who read a romantic story (and were presumably prompted to engage in some degree of romanticized or mating-related thinking) were "significantly" better at judging the men's sexual orientation than those women who hadn't.

More information

The Nemours Foundation has more on sexual orientation.