Women can more accurately identify a man's sexual orientation the closer she is to ovulation, according to a study released this month.

The study, a joint project by the University of Toronto and Tufts University in Boston, looked at the ability of 40 undergraduate women to judge whether a man is gay or straight based on viewing a photograph of his face.

In the first of three experiments, the women looked at 80 images, half of which were gay men, and were told to determine each man's sexual orientation.

The women also reported their last menstrual cycle and its duration. None were on contraceptives.

The researchers found that the nearer a woman was to peak ovulation, the more accurate judge of sexual orientation she was.

"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," Nicholas Rule, lead author from the Department of Psychology at the University of Toronto, said in a release.

The second experiment involved 34 women viewing images of other women's faces, 100 lesbian and 100 straight women.

Researchers found no relationship between fertility and accurate judgments of the women's sexual orientations.

"This effect is not apparent when a woman is judging another female's orientation," Rule said.

"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."

The study, published in Psychology Science, also found that having romantic or sexual thoughts affected the women's judgment.

In the third experiment, 20 of 40 women were asked to read a story with a romantic encounter before judging the men's orientation with photos.

Women who read the story were significantly more accurate in their judgments, the study found.