Washington, June 23 (IANS) Ovulating women seem to be more suspicious of male strangers, says a new sociological study.

Male strangers may have posed considerable risk of sexual coercion throughout human history, said Melissa McDonald, doctoral student in psychology and study co-author.

"Our findings suggest that women's prejudice, at least in part, may be a by-product of their biology," added McDonald, at the Michigan State University.

This is especially true of women who perceived such men as physically threatening, said Carlos David Navarrete, assistant professor of psychology at Michigan, who led the study.

Researchers involved two groups of women to investigate how their implicit attitudes towards men change across the menstrual cycle, reports the journal Psychological Science.

They found that fertile women were more biased against men of different races and men of different social groups than men of their own group, according to a Michigan statement.

McDonald and Navarrete said their findings are consistent with the idea that women's prejudice may reflect the workings of an evolved psychological system that once functioned to protect them from sexual coercion, particularly when they are at their most fertile.

To minimize this threat, McDonald said, women may be more biased against men who have posed the greatest risk to their reproductive choice.