Researchers at Michigan State University asked 252 female university students, both Caucasian and black, to look at photos of men’s faces, also both Caucasian and black. The women then had to link each face with either a physical adjective, such as muscular, or a mental one, such as brainy.

The women, who were not pregnant nor on hormonal birth control, also recorded their menstrual cycles.

Their study found that fertile women were less accepting of the men they perceived as being muscular if the men were of different race than their own. And because that bias jumped when women were at the peak of their menstrual cycle, when they are most fertile, that prejudice appears to be partly innate, said lead study author Melissa McDonald said.