People with racially diverse parents are perceived to be more attractive than those with parents of the same race. Michael B. Lewis, a psychologist at Cardiff University in Wales, gathered 1,205 facial photos of men and women who were black, white, or of any mixed race. Twenty men and women, unaware of the purpose of the study, then rated each photo for attractiveness. People of mixed race were in 40 percent of the photos, but they were overwhelmingly represented among those considered to be the bestlooking. Lewis suggests that this finding may be attributable to a genetic process called heterosis. "This is an idea, put forward by Darwin...that cross-breeding within species leads to offspring that are genetically fitter than their parents"—and that attractiveness "is probably the best indicator of genetic fitness."