That's been known since antiquity. Anyone who's had a class in ancient art can tell you that artists in the ancient Mediterranean conventionally used red pigments for men and lighter, paler pigments for women, though they tended to exaggerate the natural difference. The reddish bias of male skin tone is almost common knowledge among artists and anyone who's ever had anything to do with measuring skin tone. It's just due to the gender difference in heme concentration in the blood.