Several years ago, my main research interest was the difference in skin pigmentation between women and men. In a nutshell, women are paler in complexion and men browner and ruddier because the latter have more melanin and hemoglobin in their skin. This sex difference dominated skin color variability in earlier social environments; therefore, skin color may have become a visual cue for gender-specific responses (e.g., sexual attraction, gender identification, conflict readiness, social distancing, etc.).
In a rating study, I showed female subjects several pairs of male facial photos, and in each pair one of the faces had been made slightly darker than the other. The darker face was more likely to be preferred by the women in the estrogen-dominant phase of their menstrual cycle (i.e., the first two-thirds) than by those in the progesterone-dominant phase (i.e., the last third). This cyclic change in preference was absent in women on oral contraceptives and in women who were assessing pairs of female faces (Frost, 1994).
At no point in the cycle was the darker male face more popular than the lighter one. It was simply less often disliked during the estrogen-dominant phase. As I saw it, higher estrogen levels seemed to be disabling a negative response to darker individuals. This negative response might be a social-distancing mechanism that keeps conflict readiness at a higher level during social interaction with males.
My study left some questions unanswered. What component of male skin color was triggering this response? Was it ruddiness (hemoglobin) or brownness (melanin)? And exactly what feelings were being triggered?
Some recent findings suggest that the trigger may be male ruddiness and the feelings something akin to intimidation. In the 2004 Olympic Games, opponents in boxing, taekwondo, Greco-Roman wrestling, and freestyle wrestling were randomly assigned red or blue athletic uniforms. For all four competitions, the ones who wore red uniforms were significantly likelier to win. This phenomenon was investigated by Ioan et al. (2007), who asked participants to name the color of words on a computer screen and measured the response time. The men took significantly longer than the women to respond when the words were red. Reducing luminosity increased response time for both men and women, but the gender gap remained. The authors concluded:
Our data suggests that “seeing red” distracts men through a psychological rather than a perceptual mechanism. Such a mechanism would associate red with aggression or dominance and may have a long evolutionary history, as indicated by behavioural evidence from nonhuman primates and other species.
With respect to our species, they state:
In humans, the adult male is ruddier in complexion than the adult female and male hormones greatly increase blood circulation in the skin’s outer layers. Testosterone influences erythropoiesis during male puberty and a decline of testosterone with aging increases the risk of anemia. Furthermore, men with hypogonadism or those taking anti-androgenic drugs frequently have anemia. These data are consistent with a testosterone-dependent ruddiness of the male complexion, as seen in many other species where red coloration acts as a signal of male dominance.
It would be interesting to repeat the above study with female subjects at different phases of the menstrual cycle. I suspect that response time would be longer among subjects in the progesterone-dominant phase than among those in the estrogen-dominant phase. In other words, the gender gap may be due to estrogen disabling a conflict-readiness mechanism that uses ruddiness as a visual cue for male identity.
Frost, P. (1994). "Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship", Perceptual and Motor Skills, 79, 507-514.
Ioan, S., Sandulache, M., Avramescu, S., Ilie, A., & Neacsu, A. (2007). Red is a distractor for men in competition. Evolution and Human Behavior, 28, 285-293.