I’ve just found this surprising study in Psychological Science that found a link between the point in the menstrual cycle of 77 white women and various measures of race bias.

Race Bias Tracks Conception Risk Across the Menstrual Cycle. Psychol Sci. 2009 May 4. [Epub ahead of print] Navarrete CD, Fessler DM, Fleischman DS, Geyer J. Although a considerable body of research explores alterations in women’s mating-relevant preferences across the menstrual cycle, investigators have yet to examine the potential for the menstrual cycle to influence intergroup attitudes. We examined the effects of changes in conception risk across the menstrual cycle on intergroup bias and found that increased conception risk was positively associated with several measures of race bias. This association was particularly strong when perceived vulnerability to sexual coercion was high. Our findings highlight the potential for hypotheses informed by an evolutionary perspective to generate new knowledge about current social problems-an avenue that may lead to new predictions in the study of intergroup relations.

The research paper is online as a pdf if you want the full details.

The authors explain the findings as suggesting that women show a preference to their ‘in group’, those who more closely match their own background and lifestyle, when most fertile.

Menstrual cycle has been found to influence numerous preferences in women in earlier studies, including dressing attractively, preference for the type of fanciable person, including a preference for more ‘masculine’ features.

Indeed, cycles in oestrogen are known to alter dopamine function in the striatum, a deep brain structure.

pdf of menstrual cycle and race bias study.

Link to PubMed entry for same.