Being good-looking may be beneficial to a woman’s career—unless her boss is also an attractive female, as new research finds that women at the peak of their fertility tend to hold a sharper competitive edge toward other women they find attractive.

According to researchers from Wellesley College, women at their most fertile point are more likely to offer smaller raises to attractive female employees—25 percent less—than during periods of low fertility, when women become more generous with raises, offering 20 percent more.

Researchers believe this behavior comes from the hunter-gatherer days when women would compete with other women for high-quality male stock. "What we found supports the idea that, among women, competitiveness during periods of high fertility is linked to the withholding of resources from potential rivals," said Dr. Margery Lucas, professor of psychology. "Resource competition is important because women need to acquire products—clothing, makeup, accessories and so on—to enhance their attractiveness. By offering less to attractive women and keeping more for themselves, fertile women can help to enhance their own appearance and weaken a competitor's ability to do the same."

Lucas went so far as to suggest that women negotiate pay raises only after becoming privy to their female boss’s menstrual cycle.

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