Women who use contraceptive pills are 50% less competitive during the 21 days of the month when they take the drug, which contains the sex hormone progesterone, according to Thomas Buser of the University of Amsterdam. Moreover, in an experiment involving compensation schemes for a problem-solving task, women were 10.5 percentage points less likely to opt for a competitive scheme if they were at the point in their monthly cycles when fertility was low because progesterone levels were high.