A recent study of yellow baboons suggests that environmental stress leads to infertility in them. In a season when food is scarce, conception rates are likely to halve. Scientists could correlate the environmental stress to low levels of progesterone in these primates. The study strongly suggests that reducing stress levels could be a powerful therapy for infertility problems. Sam Wasser of the University of Washington, US, is busy recruiting 1,000 infertile couples to test the theory in humans and investigate how best to incorporate stress reduction as part of the treatment. Over 10 per cent of infertility problems in women stem from inadequate levels of progesterone ( New Scientist , Vol 151, No 2044).