Women exposed to the now-banned environmental pollutant polychlorinated biphenyls, or PCBs, are far more likely to give birth to girls. A long-range study of women exposed to PCBs in the 1950s and 60s has shown that the women were 33% less likely to have boy children. This may be a natural reaction that occurs in many species when the environment becomes more hazardous, since having more girls means the species can continue reproducing at a decent clip even if many babies die. [PhysOrg]