Why are some people gay? Most researchers who study sexual orientation think that both genetic and environmental factors play a role, but the relative contributions of each remain unclear. A new study of Swedish twins reinforces earlier findings that environmental influences--including the environment in the womb--may play a greater role than genes. Scientists studying complex human behaviors often turn to twin studies. Researchers look at both identical and fraternal twins to see how often they share a trait--a parameter called concordance. The greater the concordance among genetically identical twins compared with fraternal twins--who share only half of their genes--the more...