Sex‐specific concordance rates can be used to infer differences in genetic and environmental factors underlying a trait or disease. Similar concordance rates among male‐male and female‐female pairs indicate similar underlying genetic factors, whereas differences may indicate either different genes or environmental exposures to the disease. Opposite‐sex twins are particularly informative in elucidating the relative contribution of genetic and environmental factors. if the same genes contribute to an outcome in males and females, then the opposite‐sex DZ twin pair correlation should lie between the correlations of the male‐male DZ and female‐female DZ pairs. However, if the correlation in opposite‐sex DZ twin pairs is significantly lower than that of the same‐sex pairs, the same genes do not contribute to the trait in males and females. 5 Data from opposite sex twin pairs have been used as evidence for within utero “testosterone transfer” 33 , 42 , 97 for some phenotypes such as epilepsy 98 and language impairment. 99 However, meta analyses of phenotypes such as body mass index and height have not supported the impact of prenatal hormone exposure on these phenotypes in later life.
