all environmental factors that increase resemblance of relatives for non-genetic reasons, are correlated unity in same-sex relatives and φ is the correlation among environmental factors in male-female pairs. When γ<0.5 there is evidence for qualitative sex differences, i.e. for the hypothesis that different genes are expressed in men and women. When γ<0.5, the observed correlation in opposite-sex relatives will be lower than predicted from the resemblances in same-sex relatives. However, an alternative explanation for this observation might be that environmental sources of covariance between relatives differ in men and women. When φ<1, male-female pairs share fewer environmental factors than same-sex pairs.
