There are less consistent findings on sex differences in the heritability of ASD from twin research. Whereas early twin studies reported MZ twin concordance of 72%, compared with DZ twin concordance of 0%, with heritability estimate of 90%, 105 , 106 more recent work has shown lower MZ twin concordance rates (eg, 58% for male pairs and 60% in female pairs) and higher concordance rates for DZ pairs (eg, 21% for male DZ pairs and 27% for female DZ pairs). This narrowed distance in concordance rates between MZ and DZ twins yielded a substantially lower heritability estimate of 37% than that found in earlier studies. 107 These studies did not show sex differences in the heritability of ASD. Sex differences may also differ across development. For example, because females have lower persistence rates of ASD, 11 the sex ratio may differ in cross sectional studies as compared with studies of the trajectories of ASD across the life span. This may also apply to ADHD, because twin studies of ADHD have shown that the heritability of ADHD in youth is greater
