Editing Twin study (section)

=== Sex differences ===
Genetic factors, including both gene expression and the range of gene × environment interactions, may differ between the sexes. Fraternal opposite sex twin pairs are invaluable in explicating these effects.

In an extreme case, a gene may only be expressed in one sex (qualitative sex limitation).{{clarify|date=September 2022}}<!-- does this refer only to genes that can't be expressed in one sex, e.g. in the genitals, or does it also refer to genes expressed only in one of the twins, e.g. an X-linked recessive trait? --> More commonly, the effects of particular alleles may depend on the sex of the individual. A gene might cause a change of 100&nbsp;g in weight in males, but perhaps 150&nbsp;g in females – a quantitative gene effect.

Environments may impact on the ability of genes to express themselves and may do this via sex differences. For instance, genes affecting voting behavior would have no effect in females if females are excluded from the vote. More generally, the logic of sex-difference testing can extend to any defined sub-group of individuals. In cases such as these, the correlation for same and opposite sex DZ twins will differ, betraying the effect of the sex difference.

For this reason, it is normal to distinguish three types of fraternal twins.  A standard analytic workflow would involve testing for sex-limitation by fitting models to five groups, identical male, identical female, fraternal male, fraternal female, and fraternal opposite sex. Twin modeling thus goes beyond correlation to test causal models involving potential causal variables, such as sex.