Nonhuman studies would almost certainly be more definitive as gender identity (in humans) is hugely compounded with exogenous societal, cultural, political, and medical factors. As an example of the latter, until recently considered a diagnosible and treatable medical and/or psychological disorder.

Disentangling those factors would be exceedingly difficult, although longitudinal cross-cultural studies (possibly widely separated twins) might get you somewhere.

Animal model populations' medical institutional bias is somewhat lower.