It could also be that differences in academic performance (I'm assuming academic is the performance you were referring to in your comment) are not accurate metrics for identifying the effects of differential brain organization.

In scenarios like that, it's also difficult to control for the impact of different level of support for students in academic matters. For example, focused support for under-performing students compared to relatively relaxed support for students meeting expectations.

I could probably also see the impact of different psychological factors associated with being adopted by a family on a different continent being hard to control for.