[GoNative]

The native parent generally gets custody in most cases because the child has grown up, made friends, and speaks the language of that country probably a whole lot better than the other. It's generally less traumatic for the child to stay in the country they grew up in.

This issue is about the non native parent, who more than likely hasn't been granted custody, moving back to their home country and basically abducting their children with them. Because Japan is not a signatory to the convention on child abduction there is very little the custodial parent can do legally to have them returned. And this works both ways for children abducted from or to Japan.