how do they separate culture from language? i am bilingual. i switch between english and spanish depending on the cultural context. sure, my behaviour changes - but why is that connected with the language, and not with the culture?
[edit:] traditionally you work around this kind of thing by finding subjects that differ only in one variable. identical twins, for example, might better show environmental differences, because they have common genomes. but while that works for nature/nurture i have no idea how you do something similar for culture/language.