Oh I agree that behavioural differences are to some extent caused by biology, and that biology has been determined by evolution. That is obvious, for the reason

Oh I agree that behavioural differences are to some extent caused by biology, and that biology has been determined by evolution. That is obvious, for the reasons you state. What I am attacking is the idea that we can be at all certain about which behaviours are biologically influenced and which are socially constructed, or about the extent to which each factor is influential. Both factors self-evidently interact in highly complex ways to produce the behavior we observe. Unfortunately it is almost impossible to isolate either one of them, largely due to ethical problems with the experiments that would be necessary. The only thing that we can be certain of IMO is the fact that all behaviour is inextricably influenced by both nature and nurture.