And this "difference" has a biological basis.

Well is that true or not?

It's hard to tell because it's practically impossible to run double-blind experiments on the process of evolution.

But it wouldn't be the first time that scientists gave their blessing to the norms of the day when the empirical evidence didn't truly support the assertion. Politics being the mind-killer, we should probably expect that morally controversial scientific results are biased. I suspect the bias is in favor of the status quo, but I'm sure there are counter-examples.