There are definitely demonstrable differences between male and female brains. It's hard to tell how much of that develops pre-natal (effects of various prenatal hormones on foetal brain development) and how much is part of the different socialisation and childhood male and female children go through. There are areas with different neurone density or areas which have different levels of activity when performing a particular task. Nevertheless, pretty much any neuroscientist would be able to tell a male from a female human brain most of the time on a 'blind test'.
Men and women therefore tend, over a large population, to perform to a different average standard at different intellectual tasks. This of itself is not sexism, there's absolute mounds of empirical evidence to show this is true. What is sexism is assuming that because a particular person is male or female, they therefore inevitably will be inferior at a given task even if, on average, that would be true of their gender. There are women who excel beyond the average male at tasks which, over the whole population, men would be better at. And vice versa. You should judge people on their individual merits, not just make an assumption because of the presence or lack of a y chromosome.
