Why do you think women make less money than men?

It’s common knowledge that women in general make less money than men, only like 78% of what men make if I recall correctly. I don’t know whether this is adjusting for the jobs that women hold more commonly or if it actually means that women working in the same positions as men are paid less on average.

I’ve heard some theories about why this is, ranging from straight sexism to women in general do less negotiating / asking for raises.

What’s your take? Do women just tend to work in less high-paying fields? If so, why? Are hirers just sexist? Do women do less negotiating? If so, why?