What do you mean by "obviously biological"?
In general, Men have a higher diversity in IQ and Risk Taking.
The TL;DR is that Men will have a tendency to take higher risks than women, which is then rewarded more, if it succeeds, however, it is punished more severely when one fails. Punishment generally involved poverty and unemployment.
This doesn't mean women cannot take risks, just that they have a lesser inclination to.
Negotiation for a wage on your job is a form of risk taking, in case of failure you can certainly sour your work relationships which ruins your long-term income projection.
There will be exceptions but they're not the norm.