> we are not talking about physical labor here, but work that requires the use of your brain.
English is not my first language and I'm not really familiar with the use of word 'weak' to describe mental capacity. Personally, I perceive women to be just as capable as men when it comes to intellectual work.
I don't agree with you that salary measures only negotiation skills. If you are a god-tier haggler, but zero in programming, you wouldn't have big salary for long. Your compensation is based mostly on the value you provide, which is only somewhat influenced by your negotiation skills.
You yourself say that it is very hard to objectively measure the performance of an intellectual worker. I think that is not just very hard, but simply impossible. Consequently, we can't pay people equally for equal performance because we can't even measure that performance to begin with.
In contrast, direct negotiations system is fair, and works both ways. If you think you are underpaid, go on a job market and find a better paying job. If you can't, your perceived market value is lower than you think. If you can, and leave for greener pastures, your current employer will lose a skilled worker, because he underpaid you.
Rational employer will seek to maximize the value extracted from employees, and hire the cheapest candidate with sufficient skills. Rational worker will seek the best paying job. Eventually the job market reaches an equilibrium, and everyone is paid what he or she is currently worth in current economy.
So if some woman thinks that she's paid less than her male colleague, she can just test herself against the marker. If she can't find an equally or better paid job, maybe she's overvaluing herself. And if she can, well, she can simply change job for a better paying one!
(And we don't need to make some bizarre attempts to create a performance measurement system, which inevitably will be flawed and inaccurate)