Girls who need more money

An anonymous multinational hiring person writes of the difficulty they have paying new female hires as much as men:

The reason they don't keep up, from where I sit, is simple. Often, a woman will enter the salary negotiation phase and I'll tell them a number will be sent to them in a couple days. Usually we start around $45k for an entry level position. 50% to 60% of the women I interview simply take this offer. It's insane, I already know I can get authorization for more if you simply refuse. Inversely, almost 90% of the men I interview immediately ask for more upon getting the offer.

In my field it's slightly different; software engineering is not exactly oversupplied with people bearing double-X chromosomes, so I think we tend to pitch higher offers to good female candidates. Yes, this is technically discrimination, but frankly the software engineering girls I have known have had to put up with so much hyper-male-induced crap throughout their working lives to get here that they're well worth the money. If you don't know what I mean, you've clearly never spent time with the boys in this field.

But the girls all still accept the first offer we give them.

What confuses me somewhat is that a majority of the best negotiators I have known have been (hard-core career) women, so a simple sexist "girls can't negotiate" doesn't work. So where do the boys go to learn to say "you must be having a laugh!" while the girls stick with "oh, that's great, thank you very much"?