According to this theory(link)

When I had my tits cut off my IQ went up because it is a trade off you see. Who knew? Exactly how do I get to equate my new found, surgically enhanced IQ? How much smarter did I get? Is it one 1 point per pound, 5 points per pound, someone let me know; there are a few lawyers I want to let know that I am now definitely much brighter than they will ever be. Just so we are clear, the equation is 4 pounds of breast tissue removed = +x IQ points. The question is what is x. Also, if you have your breasts surgically enhanced do you lose IQ points?

Update:
The link is to Digby who links to Fair where I found the quote which I am now sharing:

"More Brain Or More Penis"

Rushton (who's gotten more than $770,000 from Pioneer) has transformed the Victorian science of cranial measurement into a sexual fetish--measuring not only head and brain size, but also the size of breasts, buttocks and genitals. "It's a trade-off: More brain or more penis. You can't have everything," he told Rolling Stone's Adam Miller (10/20/94), explaining his philosophy of evolution.