Testosterone begets testosterone, evidently.

A new study has shown that women with high levels of testosterone are more likely to conceive sons than daughters. In effect, women whose genetic makeup is more suited to raising boys tend to deliver boys more often. This doesn't mean that a mom who plays hockey and pounds beers with the guys will definitely get a son; yet findings from Dr. Valerie Grant, a reproductive scientist at the University of Auckland, imply it's more likely.

What she has come up with is a mechanism that has proved (albeit in cows, which sounds odd but is considered an acceptable model) that levels of testosterone in the follicles (which produce the egg) reliably predict the sex of the embryo. And, more startling, the egg may well come out already adapted to receive an X or Y chromosome-bearing sperm. In lay-men's terms, this means that the female has already "decided" which sex to give birth to, even before sperm get involved.