Fertilization, say biologists, is a maniacal, winner-takes-all race between millions of sperm. If the first to arrive bears an X chromosome, the result is a girl. If a Y-sperm shows up first, it's a boy.
But a New Zealand psychiatrist believes it's more complicated than that. Valerie Grant, M.D., contends that mothers, not dads, determine the sex of a baby -- and that a woman's personality may be a window to her son- or daughter-bearing potential.
Grant assessed the personalities of young women likely to conceive soon, then checked back nine months later. Those who bore sons, she found, were more assertive, competitive, and self-reliant than those who had daughters, she reports in the British Journal of Medical Psychiatry.
Her theory: Testosterone levels influence both a woman's behavior and the sex of her kids. (Yes, women have testosterone too, though far less than men.) Grant believes the hormone turns an egg into an active player in the fertilization sweepstakes, causing it to play favorites as sperm try to fertilize it. When testosterone is high, for example, an egg might turn an X-sperm away, letting a nearby Y-sperm sneak in.
As Grant sees it, there's a key testosterone level above which a woman's offspring wind up a particular sex. But our testosterone levels aren't stable: Stress and other factors influence its production. So women with average testosterone might vary on either side of that point over time, giving birth to daughters or sons as conditions change. But women with extreme levels may remain on one side of the threshold most of the time, dooming them to children of a single sex.