THE NATURE OF SPERM. Linda Hirshman's discussion of the history of religious thinking about sperm reminded me that there have been some fascinating studies coming out lately about the biology of human reproduction. And since the only people who know less about their bodies than women are men, the male readers of this site may be interested to learn that whether they know it or not, they, too, have biological clocks that tick.

Male fertility decreases markedly with age. While women have been subjected to an endless stream of anxiety-provoking magazine articles and books about their diminishing reproductive capacity over time, men are much less commonly warned that if they want to have healthy children when they marry, they, too, ought to marry and have children at younger ages. Unlike women, men never completely lose their fertility. But once they hit age 40, it becomes much, much harder for them to impregnate a woman. Says one of the new studies:

New research indicates that the genetic quality of sperm worsens as men get older, increasing a man�s risk of being infertile, fathering unsuccessful pregnancies and passing along dwarfism and possibly other genetic diseases to his children.

Just as older women are at higher risk of bearing a child with birth defects, older men are at increased risk of passing on chromosomal abnormalities due to an increase in DNA fragmentation and mutations in their sperm. This is of increasing significance because, while nature has put limits on the age at which women can bear young, men are increasingly pushing fatherhood into their later years on the theory that they can safely do so:

Since 1980 there has been about a 40 percent increase in 35- to 49-year-old men fathering children, and a 20 percent decrease in fathers under 30.

And yet their increasingly mutant sperm increase their risk of fathering disabled children as they age. So, too, do other factors make it harder for them to even have kids. According to another recent study:

A man's fertility appears to decline after the age of 40, in much the same way that a woman's ability to conceive fades after 35, according to French researchers. Their study, of nearly 2,000 couples undergoing fertility treatment, found that pregnancy attempts were 70 percent more likely to fail when the man was age 40 or older than if he were younger than 30 -- regardless of his wife's age.

So nature is less inequitable in her treatment of men and women than has long been imagined. Hirshman suggests that women should marry men younger than themselves for reasons of social equity, but, as I noted this 2002 Prospect story, there are also some very compelling biological arguments for single women in their 30s who wish to have children to seek a mate who is younger.

--Garance Franke-Ruta