Nonetheless, a virtual tidal wave of recent research has made it irrefutable: Not only does male fertility decrease decade by decade, especially after age 35, but aging sperm can be a significant and sometimes the only cause of severe health and developmental problems in offspring, including autism, schizophrenia, and cancer. The older the father, the higher the risk. But what's truly noteworthy is not that infertility increases with age—to some degree, we've known that all along—but rather that older men who can still conceive may have such damaged sperm that they put their offspring at risk for many types of disorders and disabilities.
The mechanism seems to be that as men age, their DNA does not replicate as accurately as at earlier ages. Therefore, small abnormalities in the genetic struction which are not large enough to be fatal, can cause a wide array of damage to the child:
These mutations could reflect the differences in male and female reproduction, notes Jabs (Ethylin Wang Jabs, professor of pediatric genetics at Johns Hopkins University). By the time females reach their teen years, their eggs have already been formed—just one new egg matures each month. Men, on the other hand, produce millions of sperm cells every time they ejaculate. After each ejaculation, they must literally replicate those cells, and each replication multiplies the chance for a DNA "copy error"—a genetic chink in the sperm DNA. The more ejaculations a man produces, the greater the chance for chinks to arise, leading to increased point mutation and thus increased infertility and birth defects. While a woman's reproductive capacity halts more or less abruptly after all her eggs have been used up somewhere in their forties or fifties, men experience a longer, more gradual winnowing and disintegration. "We believe that something in men's DNA replication machinery keeps becoming less efficient and less accurate with age, and the problems accumulate," says Jabs.
This is yet another unintended consequence of delaying childbirth, a delay made possible by conctraception. We are increasingly organizing society around the premise that sex is essentially a sterile activity, with childbearing thrown in as an after-thought, if you happen to like that sort of thing. Widespread access to contraception has made indefinite postponement of childbearing the norm, not the exception among the educated classes.
It is time to rethink this.