Dr. Nestler is not sure exactly how the mouse fathers’ trauma communicates itself to their offspring. It may be via sperm, or it may be through some more complicated dance of nature and nurture that involves sperm but also other factors. When instead of letting the “defeated” mice mate, Dr. Nestler’s researchers killed them, harvested their sperm and impregnated the female mice through artificial means, the offspring were largely normal. Perhaps the sperm was harvested at the wrong stage in the process, says Dr. Nestler. Or maybe the female mouse picked up some signal when she had sex with the dysfunctional male mouse, some telltale pheromone or squeak, that made her body withhold nutrition and care from his pups. Females have been known to not invest in the spawn of non-optimal males, an outcome that makes perfect evolutionary sense — why waste resources on a loser?

When it comes to the epigenetics of aging, however, there is little question that the chemical insults and social setbacks of everyday life distill themselves in sperm. A woman is born with all the eggs she’ll ever carry. By the time a man turns 40, on the other hand, his gonad cells will have divided 610 times to make spermatozoa. By the time he’s in his 50s, that number goes up to 840. Each time those cells copy themselves, mistakes may appear in the DNA chain. Some researchers now think that a percentage of those mistakes reflects not just random mutations but experience-based epigenetic markings that insinuate themselves from sperm to fetus and influence brain development. Another theory holds that aging gonad cells are more error-prone because the parts of the DNA that should have spotted and repaired any mistakes have been epigenetically tamped down. In any case, we now know that the children of older fathers show more signs of schizophrenia, autism and bipolar disorder than children of younger ones.

In a meta-analysis of a population study of more than a million people published last year, Christina Hultman of the Karolinska Institute of Sweden concluded that children of men older than 50 were 2.2 times as likely to have autism as children of 29-year-olds, even after the study had factored out mothers’ ages and known risk factors for autism. By the time the men passed 55, the risk doubled to 4.4 times that of 29-year-olds. Can the aging of the parent population explain the apparent spike in autism cases? A study published last month in Nature that used whole-genome sequencing on 78 Icelandic families made the strongest case to date that as fathers age, mutations in their sperm spike dramatically. Some of the mutations found by the researchers in Reykjavik have been linked to autism and schizophrenia in children.

In his Washington Heights laboratory at the New York State Psychiatric Institute, Jay Gingrich, a professor of psychobiology, compares the pups of young male mice (3 months old or so) to those of old male mice (12 to 14 months old). The differences between the pups, he told me, weren’t “earth-shattering” — they weighed about the same and there weren’t big gaps in their early development. But discrepancies appeared when the mice grew up. The adult offspring of the older fathers had less adventuresome personalities; they also reacted to loud noises in unusual ways that paralleled reactions evinced by schizophrenics who heard similar sounds.

Still, Dr. Gingrich said, “the differences were subtle” until he decided to pool the data on their behavior and graph it on a bell curve. A “vast majority” of the children of the older mice were “completely normal,” he said, which meant their score fell under the upside-down parabola of the curve. The real differences came at the tails or skinny ends of the bell curve. There was about a sixfold increase in likelihood that one of the “abnormal outliers,” mice with cognitive or behavioral handicaps, “would come from an older father.” Conversely, the super-high-performing mice were about six times more likely to come from a younger father. “I’m an inherently skeptical person,” Dr. Gingrich told me, but he was impressed by these results.

One unanswered question about autism and schizophrenia is how they crop up in generation after generation; after all, wildly dysfunctional individuals don’t usually flourish romantically. “I think we’re going to have to consider that advanced paternal age, with its epigenetic effects, may be a way of explaining the mysteries of schizophrenia and autism, insofar as the rates of these disorders have maintained themselves — and autism may be going up,” Dr. Gingrich said. “From a cruel Darwinian perspective, it’s not clear how much success these folks have at procreating, or how else these genes maintain themselves in the population.”

When you’re an older mother, you get used to the sidelong glances of sonogram technicians, the extra battery of medical tests, the fear that your baby has Down syndrome, the real or imagined hints from younger mothers that you’re having children so late because you care more about professional advancement than family. But as the research on paternal inheritance piles up, the needle of doubt may swing at least partway to fathers. “We’re living through a paradigm shift,” said Dolores Malaspina, a professor of psychiatry at New York University who has done pioneering work on older fathers and schizophrenia. Older mothers no longer need to shoulder all the blame: “It’s the aging man who damages the offspring.”

Aging, though, is only one of the vicissitudes of life that assault a man’s reproductive vitality. Think of epigenetics as having ushered in a new age of sexual equality, in which both sexes have to worry about threats to which women once felt uniquely exposed. Dr. Malaspina remembers that before she went to medical school, she worked in a chemical plant making radioactive drugs. The women who worked there came under constant, invasive scrutiny, lest the toxic workplace contaminate their eggs. But maybe, Dr. Malaspina points out, the plant managers should have spared some concern for the men, whose germlines were just as susceptible to poisoning as the women’s, and maybe even more so. The well-being of the children used to be the sole responsibility of their mothers. Now fathers have to be held accountable, too. Having twice endured the self-scrutiny and second-guessing that goes along with being pregnant, I wish them luck.