IN THE YEARS AFTER HER first mouse experiment, Isabelle Mansuy replicated the study more than a dozen times. Still, it took almost a decade before she was able to publish the results. Mansuy attributes the delay to the reluctance of the scientific establishment to embrace the idea of epigenetic inheritance.

The delay might, of course, be due to the science itself, not the scientific establishment. There are complaints that can be leveled against advocates of epigenetics. The most significant may be that there are holes in their reasoning. Critics point out that the existence of epigenetic changes in people and animals with a history of stress does not mean that one caused the other. The connection may be there, but so far scientists have failed to trace the causal chain. Nor have they produced a full account of how epigenetic activity can be passed down a generation. And it won’t be easy to do so. Epigeneticists can’t recreate a traumatic childhood in the lab, and they can’t take samples from the organ in which epigenetics does some of its most interesting work: the living human brain.

There’s also the possibility that something other than epigenetics may explain the results of Mansuy and others. Last year, an American geneticist named Eric Nestler described an experiment similar to Mansuy’s, but with a clever twist. After stressing male mice, Nestler extracted their sperm and, using IVF, impregnated female mice. Because the mothers never met the fathers of their pups, Nestler had eliminated the possibility that spending even a brief time with a troubled male might be enough to damage a female’s parenting skills. When he ran behavioral tests, Nestler saw only “very modest” signs of depressive behavior in the offspring, not the significant changes seen in Mansuy’s mice.

The study doesn’t disprove Mansuy’s claims. There was still some transmission of behavior and, perhaps more importantly, Nestler had applied the original stress to adult mice, which may have limited the impact it had on the animal’s epigenome. But Nestler’s results did demonstrate how much more work needs to be done before the implications of Mansuy’s work can be accepted — or rejected.

Even if Mansuy’s mouse studies are replicated, skeptics will have at least one more question. For scientists who work with rodents, the similarities between mice and rats and humans abound. To the rest of us, such comparisons can seem bizarre: what can a pair of caged rats possibly tell us about a man who beats his wife? In fact, the two positions are not completely at odds. Rodents share many genetic, physiological and behavioral traits with us — enough to have proved an extraordinarily useful way of studying humans. But they also lack so many aspects of human behavior that it’s easy to over-interpret the results of rodent studies.

“It’s a long way from differential methylation to behavior, physical health or mental health,” says Greg Miller, a psychologist at Northwestern University in Chicago. Ultimately, the claims of epigenetics need to be evaluated in humans.

But working on the human epigenome is difficult, says Meaney. Epigenetic markers do not always manifest in two places in the same way, making hard and fast conclusions rare. For all of that, he believes the science will reveal that patterns of violence like those in Yokia’s family are at least partially rooted in biology.

“Does witnessing violence produce a stable epigenetic signal that would then influence physiology and behavior?” he asks. “Yes, if I had to bet my house down, then yes.”

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IF THE EPIGENOME REALLY IS the molecular manifestation of Yokia’s ghost, studying it may lead to new ways of sparing others from similar demons. Meaney, for one, believes epigenetics speaks directly to how we structure our societies. For too long, he says, policymakers have intervened in places like Dogtown using reforms based on shoddy science, or even worse — no science at all. Epigenetics could present a smarter alternative. “What we need are objective predictors of who is at risk,” he says. The epigenomes of people in neighborhoods like Dogtown might contain that information; we could use the results to assess the impact of the environment on residents, and to identify those upon whom it takes the greatest toll.

Epigenetics could also lead to a better understanding of why people commit crime. In 1984, Richard Tremblay began tracking 1,000 kindergarten-aged boys from Montreal’s poorest neighborhoods. Today, those kids are in their mid-30s and Tremblay’s study is one of the longest, and largest, longitudinal experiments conducted.

One of Tremblay’s more surprising results is the suggestion that the peak of aggressive behavior in humans, as measured by his subjects’ willingness to resort to violence, generally occurs around the ages of two and three. As soon as children gain a decent control of their muscles, their use of aggression as a problem-solving mechanism spikes. The finding cuts across much conventional thinking on violence, which holds that people learn to be aggressive, perhaps by witnessing violent events.

“Physical aggression is a natural, normal response to solving a problem,” says Tremblay, who is now based at University College Dublin in Ireland. “As we get older, humans learn not to aggress, rather than the other way around.” (As Tremblay has noted, Thomas Hobbes made this point in 1647: “An evil man is rather like a sturdy boy, or a man of childish mind, and evil is simply want of reason at an age when it normally accrues to men by nature governed by discipline and experience of harm.”)

Roughly four percent of Tremblay’s cohort are what he calls “chronically violent offenders”: people who have committed murder and other serious crimes. In 2006, Tremblay studied blood and saliva samples from this group. Several hundred genes, including some that have been linked to aggression, were marked by patterns of epigenetic activity that differed from those he saw in the rest of the group. “It looks like there is an epigenetic basis for the transmission of violence, and not only does it look like it but it makes sense,” Tremblay said.

It’s not hard to see that such a project would generate both enthusiasm and concern. A more accurate means of honing in on the most troubled children and those at risk of committing crimes would be a boon to social services.

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BUT WE SHOULD BE CONCERNED whenever simple labels are proposed for complex problems. Even though epigenetics may be part of the reason why people abuse drugs or turn to crime, it almost certainly can’t explain the detailed trajectory of a person’s life, or their ability — or inability — to change its course. Yet if the field continues to attract attention, some scientists will claim that it can, just as some eugenicists claimed that genetics could explain criminality.

“If the funding stream for research and services moves towards trying to classify people, then you’re going to evaluate people based on their physical experiences to the detriment of not just how these people look but how they suffer,” says Jaleel Abdul-Adil, a psychologist at the University of Illinois at Chicago who works with at-risk youth.

There’s another thorny question raised by the new science: that of responsibility. We know, for instance, that smoking during pregnancy increases the risk that a child will develop asthma. Last year, scientists working with a rat model of the disease showed the effect extends at least a generation further: the grand-pups of rats exposed to nicotine during pregnancy also suffered from lung problems. So are smoking mothers hurting their grandchildren?

“And what if my alcohol consumption affects my grand-offspring?” asks Bruce McEwen, a neuroscientist at the Rockefeller University in New York. “Or if the stress of one generation affects others? If you’re going to pass on your life experiences to subsequent generations, what do we do as policymakers? Especially when we may not see effects of our interventions for generations. Right now we evaluate success on short term, but epigenetics tells us we need to rethink that.”

Yet the most immediate implication of epigenetics is less controversial — and potentially far more powerful. Recent research suggests that the epigenome is less malleable later in life, and that the epigenetic marks laid down by childhood trauma are stubborn. If that’s the case, we may be too late if we intervene only after epigenetics starts to influence adult behavior. “Things start going wrong very early, but we don’t know it, we can’t observe it,” says Tremblay. “So we wait…then it’s too late because the brain has not developed in a way that enables the individual to get control of himself.”

The idea that we should intervene early in a troubled life is not a new one. When I asked Tremblay about the policy implications of epigenetics, he cited the importance of having nurses visit families at home to advise on health and parenting — an idea that is over a century old and known to bring about real health benefits. “The interventions are quite simple,” he said. “But we know that, and have known that for a long time.” The problem, he noted, was a reluctance to pay for those nurses. If epigenetics becomes more widely accepted, the field’s first contribution could be a renewed interest — and perhaps funding — for an old idea.

Gary Slutkin is an American epidemiologist who spent much of his early life studying infectious diseases in places like Somalia and East Africa. He now works in Chicago for Cure Violence, an organization he co-founded in 1995. Slutkin sees epigenetics as a tool that can help a new generation of health workers to intervene to prevent violence — in his language, to become “interrupters”.

“Communities need more intervention, they need more care,” he told me. “It’s analogous to going to Bangladesh, where there are certain communities that have more diarrhea. So they need the intervention more. In Bangladesh, you need more training about oral rehydration and improving sanitation. For violence, you need more interrupters, that’s the new health worker.”

The results of epigenetics should force a broader rethinking of violence, Slutkin added. “If we look at it this way, we’re able to take it out of the realm of morality. We used to think of people with leprosy as bad people because we didn’t understand what was happening. Epigenetic damage is invisible, and neuronal circuits are invisible — so until we start to talk about violence as science we’re still in the Middle Ages.”