When it comes to finding culprits for everything that’s wrong with us as children and even as adults, parents are everyone’s favorite default option. And why not? Decades of research in child development and psychology have linked maternal depression to children’s mental and physical illness as well as language and cognitive deficits, shown that when the parents’ marriage is riven by conflict children grow up to be emotionally insecure and have difficulty forming loving adult relationships, and found that when parents are under significant stress their kids are more likely to have behavior problems, to have difficulty handling stress, and to be at greater risk for mental illness. If there was any doubt about the power of this parental legacy, an upcoming study should dispel it: when parents are under emotional, financial, or other forms of stress, it can alter their children’s patterns of genetic activity at least through adolescence and perhaps longer. And since some of the altered genes shape brain development, the effects of parental stress might permanently wire themselves into children’s brains.
This is the first time scientists have ever found a link between parental stress in early childhood and the condition of their children’s DNA. As such, it represents the next frontier in the study of nature and nurture: identifying how the experiences we have (nurture) affect our DNA (nature). The groundwork was laid with research on (of course) lab rats, where scientists at McGill University showed that when mother rats devotedly lick and groom their pups, it sets off a wave of changes: it activates a gene that makes a receptor for stress hormones in the baby rats’ brains, which causes more receptors to be produced, which causes fewer stress hormones to course through the rats, which causes the rats to be well-adjusted, curious, and mellow rather than neurotic, fearful, and stressed-out (as rats whose mothers did not lick and groom them become).
Child abuse and even maternal depression, studies show, can do to people what neglectful rat mothers did to their pups: silence the stress-hormone receptor in the brain. In the brains of people who were abused as children and later took their own lives, the gene for the stress-hormone receptor is more likely to be “off” than it is in people who did not commit suicide or were not abused, found a 2009 study. In people, as in rodents, when this gene is silenced the stress-response system is on a hair trigger, making it extremely difficult to cope with life’s adversity, leaving the person more vulnerable to suicide. In newborns of depressed or anxious mothers, a 2008 study found, this stress-hormone-receptor gene also tends to be silenced. That would put the children, like the rats’ pups, at risk of growing up to be hyper-sensitive to, and unable to cope with, stress.
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The new study shows that childhood experiences that fall well short of abuse, or even of having a mother who is depressed, leave their marks on our DNA. Led by Marilyn Essex, professor in the Department of Psychiatry and director of the Life Stress & Human Development Lab of the University of Wisconsin, Madison, scientists gave questionnaires to hundreds of parents, who were part of a years-long study, when their kids were infants and again when they were 3½ and 4½ years old. The questions asked about depression, financial stress (“how much trouble do you have paying monthly bills?”), marital stress (“are you concerned about how much you argue and fight?”), parenting stress (“do you often feel angry with your child?”), and feeling overwhelmed (“do you feel pulled apart by all your obligations?”). When the kids were 15, the scientists analyzed their DNA, using cells obtained by cheek swabs.
The scientists found hundreds of differences in the DNA of kids whose parents were in the top 20 percent of stress, they will report in an upcoming issue of the journal Child Development. After applying a rigorous statistical filter to make sure any “hits” were not random flukes, they found that mom’s high stress during the children’s infancy altered 139 genes, while dad’s stress during the children’s pre-school years altered 31 genes. Curiously, mom’s stress during the pre-school years, and dad’s stress during his kids’ infancy, had no genetic effects, perhaps reflecting how dads become more involved once their kids graduate from breast feeding and diapers. In another intriguing finding, although mothers’ stress affected both daughters and sons equally, fathers’ stress had more effect on daughters than sons. That jibes with the findings that when dad is emotionally or physically absent, girls tend to enter puberty earlier and develop difficult temperaments, while boys do not.
The genes that were altered—usually, silenced—by mom or dad’s stress included one involved in insulin signaling, a DNA-repair protein, a cell-signaling protein, and a protein in the cell membranes of neurons. The last two, says Essex, are “involved in brain development and behavior.” In mice, damage to the cell-signaling protein makes the animals fearful and jumpy in unfamiliar environments; in people, damage to the membrane protein causes neuronal impairment. Perhaps the most fascinating gene that was found to be altered by parental stress is one called NEUROG1; it promotes the creation of new neurons in the brain, which is crucial for neurological development, learning, and memory.
That said, the on-off status of genes in cells scraped from the cheek may not be identical to the on-off status of genes in brain cells. The point, says Essex, is not to identify the specific genes that parental stress changes, but to establish a fundamental principle: that the pathway by which parental stress affects children physically, emotionally, and cognitively wends through our DNA.
What might be the impact of these genetic changes? If they affect fundamentals of brain development, it may be permanent: although the adult brain retains enormous powers of plasticity, the window when the basic wiring takes place tends to slam shut by early adulthood. In other words, some DNA changes, particularly those in genes that choreograph brain development, may have a lifelong legacy. But that “may” is crucial. Just as parental stress can alter DNA for the worse, so later experiences might alter it for the better. That is at the top of Essex’s to-do list: identifying the influences that can undo the harmful genetic and other effects of parental stress. Of course, prevention is more effective than treatment. Kids Pick Up on Everything: How Parental Stress Is Toxic to Kids, a forthcoming book by Episcopal minister/family therapist David Code argues that the best thing parents can do for their kids is reduce their own levels of stress.
Since we already know that parents’ marital and financial stress can hurt children’s development, a skeptic might ask whether the new study advances the ball. It does. By showing how parenting exerts the effects it does—namely, by altering which of a child’s genes are turned on and which are turned off—it makes those influences much more real and concrete, much the way brain imaging studies that show junkies’ brains when they crave heroin made addiction much more real and concrete: just as a junkie can’t simply summon the willpower to kick his addiction, so a child cannot just shake off the legacy of a troubled infancy and adolescence. That legacy has altered her very DNA.
