Genes, Not Experience
Explain Why The Lives
Of Some Take Bad Turn
June 16, 2006; Page B1
Defenders of cigarettes aren't known for cutting-edge science, but give them credit for one insight.
When the link between smoking and lung cancer was emerging, smoking apologists asked whether some unknown biological factor (call it X) might have two effects, causing people to smoke and causing them to develop cancer. In that case, although the higher rate of illness among smokers might seem to implicate smoking, in fact X would be the cause of both smoking and cancer. Smoking would be an innocent bystander.
That turned out to be hogwash, of course. But the premise isn't crazy. Indeed, it has returned to center stage in studies of how life experiences increase a child's risk for depression, behavior problems and other bad outcomes.
The approach is called genetically informed research. The idea is to not be fooled into believing that a certain experience leads to a certain outcome but to probe whether a genetic predisposition (the X factor in the smoking analogy) leads to the experience and the outcome.
Take the conventional wisdom that their parents' divorce increases the risk that children will develop depression. "It turns out that the increased risk of depression in these children reflects a common genetic liability in the parents and kids," says Brian D'Onofrio of Indiana University, Bloomington. Since depressed people have more trouble getting and staying married, this genetic risk of depression raises their risk of divorce, he finds in a study submitted for publication. Parents pass that risk of depression to kids through DNA, not failed marriage.
"The same genetic risk that makes the parents more likely to divorce also makes the kids more likely to develop depression," he says.
In studies like this, scientists don't identify actual genes. Instead, they compare identical twins to fraternal twins to full siblings, half-sibs and step-sibs, who have varying degrees of genetic relatedness. If identicals are more affected by an experience than step-sibs, the experience may not have the causal power it seemed to. In the case of parental divorce, children of identical twins who suffered from depression also had increased depression, regardless of whether their parents were divorced, suggesting that parents' broken marriage is not at fault.
Children whose mother smoked during pregnancy have a higher risk of conduct and behavior problems, even criminality. This led to the hunch that nicotine exposure might change the fetal brain in a way that leads to greater impulsivity or risk-taking, says Prof. D'Onofrio. But might the kids' problems arise from inheriting genetic X factors that also led mom to smoke while pregnant?
"When we used a genetically informed approach, we found no association between maternal smoking and children's conduct problems," says Prof. D'Onofrio, who presented his results to the annual meeting of the Association for Psychological Science. Instead, likely culprits include X factors such as lower intelligence, impulsivity and aggression, which are partly genetic.
Teen motherhood is widely viewed as raising a girl's risk for dropping out of school and falling into poverty. "But who becomes a teen mother isn't random," says Sara Jaffee of the University of Pennsylvania. "Pre-existing differences might explain the different outcomes."
That is what she and colleagues found. Low IQ, conduct problems and antisocial behavior lead to both pregnancy and dropping out. Since the culprit is not pregnancy per se, efforts to help young women finish school would do well to focus not on pregnancy prevention but on learning, impulse control and conduct. All are only partly genetic and can be affected by intervention.
In the cases of divorcing parents, smoking moms and pregnant teens, genetically sophisticated studies show that what was thought to be the causal factor isn't. Instead, something inherent in the children affected by divorce, smoking or pregnancy is at work.
Other studies using a genetically smart approach, however, put the onus squarely on the bad experience. Abused children are more likely to develop antisocial behavior and even criminality, for instance. Something genetic might make parents more likely to abuse their children, and the kids -- who inherit that -- more likely to become antisocial (lying, stealing, and being aggressive, noncompliant and delinquent). But in this case, scientists led by Prof. Jaffee found, it is the abuse itself -- not some hypothetical genetic X factor -- that "plays a causal role in the development of children's antisocial behavior."
That suggests that preventing abuse can prevent its tragic consequences. Nothing inherent in abused children dooms them.
Living with a father who steals, brawls and can't hold a job raises a child's risk of similar behavior. But children also inherit genes from dad. In principle, either the experience or the genes might push them to follow in his antisocial footsteps. In this case, Prof. Jaffee finds a clear effect of environment: The longer a child lives with such a father, the greater the risk of a similar fate. That suggests that keeping families intact is not always in a child's best interest.
As Jenae Neiderhiser of George Washington University says, "the strength of genetically informed studies is that they are just as useful for identifying the effects of environment."
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