Study of twins examines effect of trauma
Twins are a bonanza for research psychologists. In a field perpetually seeking to tease out the effects of genetics, environment and life experience, they provide a natural controlled experiment as their paths diverge, subtly or dramatically, through adulthood.
Take Dennis and Douglas. In high school, they were so alike that friends told them apart by the cars they drove, they told researchers in a study of twins in Virginia. Most of their childhood experiences were shared — except that Dennis endured an attempted molestation when he was 13.
At 18, Douglas married his high school girlfriend. He raised three children and became deeply religious. Dennis cycled through short-term relationships and was twice divorced, plunging into bouts of despair after each split. By their 50s, Dennis had a history of major depression, and his brother did not.
Why do twins, who share so many genetic and environmental inputs, diverge as adults in their experience of mental illness? On Wednesday, a team of researchers from the University of Iceland and Karolinska Institutet in Sweden reported new findings on the role played by childhood trauma.
Their study of 25,252 adult twins in Sweden, published in JAMA Psychiatry, found that those who reported one or more traumas in childhood — physical or emotional neglect or abuse, rape, sexual abuse, hate crimes or witnessing domestic violence — were 2.4 times as likely to be diagnosed with a psychiatric illness as those who did not.
If a person reported one or more of those experiences, the odds of being diagnosed with a mental illness climbed sharply, by 52% for each additional adverse experience. Among participants who reported three or more adverse experiences, nearly one-quarter had a psychiatric diagnosis of depressive disorder, anxiety disorder, substance abuse disorder or stress disorder.
To disentangle the effects of those traumas from genetic or environmental factors, the researchers narrowed the pool to “discordant” pairs, in which only one twin reported maltreatment in childhood. An analysis of 6,852 twins from those discordant pairs found that childhood maltreatment was still linked with adult mental illness, although not as strongly as in the full cohort.
“These findings suggest greater influence than I expected — that is, even after very stringent control of shared genetic and environmental factors, we still observed an association between childhood adversity and poor adult mental health outcomes,” said Hilda Bjork Danielsdottir, a doctoral candidate at the University of Iceland and the study’s first author.
A twin who reported maltreatment was 1.2 times as likely to suffer from a mental illness as the unaffected twin in identical twin pairs and 1.7 times as likely in fraternal twin pairs. This effect was especially pronounced among subjects who reported experiencing sexual abuse, rape and physical neglect.
Twins may diverge in their experiences of childhood trauma for many reasons, Danielsdottir said in an emailed response to questions. In 93% of cases in which an individual subject reported a rape, the other twin had not experienced it.
Although domestic violence is “inherently familial,” she said, and was a shared experience more than half of the time, twins may have different dynamics with their parents. For example, one twin may be more likely to confront a dysfunctional parent. Danielsdottir is an identical twin herself and said she “can confirm that we have different relationships with our parents (both good).”