One twin was injured, the other was not. His adult mental health diverged.

Twins are a boon for research psychologists. In a field that perpetually seeks to uncover the effects of genetics, environment, and life experience, they provide a controlled natural experiment as their paths diverge, subtly or dramatically, throughout adulthood.

Take Dennis and Douglas for example. In high school, they were so alike that their friends told them apart by the cars they drove, told investigators in a twin study in Virginia. Most of his childhood experiences were shared, except that Dennis suffered attempted sexual abuse when he was 13 years old.

At age 18, Douglas married his high school sweetheart. He raised three children and became deeply religious. Dennis went through brief relationships and divorced twice, descending into fits of despair after each separation. At age 50, Dennis had a history of severe depression, and his brother did not.

Why do twins, who share so many genetic and environmental contributions, diverge as adults in their experience of mental illness? On Wednesday, a team of researchers from the University of Iceland and Sweden’s Karolinska Institute reported new findings on the role of childhood trauma.

Their study of 25,252 adult twins in Sweden, published in JAMA Psychiatryfound that those who reported one or more childhood traumas (neglect or physical or emotional abuse, rape, sexual abuse, hate crimes, or witnessing domestic violence) were 2.4 times more likely to be diagnosed with a psychiatric illness than those who They didn’t.

If a person reported one or more of these experiences, the odds of being diagnosed with a mental illness increased dramatically, by 52 percent for each additional adverse experience. Among participants who reported three or more adverse experiences, nearly a quarter had a psychiatric diagnosis of depressive disorder, anxiety disorder, substance abuse disorder, or stress disorder.

To separate the effects of these traumas from genetic or environmental factors, the researchers reduced the group to “discordant” pairs, in which only one twin reported childhood maltreatment. An analysis of 6,852 twins from these discordant pairs found that childhood maltreatment was still related to mental illness in adulthood, although not as strongly as in the full cohort.

“These findings suggest a larger influence than I expected, that is, even after very strict control for shared genetic and environmental factors, we still see an association between childhood adversity and poor mental health outcomes in adulthood.” said Hilda Bjork Danielsdottir, a doctoral candidate at the University of Iceland and the first author of the study.

A twin who reported maltreatment was 1.2 times more likely to have a mental illness than the unaffected twin in identical twin pairs, and 1.7 times more likely in fraternal twin pairs. This effect was especially pronounced among subjects who reported experiencing sexual abuse, rape, and physical neglect.

Twins can differ in their experiences of childhood trauma for many reasons, Danielsdottir said in a response to emailed questions. In 93 percent of cases in which an individual subject reported rape, the other twin had not experienced it.

Although domestic violence is “inherently familial,” she said, and was a shared experience more than half the time, twins can have different dynamics with their parents. For example, a twin is more likely to deal with a dysfunctional parent. Ms Danielsdottir is an identical twin and said she “can confirm that we have different relationships with our parents (both good)”.

For decades, researchers have been accumulating evidence linking childhood abuse and maltreatment to illnesses later in life. A reference point 1998 study of 9,508 adults. found a direct correlation between childhood maltreatment and heart disease, cancer, lung disease and depression, often related to behaviors such as smoking and alcohol use.

“That kind of opened everything up,” said Dr. Jeremy Weleff, a psychiatrist at Yale University School of Medicine who has researched the effects of childhood adversity.

For decades, research had focused on biomedical models of mental illness, but the findings helped spur a shift toward examining the effects of childhood experiences, including social conditions such as racism, housing and poverty.

The two lines of research have merged into research that maps the effect of trauma on the brain. TO 2022 report in Molecular Psychiatry, a Nature journal, noted specific alterations in “stress-susceptible brain regions” in people abused as children, and recommended that psychiatric diagnoses add modifiers to reflect a history of trauma.

“These terrible things that happen to children and young people change the brain, they change the brain physically, and in some ways they cause mental illness,” Dr. Weleff said. “Mental illness that has developed anyway is harder to treat, or worse, or even fundamentally different.”

By ruling out the role of genetic factors, the new findings should help dispel any remaining doubt that childhood maltreatment leads to poorer mental health in adulthood, said Mark Bellis, professor of public health at Liverpool John Moores University. in Britain, who were not involved in the study.

The findings add to “increasingly compelling evidence that it will cost us all much less if we invest in addressing” child abuse and neglect now, he added, rather than “continuing to pay for epidemic levels of damage” they cause. cause downstream.