Twins are a bonanza for research psychologists. In a field that continues to seek to penetrate the effects of genetics, environment and life experiences, they provide a natural controlled experiment as their trajectories diverge, subtly or dramatically, into adulthood.
Bring Dennis and Douglas. In high school, they looked so much alike that their friends separated them by the cars they drove said the researchers in a twin study in Virginia. Most of their childhood experiences are shared — except that Dennis endured an attempted molestation when he was 13 years old.
At 18, Douglas married his high school sweetheart. She raised three children and was very religious. Dennis cycled through short-term relationships and was divorced twice, falling into bouts of despair after each breakup. In their 50s, Dennis had a history of severe depression, and his brother did not.
Why do twins, who share so much genetic and environmental input, differ 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 of childhood trauma.
Their study of 25,252 adult twins in Sweden, published in JAMA Psychiatryfound that those who reported one or more childhood trauma — physical or emotional neglect or abuse, rape, sexual abuse, hate crimes or witnessing domestic violence — were 2.4 times more likely were more likely to be diagnosed with psychiatric illness than those who were not.
If a person reported one or more of these experiences, the likelihood 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 isolate the effects of these traumas from genetic or environmental factors, the researchers narrowed the pool to “discordant” pairs, where only one twin reported childhood maltreatment. An analysis of 6,852 twins from these opposite pairs found that childhood maltreatment was still associated with adult mental illness, although not as strongly as in the whole group.
“These findings suggest a greater influence than I expected – that is, even after very strict control of shared genetic and environmental factors, we still observed an association between childhood poverty and poor mental health outcomes in adults,” 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 more likely to suffer from a mental illness than the unaffected twin in identical twin pairs, and 1.7 times more likely in fraternal twin pairs. This effect was particularly pronounced in subjects who reported experiencing sexual abuse, rape and physical neglect.
Twins may differ in their experiences of childhood trauma for a number of reasons, Ms. Danielsdottir in an emailed response to questions. In 93 percent of cases where one individual subject reported a rape, the other twin did not experience it.
Although domestic violence is “familial in nature,” she says, and a shared experience more than half the time, twins may have different dynamics with their parents. For example, a twin may be more likely to deal with a dysfunctional parent. Ms. Danielsdottir is an identical twin, and she says she “can confirm that we have different relationships with our parents (both good).”
Evidence is growing
For decades, researchers have been accumulating evidence linking childhood abuse and maltreatment to illnesses later in life. A sign 1998 study of 9,508 adults found a direct link between childhood maltreatment and heart disease, cancer, lung disease and depression, often associated with 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 poverty.
For decades, research has focused on biomedical models of mental illness, but the findings have helped spur a shift toward examining the effects of childhood experiences, including social conditions such as of racism, housing and poverty.
These two lines of inquiry converge in research mapping the impact of trauma on the brain. A 2022 report in Molecular Psychiatry, a journal of Nature, pointed to specific changes in “brain regions susceptible to stress” in people who were abused as children, and recommended that psychiatric diagnoses should add modifiers to present a history of trauma.
“These terrible things that happen to children and young people change the brain, they physically change the brain, and in some ways cause mental illness,” said Dr. Weleff. “Mental illness that may still develop is harder to treat, or worse, or perhaps 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 worse mental health in adulthood, said Mark Bellis, a professor of public health at Liverpool John Moores University in Britain, who was not involved in the study.
The findings add to “especially irrefutable evidence that it costs us all less if we invest in tackling” child abuse and neglect today, he added, rather than “continuing to pay for epidemic levels of damage” they cause downstream.
