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 trauma 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 these experiences, the odds of being diagnosed with a mental illness climbed sharply, 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 disentangle the effects of these 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 these discordant pairs found that childhood maltreatment was still linked with adult mental illness, though 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, Ms. Danielsdottir said in an emailed response to questions. In 93 percent 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. Ms. Danielsdottir is an identical twin herself, and said she “can confirm that we have different relationships with our parents (both good).”

Growing evidence

For decades, researchers have been accumulating evidence that links child abuse and maltreatment to illnesses later in life. A landmark 1998 study of 9,508 adults found a direct correlation between childhood maltreatment and heart disease, cancer, lung disease and depression, often linked by behavior like smoking and alcohol use.

“That kind of blew it all open,” said Dr. Jeremy Weleff, a psychiatrist at the 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 propel a shift toward examining the effects of childhood experiences, including social conditions like racism, housing and poverty.

The two lines of inquiry have merged in research that maps the effect of trauma on the brain. A 2022 report in Molecular Psychiatry, a Nature journal, pointed to specific alterations in “stress-susceptible brain regions” in people maltreated as children, and recommended that psychiatric diagnoses should add modifiers to reflect 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,” Dr. Weleff said. “The mental illness that may have developed anyway is more difficult to treat, or worse, or maybe 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 the Britain, who was not involved in the study.

The findings add to “the increasingly irrefutable evidence that it is going to cost us all a lot less if we invest in tackling” abuse and neglect of children now, he added, rather than “continuing to pay for the epidemic levels of harm” they cause downstream.