Twin study reveals link between childhood trauma and adult illness

An adult who has experienced trauma in childhood is more than twice as likely to be diagnosed with a mental illness as one who did not experience the trauma, a Swedish study has found.

The study of 25,252 adult twins, published in JAMA Psychiatry, was carried out by researchers from the University of Iceland and the Karolinska Institutet in Swede. The  adult twins, aged 18-47 years, were born between 1959 and 1998 and were followed up from age 19 years until 2016, with a maximum follow-up time of 39 years.

The researchers found that those who reported an adverse childhood experience (ACE) were 2.4 times more likely to be diagnosed with a psychiatric condition. They looked at seven ACEs in total: family violence, emotional abuse or neglect, physical neglect, physical abuse, sexual abuse, rape, and hate crime.

Sexual abuse strongly increases risk of adult mental illness

If an individual reported having one or more of these adverse experiences, the odds of being diagnosed with a mental illness increased by 52% 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.

Because identical twins share genetic and environmental influences, the researchers also isolated those “discordant” pairs where only one twin reported adverse childhood experiences. 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.

“After adjusting for shared genetic and environmental factors in stringent twin analyses, the association between ACEs and clinically confirmed adult psychiatric disorders remained evident, with particularly large increases in odds after multiple ACEs or sexual abuse,” the authors write.

In the discordant pairs, 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.

Hilda Bjork Danielsdottir, the first author on the study and a doctoral student at the University of Iceland, told the New York Times: “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.”

Psychiatric disorders are ‘moderately heritable’

The study is important because we know that ACEs and psychiatric disorders cluster within families. This means that few studies examining associations of ACE exposure with subsequent psychiatric disorders have been able to disentangle the effect of the trauma from the effect of genetics or other risk factors shared by family members. Previous twin studies have shown that psychiatric disorders are “moderately heritable”, the authors say, “with 40% to 60% of individual differences in, for example, depression, anxiety, and PTSD attributable to genetic factors.”

By ruling out the role of genetic factors, the new findings confirm that childhood maltreatment leads to worse mental health in adulthood, Mark Bellis, a  professor of public health at Liverpool John Moores University, told the New York Times.

The authors say their findings suggest that interventions targeting ACEs, including primary prevention and enhanced access to evidence-based trauma therapies to individuals who experienced ACEs, “may be associated with reduced risk of future psychopathology.” They add: “However, our findings additionally indicate that family-wide risk factors (eg, genetic predisposition and socioeconomic disadvantage in childhood) also contributed to adult mental health outcomes among individuals who experienced ACEs, suggesting that there may be added value in addressing risk factors within the whole family.”