Why Does a Challenging Childhood Affect Adult Mental Health?

A recent study dissects genetic and environmental influences.

Early in my career, I worked as an intern for a year at a state mental hospital. The individuals I interacted with were grappling with severe mental illnesses, including depression, schizophrenia, bipolar disorder, and substance use disorders.

As I engaged with many of these individuals, I heard distressing stories about their childhoods: abuse, family violence, and other adverse childhood experiences (ACEs) were prevalent. It wasn't hard to see how their early experiences affected their current mental health challenges.

Differentiating Genetic and Environmental Influences

Numerous research studies have confirmed that a difficult childhood is associated with a heightened risk for mental illness in adulthood. However, it's challenging to determine whether the ACEs themselves are the cause of the mental issues later in life. It's possible that a third factor, such as shared genetics, is responsible for both the adversity and the subsequent mental health challenges.

Moreover, commonly measured ACEs, like parental mental illness and substance use, are themselves heavily influenced by genetic factors. Therefore, what appears to be an effect of ACEs on mental health could actually be caused by genetic factors that influence both the adverse experience (via the parent's genetic risk) and the child's mental health outcomes.

Likewise, various environmental factors might influence the link between adverse childhood experiences (ACEs) and mental health. ACEs typically don't occur in isolation but are linked to longer-standing factors, such as neighborhood crime rates or being raised by a highly critical parent.

It may be that these chronic forms of low-grade stress are more impactful than the less common but more severe events typically measured in ACE studies.

Robust New Study From Sweden

Researchers recently capitalized on the extensive health records in Sweden to examine the effects of ACEs on adult mental health. They utilized a nationwide sample of over 25,000 twins, allowing them to disentangle the effects of shared genetics, shared environments, and individual-specific ACEs such as physical neglect, emotional or sexual abuse, or witnessing crimes.

Adult mental health outcomes were assessed by whether the participant had received a diagnosis of a depressive disorder, anxiety disorder, substance use disorder, or stress-related disorder after age 19, as well as self-reported depression symptoms during the past week as an indicator of current mental health.

Preliminary analyses indicated that ACEs had a significant impact on adult mental health. The effects were "dose-dependent," meaning that each additional ACE increased the risk for mental illness. According to studies, each additional Adverse Childhood Experience (ACE) raises the chances of developing a mental health issue by 52%, affecting both genders equally.

Using Twin Comparisons to Disentangle Cause and Association

The twin aspect of the study was crucial in separating the effects of ACEs from genetic and overall environmental influences. Scientists studied twin pairs where one twin experienced adverse childhood experiences (ACEs) while the other didn't, to understand the impact of ACEs. They then re-analyzed the effects of ACEs on the risk for adult mental health diagnosis.

If ACEs significantly influenced adult mental health outcomes among discordant twin pairs, this finding would suggest that it wasn't the ACE itself driving the effect. For example, imagine that one twin experienced physical neglect and the other did not, yet both ended up with depression in adulthood. This result would indicate that it wasn't solely the neglect that caused the subsequent depression. Alternative explanations would include the twins' shared genetics (100% in identical twins and 50% on average in non-identical twins) and shared aspects of their childhood environment that children who experience difficult circumstances like living in stressful neighborhoods might not be detected by ACEs measures.

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