Early in my career, I interned for a year at a state psychiatric hospital. The men and women I worked with were dealing with serious mental illness, including depression, schizophrenia, and bipolar disorder, as well as substance use disorders.

As I talked with many of these individuals, I heard horrifying stories about their childhoods; abuse, family violence, and other adverse childhood experiences (ACEs) were rampant. It wasn't hard to connect the dots between their early experiences and their current mental health struggles.

Many research studies have confirmed that a rough childhood is linked to greater risk for mental illness in adulthood. However, it is hard to know whether the ACEs themselves are the cause of the psychiatric issues later in life. It's possible that a third variable, such as shared genes, is to blame for both the adversity and the subsequent mental health challenges.

After all, commonly measured ACEs such as parental mental illness and substance use are themselves significantly tied to genetic factors. For this reason, what appears to be an effect of ACEs on mental health could in fact be caused by genetic factors that impact both the adverse experience (through the parent's genetic risk) and the child's mental health outcomes (see the figure).

In a similar way, more general aspects of the environment could be driving the ACE-mental health association. ACEs typically don't happen in a vacuum........

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