There’s a well-known theory adoptees have a slightly higher likelihood of developing mental illnesses. While they experience trauma at the time of birth or separation (whether remembered or not), both environment and genetics play a role in mental illness likelihood.
Genetics play a role in anyone’s likelihood of developing mental illnesses. However, even if someone has a genetic predisposition, their environment plays a key role in gene expression. Therefore, not everyone possessing genes susceptible to mental-illness will develop one in their lifetime.
Mental illnesses can be induced by a traumatic event or ongoing emotional strain. So when adoptees experience separation or adaptation to new people and a new environment, this could potentially prompt the development of mental illnesses. Even if the adoptee is too young to remember or emotionally understand the event, it’s still processed as a traumatic early-life event, thus more prone to mental illness development later in life.
So childhood adversity and mental health can’t be fully attributed to trauma, separation or abuse, but it is due to a complex combination of genetics and environmental factors.
Additional information:
“Childhood Adoption and Mental Health in Adulthood: The Role of Gene-Environment Correlations and Interactions in the UK Biobank”
“Genetics Contributes to Mental Health Risks in Adoptees”
“The Mental Health of US Adolescents Adopted in Infancy”