Depression Is a Result of Both Genetics and Environment

Major depression (MD) is a relatively common, multifactorial psychiatric disorder that is causal for substantial disability worldwide. According to the National Institute of Mental Health, in 2015, almost 7 percent of all adults in the U.S., or an estimated 16.1 million individuals age 18 or older, reported having had at least one major depressive episode in the past year. The disorder is associated with significant work, school and health problems, substance abuse and an increased risk of death by suicide.

Previous meta-analysis (Kendler et al, 2006) of the five methodologically rigorous twin studies included blind personal interviews produced statistically homogeneous results. Specifically, that the evolving accumulative estimate of the heritability of major depression was 37%. In the current study, also by led by Kendler, colleagues examined parents and children from the following five family types:

  1. Intact—14,104 offspring
  2. Adoptive— 116,601 offspring
  3. Not-lived-with father—116,601 offspring
  4. Stepfather—67,826 offspring
  5. Triparental—29 205 offspring

The study population included 2,269,552 offspring (51.5% male and 48.5% female; median age, 42; range, 26-56 years). The diverse family subtypes allowed for quantification of outcomes for each subtype. Treated MD results were assessed from national primary care, specialist care, and inpatient registries. Data were collected from January 1, 1960, through December 31, 2016.

The question—is MD the result of nature or nurture? Unequivocally, YES. The expression of MD outcomes derived from the analysis revealed that MD arises equally from both genetic factors and child rearing experiences. In other words, both forms of cross-generational transmission added to the risk for MD among the children.

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Kendler KS , Ohlsson H , Sundquist K, Sundquist J. Sources of Parent-Offspring Resemblance for Major Depression in a National Swedish Extended Adoption Study. 1. JAMA Psychiatry. 2017 Dec 13. doi: 10.1001/jamapsychiatry.2017.3828. [Epub ahead of print]

Kendler, K. S., Gatz, M., Gardner, C. O., & Pedersen, N. L. (2006). A Swedish National Twin Study of Lifetime Major Depression. The American Journal of Psychiatry, 163(1), 109-114.