Is depression genetic?
Partly. Twin studies put the heritability of major depression at roughly 35 to 40 percent, which means genes explain part of the risk while life circumstances explain most of the rest.
'Genetic' is doing a lot of work in that question, so let me unpack it. Heritability estimates for major depressive disorder cluster around 35 to 40 percent. That means roughly that share of the variation in who develops depression is explained by genetic differences across the population. The remaining 60 to 65 percent is environmental, trauma, stress, sleep, social context, and life events.
What it does not mean: there is no single depression gene. Hundreds, possibly thousands, of variants each add a tiny amount of risk. A polygenic score for depression is real but still weakly predictive for any one person. What it does mean: if a parent or sibling has depression, your own risk is roughly two to three times the general population. That is meaningful, but still smaller than what your circumstances and access to support will do. Genetics loads the dice, environment rolls them.
One more piece of context. The hunt for 'depression genes' had a rough decade. Several early candidate genes, especially the serotonin-transporter variant, failed to replicate in larger studies. The field shifted from single genes to the cumulative effect of thousands of small-effect variants. The honest summary today: depression has a real genetic component that is complex, distributed across the genome, and not yet useful for personal prediction. Your own history and family history remain more informative than a DNA test.