The serotonin transporter gene variant significantly amplifies the risk of major depression when coupled with significant childhood stress or trauma.

“what they saw was overall there was no increase in risk having the bad variant however if it was coupled with a lot of childhood stress you had about a twenty fold increase in the risk so just to be clear on aggregate there was no difference the hazard ratio is one for with gene verses without because of the ton of variability but if you wanted to amplify it take that gene and expose it to childhood stress or trauma and you almost assure depression you get a massive boost in predictability”