The 5HTTLPR gene polymorphism significantly increases susceptibility to depression, requiring fewer stressful episodes to trigger it.

“There are these studies now of many, many thousands of individuals, these were mainly done in New Zealand, but these studies have now been done elsewhere, looking at many tens of thousands of individuals who carry particular copies of genes, what they call polymorphisms, in particular of a gene called 5HTTLPR, which is a serotonin transporter. So this is a gene that controls or regulates how much serotonin is available in the brain. If you have this gene, this polymorphism, it doesn't necessarily mean that you will be depressed, but it greatly shifts your susceptibility to depression under conditions of stress.”