Genes, Stress, and Depression
Depression is shaped in part by the environment, but the environment is shaped in part by genes
Depression is one of the most common psychological maladies afflicting our species. It’s common enough, in fact, that it’s sometimes known as the “common cold of mental disorders.” But unlike the actual common cold, depression doesn’t have a clear and unambiguous cause. Instead, it has a nebulous cloud of causes which fall into two main categories: genetic causes and environmental causes.
We tend to view the genetic and environmental causes of depression as separate and independent contributors: Depending on the person, depression may be largely due to genes, largely due to the environment, or due to a combination of both. A new study, however, pulls the rug out from underneath this conception by showing that the environmental causes of depression - specifically, stressful life events - are themselves shaped in part by our genes.
The study was conducted by Jacob Crouse and colleagues, and involved more than 14,000 Australian participants suffering from depression. Crouse et al. collected DNA samples from all of them, then calculated each participant’s polygenic scores for a range of mental-health conditions.
Polygenic scores are numbers that indicate how many gene variants an individual has that are associated with a given trait. The higher the score, the more likely that individual is to develop the trait in question.
Crouse and colleagues calculated participants’ polygenic scores for depression, anxiety, ADHD, bipolar disorder, and schizophrenia, as well as for the personality trait of neuroticism. Although their main interest was depression, they looked at the risk scores for the other disorders as well because many of the genes that contribute to one disorder also contribute to others.
If genes and life stressors really are independent causes of depression, we’d expect to find no relationship between people’s polygenic scores for any mental illness and their exposure to stressful events. But that’s not what Crouse et al. found. Instead, they found that gene variants predisposing people to most disorders also predispose them to life stressors. Life stressors, in other words, are shaped in part by people’s genes!
This surprising finding is depicted in the following two graphs. The first graph shows the relationship between six polygenic scores and life stressors in childhood. As you can see, gene variants that predict ADHD, depression, and schizophrenia also reliably predict childhood physical neglect, emotional neglect, and emotional abuse.
