It's clear that coming from more humble beginnings can make ordinary life tougher in a lot of ways, and now a new study suggests that the experience of poverty and adversity can also alter biological mechanisms related to brain function, giving rise to increased chances of developing depression.
By analysing the brains of adolescents, researchers have found that those growing up in households with lower socioeconomic status accumulated greater amounts of a chemical tag on a gene linked to depression, called SLC6A4.
These kinds of epigenetic tags – meaning they're caused by environmental or external factors – can alter the activity of our genes, and in this case, the researchers found they increased the adolescent brain's 'fight or flight' response. That response is controlled by an area of the brain called the amygdala, and the team found that participants with a more active amygdala were more likely to develop symptoms of depression later on.
"The biggest risk factor we have currently for depression is a family history of the disorder," said one of the team, psychiatrist and behavioural scientist Douglas Williamson from Duke University. "Our new work reveals one of the mechanisms by which such familial risk may be manifested or expressed in a particular group of vulnerable individuals during adolescence."
The participants involved in the study – 132 non-Hispanic Caucasians aged between 11 and 15 years old at the outset of the experiment – came from households ranging from low to high socioeconomic statuses. They were shown photos of frightened-looking faces as they underwent functional MRI brain scans, which recorded levels of amygdala activity.
The team found that adolescents from a background marked by poverty showed lower levels of serotonin transport around the SLC6A4 gene, which predicts heightened amygdala activity, and thus greater reactivity to perceived threats and risks.
The adolescents were monitored over the course of three years, and those with greater amygdala activity were found to be more likely to report symptoms of depression as time went on, particularly if they had a family history of depression.
"This is some of the first research to [demonstrate] that low socioeconomic status can lead to changes in the way genes are expressed," said one of the researchers, Johnna Swartz, "and it maps this out through brain development to the future experience of depression symptoms."
A similar study earlier in the year also found an association between childhood depression and brain connectivity. Researchers from Washington University in St Louis used fMRI scans to measure brain activity in children aged between 7 and 12 and found that adverse circumstances early in life ended up impacting the development and function of the young brain – and again, the amygdala was involved.
It's worth bearing in mind that the samples used in both these studies were relatively small, so their findings need to be considered in that light. But the good news is that the more we discover about these links between poverty and depression, the more we should be able to do about it.
The team from Duke University will now be studying activity around SLC6A4 further, looking for other evidence of biological markers that could be tied to depression. The hope is that, one day, greater understanding in this area could help inform personalised depression-prevention strategies.
Of course, the alternative treatment – eradicating poverty – is also a goal worth aiming for, as behavioural geneticist Robert Philibert from the University of Iowa, who was not involved with the study, told Sara Reardon at Nature.
"What this points out here is that if you really want to change neurodevelopment, alter the environment," he said.
The findings are reported in Molecular Psychiatry.