The results of a new study have shown that the brain's response to economic inequity can predict depression.
The researchers, from the National Institute of Information and Communications Technology in Osaka and Tamagawa University in Tokyo, both in Japan, have published their findings in the journal Nature Human Behaviour.
Experts suggest that the imaging study offers new insights into how mechanisms in the brain might explain the link between economic inequity and depression.
Depression is a global public health problem that affects more than 300 million people, and it is the "leading cause of disability worldwide." In the United States alone, depression affects around 7.6 percent of people aged 12 and older and cost the nation $210.5 billion in 2010.
People living below the poverty line in the U.S. are more than twice as likely to have depression than people living at or above it.
Economic inequity and depression
In their study report, the researchers write that economic inequity has become "an increasing concern for society," and they cite previous studies that have linked it to depression and other psychiatric diseases.
They refer to large studies, such as Whitehall II, that have found links between "economic gaps and major depression, where economic and material disadvantage are crucial in explaining depressive symptoms."
However, because the brain mechanism underlying the link is less well understood, they decided to investigate it further.
The team used functional MRI to scan brain activity of healthy individuals as they played a computer game wherein they were asked to accept or reject offers to split a pot of money between themselves and a "virtual partner."
There were three types of offer: one in which the two players received equal shares of the money ("equity"); another in which the participant got more ("advantageous inequity"); and another in which the virtual partner got more ("disadvantageous inequity").
Individualists and prosocials
At the time of playing the game, the participants were also asked to complete two other tests: the Beck Depression Inventory (BDI) and a test that measures their "social value orientation." They also completed the BDI a year later.
The team found that the participants with "individualist" social orientation tended to be more selfish and keep the money in unfair advantageous offers.
In contrast, the "prosocial" participants tended to be more self-sacrificing and rejected offers wherein the money was not split equally.
The researchers then analyzed measures of brain activity in the amygdala and hippocampus in response to types of offer and depression scores.
It was found that for the disadvantageous offers, the response in the amygdala and hippocampus predicted current depression scores and changes in symptoms 1 year later. This was true of both prosocials and individualists.
But when the team analyzed the results for advantageous offers, they found that the amygdala and hippocampus responses only predicted depression in prosocials.
'Critical effect on mood'
The researchers note that these various predictions "were not possible using participants' behavioral and socioeconomic status measures."
They suggest that the findings show that even healthy people's response to inequity may have "far-reaching implications for their mood."
They call for further studies to build on the results to find ways to better identify and protect people from mental illness.
In an accompanying commentary, Megan Speer and Mauricio Delgado - both of Rutgers University in Newark, NJ - describe the research as "thought-provoking" and suggest that it "highlights potential risk factors that can precipitate or exacerbate the debilitating nature of depression."
"These findings suggest that sensitivity to economic inequity has a critical effect on human mood states, and the amygdala and hippocampus play a key role in individual differences in the effect."