The researchers also found that people in the lowest income brackets had higher overall rates of attempted suicide, mood and personality disorders than those with the highest income.
Researchers and policy makers need to figure out how to "meet the basic needs of individuals in that (low-income) bracket," Dr Jitender Sareen, the study's lead author from the University of Manitoba in Canada, told.
"They're not necessarily interested in, say, a psychological intervention," he said. "They're more wanting basic needs met."
Sareen said the findings, published in the Archives of General Psychiatry, are consistent with previous studies that have shown a link between low income and higher rates of mental disorders. However, the direction of that link has been unclear - it's also possible that having a mental disorder means people are likely to make less money.
In the current study, Sareen and his colleagues tried to clear up that link by analysing surveys of almost 35,000 adults in the US conducted on two separate occasions that were 3 years apart. Interviewers assessed study participants for a range of mental disorders, as well as asking them about their total household income.
Among all participants, about one in five had some kind of mental disorder at the time of the second survey.
In general, lower income was linked with a higher rate of almost all mental disorders the researchers analysed, including mood disorders, anxiety disorders, and substance abuse disorders.
Low income earners and personality disorders
People in the lowest income bracket making less than $20,000 (R134,539) per year were also more likely to have a personality disorder, such as borderline or antisocial personality disorder, than those in the highest income bracket, with a household income of $70,000 (R470,920) per year or higher.
Those with the lowest income were also more likely to have attempted suicide at some point.
Taking an income hit in the three-year span between surveys meant that participants had a greater chance of being diagnosed with a mood disorder or a substance abuse disorder at the second visit than those who stayed in the same income bracket.
However, having a mental disorder at the first interview was not linked to a drop in income in the next three years.
Money linked to mental health
Andrew Oswald, an economist at the University of Warwick in the UK who has studied the link between money and mental health, said that the study supports the idea that low income is causing a higher rate of mental disorders, and not the other way around.
But that's very difficult to prove, he said.
"What we know for sure is that there's a very strong correlation between these two things," Oswald, who was not involved in the current study, told.
And it seems very possible, he said, that making more money could help alleviate mental strain, as well as mild depression or anxiety - though maybe not more severe personality disorders.
With this new information, policy makers can focus their attention on preventing and treating mental disorders in those with the greatest risk, the authors said.
The challenge now "is to try to figure out what are the kind of interventions that are going to be most helpful" for low-income people who may be suffering from mental illness, Sareen said.(Reuters Health/ March 2011)