It has been a chicken-and-egg question for decades: Does the misery of poverty breed mental illness, or does the burden of mental illness cast people down into poverty? The two clearly tend to go together, but which causes which?
This week, a Massachusetts researcher published possibly the broadest study yet on the question, examining tens of thousands of hospital records statewide to see whether patients who were hospitalized for mental illness then ''drifted down" to less affluent ZIP codes.
The study, which followed patients from 1994 through 2000, turned up surprisingly little such downward drift, said the researcher, Christopher G. Hudson, a professor at Salem State College and expert in mental health policy.
So, he said, ''the news here is that there is now increasingly strong evidence that socioeconomic status is indeed a very important dimension of mental illness, though obviously not the only dimension."
The study also highlighted the striking contrast between the state's richer and poorer communities when it comes to the rate of mental illness. It documented a rate of about 4 percent of the population with mental illness serious enough to lead to repeat hospitalizations in the richest communities, compared to about 12 percent or 13 percent in the poorest -- and those are very conservative figures, Hudson said.
Hudson acknowledged that the study, which focused on 34,000 patients who had been hospitalized at least twice, had some methodological issues. In particular, ZIP code is far from a foolproof way to determine a person's economic level. And he did not conduct diagnostic interviews with any patients.
David Duncan, a mental-health epidemiologist based in Kentucky, noted another possible flaw: When downward economic drift occurs among the mentally ill, it is believed to happen mainly before they are ever hospitalized. So, a study following them only after a first hospitalization might come too late to catch their drift, he said.
The study used powerful statistical tools to test five hypotheses about the link between mental illness and poverty, including the ''downward drift" idea. The theory that stressful economic conditions bring on mental illness was the only one that really fit the data, Hudson writes in the current issue of The American Journal of Orthopsychiatry, published by the American Psychological Association.
That comes as no surprise to Deborah Belle, a psychology professor at Boston University who studies poverty-related stress.
''There are so many plausible causal links between poverty and mental illness, particularly depression, which I know best," she said.
Among them: Poor people are likelier to face threatening, humiliating and entrapping life events, she said. Poverty can undermine their self-image and social connections, and leave them feeling deprived of control over the most basic aspects of their lives.
Hudson said that his findings jibed with his own experience as a community organizer and social worker among the demoralized poor in Chicago and on two Indian reservations. Their circumstances, he said, were like ''straightjackets, especially suited to induce mental breakdown."
Ever since a seminal study was conducted in 1930s Chicago, researchers have documented the tendency for poverty and mental illness to go hand in hand.
But they continue to dispute which causes which, and the answer may vary depending on the mental illness. Schizophrenia, which is often highly debilitating, may be more likely than other diseases to cause downward drift, for example. Depression and anxiety disorders may be likelier than other diseases to be exacerbated by poverty.
Hudson's study found that, for all types of mental illness, the data best fit the model of poverty as a cause rather than a result, but he did see some downward drift for schizophrenics, he said.
Asked whether his study translated into policy advice, he said that it showed how important it was for states to allocate mental health funding to towns based not simply on population -- as happens in many places -- but on the level of illness. It also suggests the need for more mental-health outreach in poor communities, and for programs linking mental health services with help obtaining jobs, housing and education, he said.
Hudson does not deny that biology and heredity play a role in mental illness; even the richest towns have a baseline level of mental illness that likely reflects that biological factor, he said. Other environmental factors like troubled families and life trauma are also linked to mental illness.
''We know that economic circumstances, the lack of supports, and the stresses that people are subject to sometimes overtax their cognitive and emotional and mental abilities."
And broadly speaking, he said, his study suggests that ''poverty is at least as important as innate or biological factors."
Carey Goldberg can be reached at email@example.com.