Why Don’t Depressed People Live as Long as Others?
Jason Houle, PhD, is a Robert Wood Johnson Foundation Health & Society Scholar at the University of Wisconsin. He recently published a study online in the journal Psychosomatic Medicine that finds association between depressive symptoms and mortality is due to later health problems, not prior physical health conditions.
Human Capital Blog: Why did you decide to look at this particular topic?
Jason Houle: I first started looking at this topic in graduate school, when I took a course on event history models (a quantitative method often used when studying mortality). Up to that point, most of my research focused on the social determinants of mental health, but I had become increasingly interested in the link between mental and physical health. While there’s a long literature on how depression influences physical health (and vice versa), as a demographer, I was really interested in the link between depression and mortality. When researching this topic, I discovered a rather large literature that showed that people who experience depression tend to die younger, on average, than those who do not. However, it wasn’t clear from prior research why, exactly, depressed people tend to die younger than those who are not. Though it makes sense that depression is linked with mortality, the reasons behind it remained a puzzle, and I thought it would make an interesting project.
HCB: What did your study look at? What are some of the theories about the association between depressive symptoms and mortality?
Houle: There are several hypotheses in the literature, which have received varying amounts of support. In my study, I test three of these hypotheses. A common hypothesis in the literature is that the association between depression and mortality is simply spurious—that is, depressed people tend to die younger in part because of risk factors that both lead them to be depressed and lead them to die younger. For example, those with chronic conditions or functional limitations are more likely to become depressed, and they are also at a higher risk for mortality. In addition, the social determinants of health that we as Robert Wood Johnson Foundation Health & Society Scholars care so much about—such as socioeconomic status, exposure to stress, and social support—are likely drivers of both depression and mortality. Notably, many measures of the social determinants of health are absent from prior research on this topic.
Still others have suggested that the link is not spurious, but that it’s causal. While I am unable to assess causality in this study, there’s two hypotheses related to this issue: The first is that, regardless of one’s own physical health status, depression is an independent risk factor for mortality—much like smoking, or hypertension. A related hypothesis is that depression is associated with mortality because depression can lead to a range of physical health problems (such as functional limitations), which then increases the risk of mortality. That is, later physical health conditions mediate the link between earlier depression and mortality.
Prior work hasn’t been able to adjudicate support for these three hypotheses, in part because it tends to use research designs where depression, health, and confounders are measured at one point in time with a mortality follow-up appended several years later. So what I do in this study is use repeated measures of depressive symptoms, health, and other confounders over time to try to disentangle this relationship.
HCB: What did you find?
Houle: In short, the study findings show support for the latter hypothesis—that the association between depressive symptoms and mortality is mediated by later physical health conditions. I found that the functional limitations had the strongest mediating effects, which may suggest that depressive symptoms are linked to mortality in part because those who experience depressive symptoms are more likely to develop functional limitations. I would add that these are not small effects—on average, those who report very low levels of depressive symptoms tend to live around eight years longer than those with the highest reported levels of depressive symptoms.
HCB: What kind of implications do your findings have for patients and providers?
Houle: I would start with the obvious caveat that more work is needed to fully understand the link between depression and mortality. That said, according to the World Health Organization, depression is a massive contributor to the global burden of disease. If these findings are any indication, depression prevention and improving the health of people with depression could potentially reduce the number of deaths attributed to depression in the population.
HCB: Do you plan to look at this subject further?
Houle: This was mostly just a side project for me, but while this study does advance the literature on this topic, it also raises a bunch of additional questions. For example, though I test three hypotheses, there are still key hypotheses that are left untested—notably, the idea that depression may influence health or vice versa is likely too simple. Many have noted that the way that depression influences mortality likely occurs through complex feedback loops, whereby depression and physical health have synergistic effects on one another to increase the risk of mortality. Unfortunately, we lack the data to provide an adequate test of such a hypothesis. Second, I think future work should examine the role of health care and health insurance. Do depressed people get sick and die younger in part because they are less likely to get treatment for their health problems? It’s certainly plausible.