The Secret Link Between Evolution and Depression

Depression is totally natural. Historically, we needed it. At least that's what many evolutionary psychiatrists say. They argue that negative emotions, like sadness, regret, and anxiety, wouldn't exist if they weren't in some way useful to our species' survival.

Depression could have evolved to regulate effort expenditure. Because it makes us less active and less motivated, depression may have protected ancient humans from exerting unnecessary (and finite, costly) energy. As John Eagles, a psychiatrist and professor at Royal Cornhill Hospital in Scotland puts it, depression could have been "an adaptive means of inactivity."

It may have also promoted protective, even submissive behaviors, thereby increasing our likelihood of longterm survival. This helps explain why women are more likely to be depressed during their fertile reproductive years. Depression kept pregnant women and mothers inactive and sheltered from danger while also ensuring they stayed close to home to bond with their children and mate. Depression was less adaptive for reproductive-age men, on the other hand, who historically needed to be providers and protectors. Accordingly, today the sexes are evenly affected by depressive symptoms until puberty but, after puberty, women are more likely than men to be depressed.

Depression may have literally protected us, too. One study found an association between certain gene variants related to depression risk and key immune responses to infection that likely enhanced our ancestors' health. Finally, research suggests that variations in mood stimulate creativity. The negative emotions needed for such variation could induce life-saving innovations—a clear evolutionary advantage.

Though it's largely hypothetical for now, the evolutionary argument makes sense. But it has some limitations. First, surely severe, suicidal depression couldn't possibly have been a reproductive advantage. So where did that come from? Secondly, if depression evolved to keep us close to our safe communities, why do even mild depressive symptoms so significantly impair social functioning? But what most confounds evolution as the sole explanation for depression is this: If we evolved to be depressed, why is depression becoming more common?

Despite tremendous economic growth and improved standards of living throughout the 20th century, Americans born after 1955 are three times more likely than their grandparents' generation to suffer from major depression. Chinese people born after 1966 are 22.4 times more likely to report a depressive episode than those born before 1937. Forty years ago, the average first depression episode occurred around age 30. Now depression typically first shows up in teenagers. Three hundred and fifty million people worldwide are depressed, making depression a leading cause of disability.

Though it has probably always existed to some degree, depression appears to be a particularly modern problem. Amish people living in Pennsylvania, without electricity in their houses and using horses and buggies for transportation, are between a fifth and a tenth as likely to suffer from depression as neighboring modern Americans. Likewise, adopting a contemporary American lifestyle may explain why Mexican Americans born in America have higher rates of depression than do Mexican immigrants. Even dwelling in urban areas, as opposed to rural areas, increases the prevalence of psychiatric disorders.

Some argue that we're statistically more depressed than we used to be simply because more of us know about depression, particularly in the first world. Since depression became a diagnosable clinical disorder in the 1950s, its occurrence has skyrocketed. As Ed Diener, an author and psychology professor at the University of Illinois, writes, depression is "the flavor of the month." "So what people label as depression now may not have been so labeled before."

But this explanation only takes us so far, because many studies never ask participants point blank whether they are or were depressed. Instead, they ask questions about particular symptoms of depression, such as "was there ever a time in your life when you cried every day for two weeks straight?" According to these studies, even depressive symptoms have increased substantially, though there's some indication that they're less severe than they were 50 or 100 years ago.

So the label "depression" may be booming for cultural reasons, but symptoms of depression are proliferating too, and it's not totally clear why. Brandon Hidaka, a researcher at the University of Kansas who wrote a paper on "Depression as a disease of modernity," published in the Journal of Affective Disorders, has some informed guesses.

Though our average standard of living has never been better, modern life entails an unprecedented set of ailments. For one, global inequality is widening. "Since social status is relative, that means that with rising income inequality, a greater portion of the population is at a low social rank," Hidaka explains, which feels bad. Rising inequality paired with a consumerist culture that encourages us to spend money we don't have instead of spend time with people we love is a depressing combination.

Another contributing factor for America is our urban transience. Moving is associated with a higher risk of depression for females, Hidaka tells me. But, for anyone, moving disrupts social networks, which are critical to our sense of wellbeing, and causes stress and family conflict.

More broadly, our culture's unnatural individualism may yield higher rates of depression. On average, people are happier with others than when alone. For example, renowned psychologist Daniel Kahneman has found that people on balance feel more positive when with others than when alone for 14 out of 15 daily activities, like exercising, resting, commuting, and working around the house. Whereas social support buffers against depression, a lack of it exacerbates our vulnerability, Hidaka explains. For many of us, the Internet keeps us company—but it fails. Internet use is correlated with "less family communication, smaller social circles, more depressive symptoms and greater feelings of loneliness," Hidaka writes.

Add loneliness to what Hidaka calls our culture's "famine of time," and you get lonely, busy, depressed people. We are depressed and unwell because, Hidaka believes, "we are lonely, overworked, and underpaid." Finally, though we have far superior medical treatment than earlier times in human history, our brain chemistry isn't functioning as well due to inadequate diets, chronic inflammation, autoimmune disorders, and insufficient sunshine, physical activity, and sleep. "Depressive symptoms are more severe and more common than they used to be in the past because our bodies are not functioning as well," Hidaka explains.

In short, our modern depression may be a product of inequality, poor health, high stress, fraying social networks, and tech-induced social isolation. There may, indeed, be a psychologically distressing mismatch between the activities we evolved to do and adapted to enjoy and what we do now. Modern life may feel awful to our ancient brains. As Carl Jung wrote, "Every civilized human being, however high his conscious development, is still an archaic man at the deeper levels of his psyche."

So are we depressed because it's natural to be depressed, or because we're now living unnaturally? Most likely, depression is some combination of evolution and modernity. Moreover, one study explains that, in our fast-paced society, individuals predisposed to depressive traits may have a particularly hard time adapting and coping. Put another way, Hidaka concludes, "the modern man would likely be much more resilient to the toils of living if he were physically fit, well-rested, free of chronic disease and financial stress, surrounded by close family and friends, and felt pride in his meaningful work."

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