“The flesh is sad, and I've read all the books.” The famous opening of Mallarmé’s poem “Sea Breeze” is a sigh of resignation. Not even the knowledge gleaned from all the books in the world can loosen the grasp of human sadness.
Of course reading all the books is impossible; merely reading the books on ways to find happiness would be a daunting prospect. Yet despite the countless titles promising infallible strategies for attaining happiness, depression is reaching epidemic proportions. More than 30 million adults in America now suffer from depression. The World Health Organization estimates that by 2030 depression will surpass cancer and heart disease to become the leading cause of worldwide disability and death.
Dominant models of depression tend to treat the condition as a defect or deficiency. Psychiatry emphasizes a deficiency in the brain, cognitive therapy sees the shortcoming in our thoughts, family or couples therapy focuses on defects in our relationships.
The deficiency approach regards depression as the malfunctioning of the human organism; it's what happens when something is missing or flawed. In The Depths: The Evolutionary Origins of the Depression Epidemic, psychologist Jonathan Rottenberg presents a compelling inversion of conventional wisdom, arguing that depression is not only a natural response to certain conditions, it's a state that often promotes our very survival.
One sign that depression is an adaptive behavioral response is its widespread presence in the animal kingdom. Depressed dogs and depressed humans both show decreased interest in food and sex, a drop in energy levels, sleep disruption, and lower levels of self-care and personal grooming. Though other species lack our capacity to signal these sensations in language, behavioral symptoms in animals mirror human experiences of depression with striking fidelity. We also share triggers of depression with other species: starvation, disease, and the death of a close relative can induce depressed behaviors in multiple species.
To see the adaptive benefits of depression, it helps to consider certain cruel but illuminating studies. In one, mice are hung upside down by their tails for six minutes while the intensity and duration of their struggle is measured. Initially they attempt to escape, but over time their efforts diminish. Rather than continuing to expend energy on an impossible goal, the mice reduce the risk of exhaustion and injury through partial or total immobility. Their moods system demobilizes effort in the face of an impossible task and thus increases the likelihood of survival.
In another test, mice or rats are repeatedly dropped into water. Their first response is to swim vigorously, but after multiple immersions they conserve energy by floating and making only minor motions to stay above water. If they swam just as strenuously on the tenth immersion as on the first, the risk of drowning would increase dramatically. More broadly, animals without an evolved mechanism to decrease effort in certain situations would be less likely to survive. Depression, Rottenberg argues, is precisely such a mechanism.
Experiments with human subjects tend to lack such stark brutality, but there are certain exceptions. Near the end of World War II, for instance, 36 conscientious objectors volunteered to be systematically starved for six months to help researchers develop strategies to treat the mass starvation the Allies expected to encounter in postwar Europe. For most of the men starvation caused more than just weight loss: it also induced many of the symptoms of significant depression. Animals that responded to famine or danger with exuberant moods that motivate exploration and expend energy would fare worse than those with a behavioral shutdown mechanism to conserve energy in environments of scarcity or hostility.
It's easy to forget that humans have spent approximately 1000 times longer living and evolving as hunter-gatherers than in any other lifestyle. Starvation, disease, war, and predation were common threats for a majority of our evolutionary history. Even in less dire circumstances, however, mild depression can still confer benefits. A variety of studies indicate that low mood narrows and directs our attention to perceive threats and obstacles. It also helps conserve energy, facilitates disengagement from impossible goals, and improves our capacity to detect deception and to assess the degree of control we exercise over our environment. Some studies even suggest that low mood can improve skill in persuasive argument and sharpen memory.
Does this mean the psychologist enlightened by an understanding of these evolutionary benefits will greet the onset of depression in friends, family, and clients with appreciative acceptance? It's useful to remember that many incredible adaptations have major attendant costs, and depression also follows this pattern. The evolution of larger human brains increased the risk of death in childbirth for mothers; upright walking helped our ancestors to spread across the globe, but it also causes major back problems.
Depression can be a useful response in particular conditions, but it can also be a debilitating condition that mars quality of life and even interferes with evolutionary goals of survival and reproduction. The behavioral mechanism that helps us disengage from impossible goals can become a generalized condition that inhibits the pursuit of any goals, even perfectly attainable ones. The symptoms of depression often last far longer than its initial causes; improved circumstances, in other words, do not always result in relief from symptoms.
In some cases it appears that depression is an adaptation that has long outlasted its utility. Rottenberg presents an instructive analogy with the freeze reflex of deer. Before headlights existed, freezing would help deer avoid detection by a predator. Now, however, the impulse to freeze when threatened often causes their death. Humans can also respond to perceived threats in maladaptive ways that decrease our well-being and even jeopardize survival. Anxiety that might have been useful in the context of a lion hunt can easily become debilitating when triggered by a sales presentation. Depression too can be both a valuable defense and a devastating vulnerability.
Though depression has origins deep in evolutionary history, contemporary factors may be increasing our susceptibility to the condition. Americans get less sleep and less exposure to sunlight than they did a century ago. Cultural expectations of wealth and happiness are also rising. In 2000, half of high school seniors wanted to pursue an advanced degree, a percentage that has doubled since 1970. Many American teens now consider fame or wealth reasonable life goals. Given that a major function of depression is to promote disengagement from impossible goals, it's alarming to consider how a cultural acceptance of extravagant aspirations could be contributing to the depression epidemic.
Even that most fundamental American activity of pursuing happiness can become a self-defeating drive. Other species experience bad feelings, but humans are unique in their capacity to feel bad about feeling bad. Cultural stigmas surrounding depression certainly don't help to alleviate this meta-level of suffering. Humans are also singular in the confidence we place in our powers of reason. But attempts to reason one's way out of depression often backfire: the search for causes that explain a low mood often ends up identifying several new reasons to feel down.
Despite the dark subject of his book, Rottenberg's search for the fundamental sources of depression is strangely consoling, even inspiring at points. By accounting for depression in evolutionary terms, he decisively discredits any lingering explanations of depression as a character flaw. He also achieves something equally powerful: a nuanced assessment of the ever-shifting advantages and costs of depression in various circumstances. Depression is not an experience anyone would choose, but this doesn't mean that natural selection hasn’t favored the evolution of a condition that continues to harm and benefit us all in some way.