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Nature

February 3, 2005

Blue Monkey or Yellow Science?

What causes depression? Don't make the mistake of looking to monkeys for the answer.

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What causes depression? Don't make the mistake of looking to monkeys for the answer.

According to a recent study, it is the result of social stress on low-ranking community members. By researching captive long-tailed macaques (a good description of the species is here), Wake Forest scientists, led by Carol Shively, found that females at the bottom of the social hierarchy were more prone to exhibit depressive symptoms than other community members. The scientists analogized in a press release: "In addition, depression is higher among people with low levels of education and income. Some female monkeys face social stress that is similar to the stress that humans with low socioeconomic status experience. In some cases, these monkeys were also more prone to depression."

This study follows previous research done with male macaques, where researchers found similar results. The story goes something like this: Low-ranking monkeys get picked on and harassed whenever they are around other high-ranking monkeys, which in turn increases cortisol levels that could ultimately kill their brain cells. So it makes evolutionary sense for them to withdraw from the group and suffer "depression" while they wait for circumstances to change. In addition, those primates who had the genetic predisposition to become "depressed" were ultimately more-successful than others in reproducing, and over millions of years this predisposition became entrenched in the human genetic code. Now, when people feel they have become "low ranking," they become depressed.

The idea that depression is somehow evolutionarily adaptive, known as Rank Theory ( a good synopsis is here), has had a powerful impact. Psychology Today, for example, features a how-to guide to using an "appreciation of the evolutionary perspective on depression to help those who are acutely suffering." In an interview about rank theory with US News & World Report, Shively explained that the same genes that cause depression can be helpful: "Low energy, for instance, stops you from banging your head against a brick wall." But this theory has grave flaws.

Rank theory comes from the field of evolutionary psychology (which is explained well here), in which researchers attempt to understand how the human mind works based on evolutionary pressures faced by our progenitors. The field of evolutionary psychology has major flaws, though, and the Wake Forest researchers—who are attempting to make decisive judgments about the human brain based on captive studies of just 36 members of a species with which humans last shared a common ancestor 25 million years ago—are guilty of one of the field's biggest flaws: Adaptationism, which assumes the origin of species to be the result of natural selection.

Adaptationism, wrote Stephen Jay Gould and Richard Lewontin in 1979, "regards natural selection as so powerful and the constraints upon it so few that direct production of adaptation through its operation becomes the primary cause of nearly all organic form, function, and behaviour." It fails to allow for random genetic drift (which is explained further here) or for the fact that many adaptations as the result of natural selection can later on take on completely different—and perhaps evolutionarily unfavorable—functions. No one would claim that schizophrenia is a result of evolutionary advantages conferred upon a select group of our forebearers from the Miocene.

Another major problem with the Wake Forest study is the choice of depression, an amorphous, evolving concept that is hard to measure. Stress is one thing—it can be measured by cortisol levels. Depression is quite another. The definition of depression in humans has changed substantially over the years, as have the ways psychiatrists diagnose and treat the disease. Today, 19 million people in the United States are considered to be suffering from depression (see here for more stats), but psychiatrists admit that many questions about the disease remain, including why many more women than men have been diagnosed but more men than women become suicidal. Additionally, the evolution of psychiatry itself has made it impossible to know whether depression has always been a major problem in the world—the DSM used by psychiatrists to diagnose cognitive disorders has changed drastically within the last 50 years. In the macaque study, the researchers based decisions about which monkeys were depressed based on body position and attentiveness—hardly definite indicators.

Furthermore, depression is hardly genetic destiny. A recent study found for the first time a mutant gene that suppresses serotonin release in humans—but even then, the mutation was present in fewer than 10% of the depressed subjects. Genetics, psychology, and environment all are major factors in depression—and it is very difficult to understand the impact of each one individually.

In other words, scientists have a long way to go before we understand the underpinnings of human clinical depression—and it seems unlikely that studying our distant primate relatives will be the key to unlocking the mystery. I don't know whether this study makes me depressed or simply makes me want to bang my head against the wall.







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