Happiness has had a tough time of it lately. The backlash against the seemingly endless stream of books about the subject (Amazon returned 426,789 titles when I used that search term, including one that calls happiness "life's most important skill") had already set in last year. At the time, I pointed out that "among people with late-stage illnesses, those with the greatest sense of well-being were more likely to die in any given period of time than the mildly content were. Being 'up' all the time can cause you to play down very real threats," and channeled the arguments of scholars who lamented the medicalization of the normal human emotion of sadness. (Click here to follow Sharon Begley)

These and other critiques of happiness and the happiness industry, however, came mostly from psychologists, philosophers, and sociologists who are concerned about the effect of a message that says modest levels of well-being aren't enough, and that we all practically have a duty to be really, really happy—and that what was once considered normal sadness is something to be smothered, even shunned. I was therefore interested to see a new scientific paper taking a more brain-based perspective. (My thanks to The Psych Student blog for drawing my attention to it.)

Writing in the journal Psychological Review, postdoctoral fellow Paul Andrews of Virginia Commonwealth University and psychiatrist J. Anderson Thomson Jr. of the University of Virginia present research suggesting that depression is present in the species, and in individuals, for a purpose, and we're playing with fire if we try to eradicate it. In evolution-speak, depression is an "adaptation," they argue. That is, it evolved because it made individuals who experienced it fitter, under natural selection, than individuals who did not experience it. Andrews and Thomson—who is best known for research on the psychology of religious belief, and who has also studied whether antidepressants threaten love and fidelity—offer as evidence the presence of a molecule in the brain called the 5HT1A receptor. This serves as a docking port for the neurochemical serotonin, which the Prozac/Zoloft/Paxil class of antidepressants targets. Human brains are not the only ones with the 5HT1A receptor. Rats also have it.

Here's the really interesting part: the rat version is 99 percent identical to the human one. This suggests that in the evolution from the shared ancestor of rats and people (hold those creationism letters!), natural selection did not mess with the receptor much. That leave-well-enough-alone history tends to happen when the function of some trait is so important that tinkering with it evolutionarily would produce more harm than good. What kind of harm? Rodents that have a mutation causing them to lose this receptor exhibit fewer symptoms of depression when they suffer some stress, a 1998 paper reported. In other words, losing the receptor that promotes depression in response to stress is something evolution thought would be a very bad move. Ergo: depression is not something to be thrown out lightly.

Why not? Because, argue Andrews and Thomson, depression alters thinking and behavior in beneficial ways. For instance:

*People in the grip of depression tend to ruminate, to turn an issue over and over in the mind. If they're ruminating on why they can't get a date, that might seem bad—since it keeps the person depressed. But this way of thinking, note the scientists, is "often highly analytical." That can be useful, producing solutions to what tipped the person into depression in the first place, not to mention "Eureka!" moments such as discovering fire. Evidence: people who felt depressed before tackling challenging math problems tend to score higher than happier test-takers, Andrews and Thomson reported in a 2007 study.

*Depression tends to focus thinking. That 5HT1A receptor, it turns out, also supplies neurons with fuel, allowing them to fire without flagging. That includes neurons in the ventrolateral prefrontal cortex, which have to fire continuously to keep the mind from wandering. (It's an attention circuit.) Focused thinking, like analytical thinking, might help someone overcome depression.

*Depression tends to make sufferers seek isolation, and keeps them from deriving pleasure from sex, food, or life itself. Obviously this can be crippling (and even fatal) to the sufferer. But it may also be adaptive: these behaviors foster the kind of focused and deliberative thinking that might solve the problem that triggered the depression in the first place. Evidence: a 2006 study found that when people suffering from depression engage in expressive writing, which forces them to focus on their troubles, their depression tends to lift sooner than otherwise. A 2008 study reached the same conclusion.

Thomson, who has a private psychiatry practice in Charlottesville, Va., has described how his views on the mind have been shaped by evolutionary psychology, the discipline that (among other things) floats hypotheses about the evolutionary roots of illness. Evo psych changed the way he viewed life and practiced psychiatry, Thomson has said. He's putting theory into practice, going so far as to suggest that people suffering from depression might benefit from more rumination, not less. "Therapies should try to encourage depressive rumination rather than try to stop it, and they should focus on trying to help people solve the problems that trigger their bouts of depression," he and Thomson write in a popularized version of their paper.

The question of whether depression has adaptive value is obviously far from settled. One criticism: the argument that depression fosters rumination and analytical thought seems problematic, since many people with depression report that although they indeed ruminate on their problems, their thinking is far from clear, focused, and analytical, and thus provides little insight into—let alone a remedy for—their illness. Nevertheless, as philosophers and sociologists harrumph about the pernicious effect of the happiness industry, it's useful to throw some neuroscience into the mix.

Sharon Begley is NEWSWEEK's science editor and author of The Plastic Mind: New science reveals our extraordinary potential to transform ourselves and Train Your Mind, Change Your Brain: How a New Science Reveals Our Extraordinary Potential to Transform Ourselves .

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