Science

Evolution Could Explain Why Psychotherapy May Work for Depression


A consensus has emerged in recent years that psychotherapies—in particular, cognitive behavioral therapy (CBT)—rate comparably to medications such as Prozac and Lexapro as treatments for depression. Either option, or the two together, may at times alleviate the mood disorder. In looking more closely at both treatments, CBT—which delves into dysfunctional thinking patterns—may have a benefit that could make it the better choice for a patient.

The reason may be rooted in our deep evolutionary past. Scholars suggest humans may become depressed to help us focus attention on a problem that might cause someone to fall out of step with family, friends, clan or the larger society—an outcast status that, especially in Paleolithic times, would have meant an all-but-certain tragic fate. Depression, by this account, came about as a mood state to make us think long and hard about behaviors that may have caused us to become despondent because some issue in our lives is socially problematic.  

A recent article in American Psychologist, the flagship publication of the American Psychological Association, weighs what the possible evolutionary origins of depression might mean for arguments about the merits of psychotherapy versus antidepressants. In the article, Steven D. Hollon, a professor of psychology at Vanderbilt University, explores the implications of helping a patient come to grips with the underlying causes of a depression—which is the goal of CBT, and is also in line with an evolutionary explanation. The anodyne effects of an antidepressant, by contrast, may divert a patient from engaging in the reflective process for which depression evolved—a reason perhaps that psychotherapy appears to produce a more enduring effect than antidepressants.  Scientific American spoke with Hollon about his ideas on the topic.

[An edited transcript of the interview follows].

You described in your recent article the idea that humans evolved a propensity toward depression as a means to restore emotional and psychological equilibrium. That allows people to stay well integrated within their social milieus. So, can you explain how depression may be a product of evolution that can actually protect us?

In the late 2000s, I read a paper by the evolutionary biologist Paul Andrews. It was masterful, very thoughtful—and I totally disagreed with it. The main premise was that depression was an evolved adaptation that serves to make people ruminate.

Why did you disagree?

For clinicians, we think of rumination as a terrible thing that at best is a symptom of depression and at worst leads to something that deepens the depression. We’ve always thought of it as a kind of exhaust out the tailpipe that is not really helpful.

But the work of Andrews and his colleague J. Andrew (Andy) Thomson recounted that in our evolutionary past, what got you depressed was some kind of major problem—probably a social problem—that might get you excluded from the tribe. And what you had to do is sit down and think about things.

Most of us can think of anxiety as being a useful function, because anxiety takes us away from danger. It’s quick, it’s rapid, the reaction that occurs after stepping on a snake that might be poisonous when you’re out in the woods. But most folks don’t think of depression as having any function. It’s just something unpleasant. The trick is to figure out what the purpose of depression is—and when Andrews and Thomson looked at what goes on when you get depressed, they found that a lot of energy went to the brain.

And the reason for that is to help us to think more carefully about the things that are going wrong, and first to understand what’s the cause. That answers the question: How come I’m feeling so bad? And the second thing is: How can I do a good job of figuring out a solution to a problem? 

So, you don’t have to move rapidly in depression; the bad thing has already happened. You don’t have to get out of the way of a poisonous snake or a leopard. But you do have to solve some kind of complex social problem, and rumination is what gets you there. So as opposed to being an unpleasant byproduct of being depressed, rumination is actually the reason why depression evolved. And it helps you solve complex social problems.  

You seem to now be giving some credence to the ideas of Andrews and Thomson. How does this line of thinking actually play out in people’s lives? At what point does depression and the social problems that need to be ruminated upon begin?

These complex social problems often gear up in adolescence when young people start to ask: Am I going to have a boyfriend and girlfriend? How do I get a boy or girl to like me? Am I going to do okay in school? Are my parents happy with me? Am I going to get to go to college? Will I be able to find a job?

How does your expertise in psychotherapy—and specifically cognitive behavioral therapy—jibe with the evolutionary theory of depression?

Cognitive therapy in this context becomes a bit of a natural. It teaches people how to ruminate more effectively. Cognitive theory holds that people got depressed because they hold inaccurate beliefs about themselves. This can be combined with the additional notion that people can get stuck. For example, if something bad happens, you start thinking that you’re a failure, you’re a loser. For most people, depression motivates them to think more deliberately about the causes of their problems and the solutions they can apply. In most instances in our ancestral past this worked well enough; most depressions remit spontaneously even in the absence of treatment. Cognitive therapy, at the least, hurries the process along and, at the most, helps unstick that subset of individuals who get stuck making negative ascriptions about themselves, typically about personal competence or lovability.

The solution is to essentially teach them the scientific method so they get unstuck. We ask a patient to ask themselves: what do you think is the cause of the problem? What other explanations could there be? What’s the evidence supporting one or the other? And especially we encourage patients who get stuck to pit what are called their stable trait theories—“I am incompetent” or “I am unlovable”— against a more behavioral explanation: “I chose the wrong strategies.”

One area that you’ve worked on is whether CBT has a more enduring effect than drugs, and you’re interested in how that might provide evidence for the evolutionary basis for depression.

Basically, we have good clinical evidence that cognitive therapy is at least as effective as medications in the short run, and more enduring in the long run. CBT may get people thinking carefully about their problems in a way that facilitates coming to a resolution, whereas medications may just anesthetize the stress that underlies a depression.

Are you going to test that idea in some way?

I’ve got colleagues in Vietnam, where they’re quite interested in a study we want to do in which we compare folks treated to remission with CBT versus folks treated to recovery with medication—and compare those against a control that uses Chinese herbal medicine, which is widely believed there to be effective. And if it’s really the case that antidepressant medications suppress symptoms in a way that worsens the underlying course of depression, then those patients should be more likely to have recurrences when we take them off the medications than when we take them off the Chinese herbal medicine. If it’s really true that CBT truly has an enduring effect that protects against depression, then patients treated to recovery should be less likely to recur following treatment termination than patients who recover on Chinese herbal medicine. In essence, the Chinese herbal medicine serves as an ideal nonspecific control because it provides neither the coping skills taught in cognitive therapy nor the pharmacologically active serotonin-related ingredient provided by antidepressant medications. We have a trial that we want to do that should answer the question, but it hasn’t yet been done.

Doesn’t some evidence exist along these lines already, though?

There are over half a dozen studies that indicate that patients treated to remission with cognitive therapy are less likely to relapse following treatment termination than patients treated to remission with antidepressant medications—and a pair of studies that suggest that this enduring effect may extend to the prevention of recurrence. What we do not know is where all this fits within the proposed evolutionary context: whether cognitive therapy has an enduring effect, or antidepressant medications may be detrimental in terms of prolonging the life of the underlying episode—as evolutionary theory suggests. What is needed is a nonspecific control that neither has enduring effects or the anesthetizing effects caused by the medication. Whether cognitive therapy truly has an enduring effect, or antidepressant medications have a detrimental effect, remains to be determined. The comparison of each to a nonspecific control like Chinese herbal medicine should allow us to determine in absolute terms which is which.

You’ve talked about the difficulties in trying to measure whether there really are enduring effects in this type of trial.

It also is possible that the enduring effects observed for cognitive therapy (relative to antidepressant medications) have to do with changes that occur during the course of a clinical trial. Although we randomize patients to cognitive therapy versus antidepressant medications at the outset of the trial, we typically lose about 15 percent of the sample because of attrition, and another 25 percent as a nonresponse to either intervention. That means that only about 60 percent of the sample initially randomized makes it into the comparison of subsequent rates of relapse. If different kinds of patients remit to cognitive therapy than to antidepressant medications, that could bias any subsequent comparisons.

Do you think that these insights about CBT could have an impact for severe depression?

I don’t know, and wouldn’t necessarily assume that they would. For psychotic depression, you’d go to electroconvulsive therapy first. I’m not sure that the analytical rumination hypothesis will apply to psychotic depressions, or that it needs to. For every evolved adaptation there are instances in which the mechanism evolved breaks down and the condition can be considered to be an actual disease or disorder.


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