Some of the common characteristics of depression are:

These features of depression are exactly what make it so paralyzing: they make it difficult to continue with life as usual.

For decades, the standard explanation has been that something has gone wrong in the brain—that depression stems from a chemical imbalance that can be corrected with drugs.

But the chemical imbalance theory has little to support it. Despite decades of research, nobody has found solid evidence that depressed people have lower serotonin than anyone else—or any other known brain dysfunction.

There is a powerful alternative, one that dispenses entirely with the broken-brain metaphor. And it offers a surprising answer to why depression is so paralyzing.

Depression as Adaptation

In 2000, the psychiatrist and evolutionary biologist Randolph Nesse published a paper titled “Is Depression an Adaptation?” Since then, it’s been the subject of numerous scholarly papers (see here for a recent review).

Just as there are physical adaptations, like hands for grasping, there are psychological ones. The purpose of rage is to enforce boundaries. The purpose of sexual desire is procreation. The purpose of pain is to alert us that something is wrong with our body.

You might think depression happens when one of these psychological adaptations breaks down—a defect or dysfunction in our normal capacity for sadness. But Nesse proposed something radical: that low mood—and perhaps some depression—isn’t a breakdown at all, but a response shaped by natural selection to help us cope when our efforts in life aren’t working.

If so, it has a purpose, or function, and sometimes we stay sick because we don’t know what its purpose is.

What’s the Purpose of Depression?

If depression has a purpose, what is it? At the most general level, we can say that depression is your brain’s functional signal that something in your life is not going well and needs more attention. Its purpose is to disrupt a life path that isn’t suited to you, such as a job, a relationship, or a long-term goal.

Consider a middle-aged woman: a successful entrepreneur with a happy family. Suppose she suddenly sinks into a deep depression—”out of the blue,” as we like to say. Everything looks fine from the outside, but she’s paralyzed by hopelessness and futility. Surely this is a brain defect?

Not necessarily. Her depression may be a signal. It’s a message from the depths of her mind, just like pain. It’s a message that there’s some aspect of her life that she’s deeply unsatisfied with, but that she’s managed to set aside until now.

Why Is It So Paralyzing?

This brings us back to the puzzle. If depression is there to help you make positive life changes, why are depressed people so often incapable of making those changes? Why would a signal designed to help us change take away the energy we need to start a new career or relationship, or to pursue a new life goal?

The reason depression is so paralyzing is that it leads us to change by making us withdraw from a situation that no longer suits us. By sapping motivation and pleasure, it disrupts our ability to continue with business as usual. It’s the brain’s way of saying “I quit.” It’s a quiet form of rebellion.

In the best of cases, this period of depression is meant to lead us to insights about how we can grow, and about what a better life would look like.

What Does This Mean for Us?

This paradigm gives us a powerful tool for working through depression: Instead of fighting it off as the enemy, we want to listen to what it’s trying to say.

The first question this paradigm prompts us to ask is: What aspect of my life is it pointing to? What needs to change? The answer might be obvious—but it might not be. In that case, it might help to find a therapist who’s open to this paradigm.

A good therapist can not only help you figure out the source of your depression but also help you come up with a reasonable action plan that you can carry out without uprooting your whole life.

Depression Essential Reads

For example, the woman I described earlier doesn’t necessarily need to leave her husband, quit her business, and move out to the woods.

The solution might be more modest: to hire an assistant to help with the business; to arrange to spend one weekend a month pursuing a creative endeavor she’s left behind; to work with her family to come up with a better household division of labor.

Is All Depression Functional?

The evolutionary theorists don’t think every single instance of depression has a function or purpose. They recognize that in some cases depression might stem from a brain dysfunction.

But current evidence suggests that about 80 percent of depression cases are attributable to a life event.

Sometimes, then, we won’t know whether a person’s depression is functional or dysfunctional. The solution isn’t to abandon the function paradigm but to think of it as a first-line intervention for otherwise healthy adults.

If changes aren’t happening within a few months, that may be a good time to consider other options, like antidepressants. This simply follows a basic principle of medicine: Try the least invasive treatment first.

My focus here is on mild to moderate depression. Severe depression can lead to suicidal ideation or even suicide attempts. In those cases, the first priority is getting the person to safety, then working out the best approach in coordination with a doctor.

If you or someone you love is contemplating suicide, seek help immediately. For help 24/7 dial 988 for the 988 Suicide & Crisis Lifeline, or reach out to the Crisis Text Line by texting TALK to 741741. To find a therapist near you, visit the Psychology Today Therapy Directory.