We use the term “normal” so casually and so often that it seems utterly…normal. But in a compelling Trends in Cognitive Sciences paper published earlier this year, Yale University neuroscientists Avram Holmes and Lauren Patrick argue we must move beyond the traditional concept of “normal” because it doesn’t exist—at least, not as a single, fixed entity. Instead, they contend, it represents a wide spectrum of healthy variability. They shed light on why this variability exists, and what it contributes to the process of evolution.

It’s a position that is increasingly embraced by cognitive neuroscientists, and it has clinical implications. The current assumption is mental pathology reflects a deviation from or disruption of “normal” behavior, which is in turn defined as the average behavior across the larger population; if you don’t behave like most people, then something’s gone wrong.

Consider depression. In the Diagnostic and Statistical Manual of Mental Disorders, depression is defined by a constellation of symptoms: low mood, diminished interest in or ability to feel pleasure, changes in weight or appetite, sleeping too little or too much, feeling sluggish or hyperactive, etcetera. This diagnostic line in the sand draws a contrast to what is viewed as “normal” mental behavior—things like normal mood, normal interests in and ability to feel pleasure, normal appetite, sleep, energy and so on.

The systematic lumping of non-normal behaviors into a discrete diagnosis gives the impression mental illness is a cognitive stepping out of bounds—that the mind has broken and requires fixing. This concept of a fixed “normal,” Holmes and Patrick argue, is an erroneous and unrealistic (could one say “unhealthy”?) misconception that doesn’t reflect the healthy variation produced by evolution.

Evolution, they maintain, doesn’t converge on a stable “healthy”—or even a narrow range of healthy values within a given trait. Instead, there is a large range of healthy variability that depends greatly on the environment. And environments are rarely stable.

To illustrate this, they describe how for decades, biologists studied the foraging behaviors of the great tit (a bird, Parus major) with the assumption there was a single “healthy” foraging strategy that individual birds deviated from at their own peril.

But this healthy-versus-unhealthy dichotomy was proved untrue when biologists realized the species’s optimal temperament and foraging style varied from year to year, depending on the environment. When food was scarce, adventurous and aggressive birds were more successful at finding food and so outlived their more mellow neighbors. When food was abundant, these same adventurous and aggressive birds—doing the same things—were more likely to put themselves in harm’s way and so were less likely to outlive their neighbors who stayed close to the nest.

The take-home: behaviors are not simply healthy or unhealthy but rather healthy or unhealthy within a specific environment. Behavioral variability allows a species to flexibly meet ever-changing conditions.

As you might expect, the same applies to our own species. Sensation-seeking and impulsive behaviors can predispose someone to substance abuse and (one imagines) are problematic for people trying to hold down a nine-to-five desk job. But studies have shown these same traits are essential to social behavior, physical activity, environmental exploration and finding a partner.

These traits, Holmes and Patrick point out, are further fleshed out in brain-imaging studies—not as discrete categories (person A has an unhealthy, sensation-seeking brain; person B’s brain is healthy) but rather as characteristics that vary in each individual. Studies have shown each person’s unique genes and environment interact to produce brain networks with unique properties that coalesce into complexly unique behaviors. Just as fingerprints vary by person, so do brains.

So, they contend, the construct of “mental illness” should reflect not simply our genes, brains and behaviors, but our environments as well. Returning to the great tits, one could imagine that during times of plenty an aggressive great tit might perch on their therapist’s couch, struggling to chillax. During a famine, however, a mellower great tit might need a nudge out of the nest. Churchill was a superb wartime prime minister, but less effective in peacetime.

The construct of “healthy,” therefore, should reflect a wide range of “normal” behavior—so wide that it would likely destroy our tidy and compact notion of “normal.”

It is this sort of complexity—the very variability of the real world—that Holmes and Patrick invite us to embrace. Unfortunately, complexity is…well, complex. It’s something our brains, in their effort to adapt, simply don’t do well. This resistance against complexity reflects an adaptive strategy; when faced with it, we tend to reduce our environment to actionable—often binary—decisions. (“To be or not to be—that is the question!”) But as the scientific environment evolves, we scientists, too, should adapt our strategy as we forage about the mental jungle for truth.

Anything less would be unhealthy.