Humans are more alike than different, and that includes our near-universal tendency to overweight our differences.

Think about it this way: what would turtles obsess over? Differences between turtles. Seemingly trivial differences to us—say, a 1 cm variation in shell circumference, or a slight shift in skin tone—might be all they talked about. By contrast, differences among humans, if noticed at all, wouldn’t take up much of their time. To a turtle, someone 6'0" would look pretty much the same as someone 5'6". An extrovert would act more or less the same—sleeping, working, eating—as an introvert.

This focus would be adaptive for turtles—ecologically rational, we might say—but it wouldn’t make for very good science. Not only would non-turtle species be relatively neglected, but in their obsession with differences, turtles would become blind to their similarities.

By and large, the field of (human) psychology has fallen prey to the same instinct bias by marveling at, and expounding upon, the many ways humans can be different. This is all well and good, except for the fact that science is supposed to be reductive. The goal is to reduce many points of data to as few rules or theorems as possible. Simplicity from complexity is the motto.

Yet psychology has mostly produced complexity, and not only because the subject matter is uniquely complex. It’s also because psychologists are human, and humans are wired to look for uniqueness. In some cases, we’re even put off by similarity. It feels boring, impersonal, or politically charged to suggest that people are mostly the same. But to a large degree, they are. (See Don Brown’s Humans Universals for a start.)

The progress of science depends on identifying sameness beneath apparent variation. Psychology may eventually follow other disciplines in this trajectory, but not until it outgrows some of its all-too-human instincts. The first step is admitting that the organ between all of our ears, while complex and far from completely known, is probably more similar than different.

Some phrases for the earnest scientist to avoid like the plague:

• “Well, it depends.” Ok, what does it depend on?

• “Everyone is different.” In some ways, yes. In others, we are similar. For example, the adult human bladder can hold between 300 and 500 milliliters. The range outside of this is much larger than the range within. So why focus on difference when similarity is there for the taking?

• “It’s complicated.” How?