Once there was a movement to retrain people to correct a "disease." The best evidence showed there was no great spread of the disorder, no gentle normal distribution; it was more of a yes-or-no condition. Those with the affliction could pretend to be otherwise, but the results were grim. What was in the past described as a mere affectation or a neurotic choice, society now accepts as an intrinsic trait. We've come a long way in our understanding of left-handedness.

Lateralization—dividing the brain's into tasks left and right—is a common feature of all vertebrates, and it goes much deeper than preferring one hand over another. Dividing tasks between the halves of the brain, the hemispheres, improves capacity—like having a pastry chef and a sous-chef rather than two generic line cooks. One side scans for prey, the other figures out how to capture it; one side hunts, the other socializes—by splitting responsibilities, there is less chance for conflict. After all, how would two identical halves of the brain decide who is in charge? By working as a team, each tackling a specific aspect, the hemispheres can process complex situations in parallel.

For most people, the left side takes care of math, logic, controlling our dominant hand, and speech. The right side handles emotion, shape recognition, and where things are in space. These preferences start very early on. By 7 weeks, the right hand of most fetuses is more developed than the left; by 15 weeks, most fetuses suck their right thumb.

For the 10 percent of people who are left-handed, both speech and dominant-hand control migrate over to the right hemisphere. While the exact cause of left-handedness is debated, the strong underlying biological basis is not. Left-handed adults favor their left sides as fetuses. We can observe differences in brain structure by MRI between left- and right-handed people. But studies to figure out the underlying genetics have proven tricky. Mice also favor a paw; when attempting to breed right- or left-pawed mice, a scientist instead ended up with either ambidextrous mice—without a favored paw—or mice that strongly favored one paw, but in a half-and-half mix.

So why are 90 percent of people right-handed? Perhaps it was communication. Before speech evolved, our ancestors probably conversed with hand gestures. The machinery to decipher complex shapes is on the right side of the brain, so those using the left brain—controlling the right hand—to make the shapes could out-compete lefties by better specializing their minds.

As for the 10 percent who are southpaws, just being different than most—think of left-handed baseball pitchers or boxers—gives them some advantages. And on a population level, having diversity—some minds with speech taking space from logic, and others borrowing from emotion processing centers—is a benefit. Human beings' greatest strength as animals is our overwhelming variety, our great mixture of genes; the mix of handedness is one small part in making us an adaptable creature. What's not to love?