Cesare Lombroso, the father of modern criminology, owes his career to a human skull. In 1871, as a young doctor at a mental asylum in Pavia, Italy, he autopsied the brain of Giuseppe Villela, a Calabrese peasant turned criminal, who has been described as an Italian Jack the Ripper. “At the sight of that skull,” Lombroso said, “I seemed to see all at once, standing out clearly illuminated as in a vast plain under a flaming sky, the problem of the nature of the criminal, who reproduces in civilised times characteristics, not only of primitive savages, but of still lower types as far back as the carnivora.”

Lombroso would go on to argue that the key to understanding the essence of criminality lay in organic, physical, and constitutional features—each defect being a throwback to a more primitive and bestial psyche. And while his original insight had come from a skull, certain telltale signs, he believed, could be discerned long before an autopsy. Chief among these was left-handedness.

In 1903, Lombroso summarized his views on the left-handed of the world. “What is sure,” he wrote, “is, that criminals are more often left-handed than honest men, and lunatics are more sensitively left-sided than either of the other two.” Left-handers were more than three times as common in criminal populations as they were in everyday life, he found. The prevalence among swindlers was even higher: up to thirty-three per cent were left-handed—in contrast to the four per cent Lombroso found within the normal population. He ended on a conciliatory note. “I do not dream at all of saying that all left-handed people are wicked, but that left-handedness, united to many other traits, may contribute to form one of the worst characters among the human species.”

Though Lombroso’s science may seem suspect to a modern eye, less-than-favorable views of the left-handed have persisted. In 1977, the psychologist Theodore Blau argued that left-handed children were over-represented among the academically and behaviorally challenged, and were more vulnerable to mental diseases like schizophrenia. “Sinister children,” he called them. The psychologist Stanley Coren, throughout the eighties and nineties, presented evidence that the left-handed lived shorter, more impoverished lives, and that they were more likely to experience delays in mental and physical maturity, among other signs of “neurological insult or physical malfunctioning.” Toward the end of his career, the Harvard University neurologist Norman Geschwind implicated left-handedness in a range of problematic conditions, including migraines, diseases of the immune system, and learning disorders. He attributed the phenomenon, and the related susceptibilities, to higher levels of testosterone in utero, which, he argued, slowed down the development of the brain’s left hemisphere (the one responsible for the right side of the body).

But over the past two decades, the data that seemed compelling have largely been discredited. In 1993, the psychologist Marian Annett, who has spent half a century researching “handedness,” as it is known, challenged the basic foundation of Coren’s findings. The data, she argued, were fundamentally flawed: it wasn’t the case that left-handers led shorter lives. Rather, the older you were, the more likely it was that you had been forced to use your right hand as a young child. The mental-health data have also withered: a 2010 analysis of close to fifteen hundred individuals that included schizophrenic patients and their non-affected siblings found that being left-handed neither increased the risk of developing schizophrenia nor predicted any other cognitive or neural disadvantage. And when a group of neurologists scanned the brains of four hundred and sixty-five adults, they found no effect of handedness on either grey or white matter volume or concentration, either globally or regionally.

Left-handers may, in fact, even derive certain cognitive benefits from their preference. This spring, a group of psychiatrists from the University of Athens invited a hundred university students and graduates—half left-handed and half right—to complete two tests of cognitive ability. In the Trail Making Test, participants had to find a path through a batch of circles as quickly as possible. In the hard version of the test, the circles contain numbers and letters, and participants must move in ascending order while alternating between the two as fast as possible. In the second test, Letter-Number Sequencing, participants hear a group of numbers and letters and must then repeat the whole group, but with numbers in ascending order and letters organized alphabetically. Lefties performed better on both the complex version of the T.M.T.—demonstrating faster and more accurate spatial skills, along with strong executive control and mental flexibility—and on the L.N.S., demonstrating enhanced working memory. And the more intensely they preferred their left hand for tasks, the stronger the effect.

The Athens study points to a specific kind of cognitive benefit, since both the T.M.T. and the L.N.S. are thought to engage, to a large extent, the right hemisphere of the brain. But a growing body of research suggests another, broader benefit: a boost in a specific kind of creativity—namely, divergent thinking, or the ability to generate new ideas from a single principle quickly and effectively. In one demonstration, researchers found that the more marked the left-handed preference in a group of males, the better they were at tests of divergent thought. (The demonstration was led by the very Coren who had originally argued for the left-handers’ increased susceptibility to mental illness.) Left-handers were more adept, for instance, at combining two common objects in novel ways to form a third—for example, using a pole and a tin can to make a birdhouse. They also excelled at grouping lists of words into as many alternate categories as possible. Another recent study has demonstrated an increased cognitive flexibility among the ambidextrous and the left-handed—and lefties have been found to be over-represented among architects, musicians, and art and music students (as compared to those studying science).

Part of the explanation for this creative edge may lie in the greater connectivity of the left-handed brain. In a meta-analysis of forty-three studies, the neurologist Naomi Driesen and the cognitive neuroscientist Naftali Raz concluded that the corpus callosum—the bundle of fibers that connects the brain’s hemispheres—was slightly but significantly larger in left-handers than in right-handers. The explanation could also be a much more prosaic one: in 1989, a group of Connecticut College psychologists suggested that the creativity boost was a result of the environment, since left-handers had to constantly improvise to deal with a world designed for right-handers. In a 2013 review of research into handedness and cognition, a group of psychologists found that the main predictor of cognitive performance wasn’t whether an individual was left-handed or right-handed, but rather how strongly they preferred one hand over another. Strongly handed individuals, both right and left, were at a slight disadvantage compared to those who occupied the middle ground—both the ambidextrous and the left-handed who, through years of practice, had been forced to develop their non-dominant right hand. In those less clear-cut cases, the brain’s hemispheres interacted more and overall performance improved, indicating there may something to left-handed brains being pushed in a way that a right-handed one never is.