Around 90% of people are right-handed — and it’s been this way since at least the Paleolithic. Now, for the first time, researchers have identified regions of the human brain that are directly linked with left-handedness, and found that being a leftie is associated with both positive and negative traits.

The skewed preference for right-handedness is a uniquely human feature, researchers say. However, we still don’t know why it happens or what other effects it has. While left-handedness seems to run in the family, studies have been unable to show whether left-handedness is strictly under the genetic influence. This being said, several connections have been found between left-handedness and genetic conditions, including schizophrenia. Some results have suggested that genes are responsible for about 25% of handedness — but overall, neurologic studies on human handedness have been unable to shed light on which genes are connected to handedness.

Researchers used data from the UK Biobank, a prospective cohort study of half a million volunteers that gathers a huge range of data on their health and habits. Data from around 400,000 participants was analyzed, with 38,000 of them being self-described lefties. They identified four genetic hotspots associated with left-handedness.

“For the first time in humans, we have been able to establish that these handedness-associated cytoskeletal differences are actually visible in the brain,” said lead author Professor Gwenaëlle Douaud, who is herself left handed.

Most mutations they identified lie in areas connected to the cytoskeleton — the intricate cellular scaffolding that directs the inside of our body’s cells. Mutations in this area have been connected to changing chirality in other species. In snails, for instance, similar mutations can lead to anti-clockwise — essentially, the “left-handed” version of the shell. For snails, this is a huge problem because they can only mate with those who have the same shell chirality, due to the way their genitals are positioned.

While humans have no such issues, researchers found evidence that these cytoskeleton modifications are also affecting the way white matter is structure in the brain. There are well-established associations between left-handedness and several neurodevelopmental disorders — now, the team has found a smoking gun regarding the source of these changes, helping explain why lefties are at a slightly higher risk of some mental conditions, including Parkinson’s.

But it’s not all bad — quite the opposite.

The imaging–handedness analysis revealed an increase in functional connectivity between left and right language networks in left-handed participants. While researchers don’t have the data to back it up, they suspect that this gives lefties may have slightly better verbal skills.

However, this is still only a piece of the puzzle when it comes to understanding handedness. This study has only identified a part of the genetic differences linked to left-handedness, and only in British population — there’s still much more left to discover.

The study has been published in Brain.