With just 10% of the population being left-handed, it can be easy for everyone else to forget we're living in a right-handed world. But aside from making it tough to cut a straight line with a pair of scissors designed for righties, being a southpaw can also have some subtle effects on our physical and mental health. The brains and bodies of lefties may operate differently than those of right-handed people (and in mixed-handed people, who may have different dominant hands for different tasks). "Handedness seems to be determined very early on in fetal development, when a lot of other things about your future are being determined as well," says Ronald Yeo , PhD, professor of psychology at the University of Texas-Austin.
Here's a look at some of the most common facts about being left-handed, and what it might really mean for your health.


It's not just genetics
Scientists aren't exactly sure why some people are left-handed, but they know that genes are responsible about 25% of the time, says Yeo. Left-handedness does tend to run in families, he says, "but noticeably less than other inherited traits, like height or intelligence." In fact, identical twins, who share the same genes, can sometimes have different dominant hands. There are plenty of theories on what else might determine which hand you write with, but many experts believe that it's kind of random, says Yeo.
It's linked to stress in pregnancy
In one British study, the fetuses of super-stressed pregnant women were more likely to touch their faces more with their left hands than their right. This could be the first signs of a left-handed child, say the researchers. Other evidence supports that theory. In one 2008 Swedish study of moms and their 5-year-old children, women who were depressed or stressed during their pregnancies were more likely to have mixed- or left-handed kids . In other studies, babies with low birth weight , or born to older mothers, were more likely to be lefties as well.

It's more common in twins
Identical twins are sometimes mirror images of each other—one twin has a mole on her right cheek and the other has a mole in the same spot on her left cheek, for instance. It was once believed that twins' genetic makeup should be "mirrored" as well—therefore, one twin should be left-handed and the other should be right. (It was also once thought that all left-handed people started out as twins, and that their rightie siblings died in the womb.)
Neither of these is true, but left-handedness is about twice as common in twins than in the general population. A 1996 Belgian study found that about 21% of twins , both fraternal and identical, are left-handed

It doesn't make you "right-brained"
Most right-handed people use the left hemisphere of their brains to process language, but that doesn't mean most lefties are "right-brained"—that's just a common myth, says Gina Grimshaw, PhD, director of the Cognitive and Affective Neuroscience Laboratory at the University of Wellington in New Zealand. About 98% of right-handers are left-brained, she says, but so are about 70% of left-handers. Only about 30% are right-brained or bilateral-brained (in which both halves are equally capable). "Most left-handers seem to have similar language processing to right-handers," Grimshaw says. For other one-sided brain functions, such as attention, emotion, music, and face perception, she says, there are less data. "But for the most part, left-handers do not differ obviously from right-handers. They certainly don't have reversed brains."