— MSSAT Better Blokes (@BetterBlokesNZ) November 8, 2015
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 percent 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 five-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.