Why do some people have left-handedness?

Left-handedness is not caused by a single gene or a single event. Research suggests that many genes contribute small effects, and brain development also plays a role in which hand becomes dominant. Cultural pressures have reduced left-handed writing in some societies, but handedness itself is a stable trait. Globally, about one in ten people is left-handed, although the exact proportion varies by population and how it is measured. Scientists still study why human handedness is so unevenly distributed.