Humans are unusually right-handed compared with other primates
About nine in ten people favour their right hand, a bias so strong that a study of 41 monkey and ape species found humans to be outliers. Most other primates split evenly between left and right. Researchers suspect our lopsidedness is tied to walking upright and to our big brains.
Roughly 10 percent of people are left-handed, and a similar share are mixed-handed, switching hands between tasks. True ambidexterity is rare, at about 1 percent. A 1975 survey of 7,688 American schoolchildren found 10.5 percent of boys and 8.7 percent of girls were left-handed. Preference shows up early: fetal hand use in the womb predicts handedness after birth, and in a 2013 study 39 percent of infants aged 6 to 14 months and 97 percent of toddlers aged 18 to 24 months already preferred one hand.
Genes play a part, but not a simple one. Two left-handed parents have only a 26 percent chance of a left-handed child, and twin studies put genetic influence at about a quarter of the variation, leaving three quarters to environment. Early theories proposed a single gene, but genome-wide studies now point to at least 40 contributing regions, including genes that set up the body's left-right asymmetry. One popular explanation links handedness to speech: the left hemisphere usually controls talking, and since it also runs the right hand, fine motor skill may cluster there.
Left-handers occupy both ends of some distributions. People with intellectual disability are roughly twice as likely to be left-handed, yet a 2005 study found about 20 percent of Mensa members were left-handed, double the usual rate. Average IQ differences are negligible, about one to one and a half points.
Everyday life favours the majority. Many tools are designed for right-handers, some cultures regard use of the left hand as disrespectful, and in many countries left-handed children are made to write with their right. In sports where a player aims at a target an opponent is defending, left-handers gain an edge, because rivals are used to facing right-handers.
Source: Handedness
