The history and geography of human handedness

Summary About 90% of people are right-handed and 10% are lefthanded. Handedness is associated with functional lateralization for cerebral dominance, and may also be associated with various types of psychopathology. Broadly speaking, the vast majority of humans seem to have been right-handed since the emergence of the genus Homo, some three to four million years ago. Likewise, in all societies studied, there is a large excess of right-handers. However, there have been few studies exploring the detailed history and geography of handedness, not least because adequate pre-twentieth-century historical data are difficult to find, and very large sample sizes with consistent measurement methods are required for geographicalstudies.Thischapteroverviewsthevarioussetsofdatathat provide insight into handedness’s history and geography. It is probable that about 8% to 10% of the population has been left-handed for at least the past 200000 years or so. Detailed data only began to become available for those born in the nineteenth century, and there is growing evidence that the rate of left-handedness fell precipitously during the Victorian period, reaching a nadir of about 3% in about 1895 or so, and then rising quite quickly until an asymptote is reached for those born after about 1945 to 1950, with 11% to 12% of men and 9% to 10% of women typically being left-handed in Western countries. The sex ratio seems to remain constant, not only during historical changes but also with geographical differences, and is presumably the result of a biological rather than a cultural process. Geographical differences in handedness are clearly apparent both between continents (as in Singh & Bryden’s, 1994, comparison of Canada and India) and within continents: rates in Europe seeming to be highest in Britain, Holland, and Belgium, and falling away towards the east and south, and within countries, seen well in Stier’s (1911) study of the German Army, in Leask and Beaton’s (2007) study of the United Kingdom, and between the various states of the USA, in the very large Gilbert and Wysocki (1992) database. Ethnic differences in handedness are related to geographical differences, with left-handedness generally being more common in White, Asian and Hispanic populations – a difference seen both in the UK, and historically in the United States, wherethedifferencebetween ethnic groupshas grown smaller during the twentieth century, but was still present even for those born in the 1970s. Migration studies in the UK show that the lower rate of left-handedness in those from the Indian sub-continent is similar in those born in the UK and those born outside the UK, implying that genes rather than environment are the primary source of the difference. Different rates of left-handedness can reflect either environmental or genetic differences between societies, and rates alone cannot distinguish the two processes. However, a mathematical model shows that effects of different social pressure or gene frequencies can be distinguished if family data on handedness are available. That model suggests not only that geographicaldifferencesbut alsohistorical differences primarily reflect changes in gene frequency rather than direct social pressure.