Rates of left-handedness vary by region of the world and even by region within countries. Population studies indicate that more left-handers reside in North America, Western Europe, Australia and New Zealand than in South America. Italy, the United Kingdom and the United States show regional differences in the incidence of left-handedness. For example, Italians living in the north of Italy are more likely to be left-handed than those living in the south. The rates of left-handedness rise as one moves from the eastern European countries to those in the west of the continent. One can expect to find more left-handers in France than in Russia, for example.
There are historical as well as geographical fluctuations in the prevalence of left-handedness. Substantial archaeological evidence indicates that the two types of human handedness, right and left, have existed for millennia. Through the centuries, right-handedness has remained the majority human handedness type with left-handedness continuing as a consistent minority presence of about 10%. However, there was a drop in the prevalence of left-handedness in the 19th century when rates fell to historic lows of 2-4% of the population. What accounted for this precipitous reduction in rates of left-handedness?
I.C. McManus, a prominent expert on theories of handedness, proposes an explanation for this historical dip. He argues that left-handers were ostracized in the 19th century leading to a decrease in their desirability as mating partners. As a consequence of this social stigmatization fewer left-handed children were born to left-handed parents. Presumably, left-handed parents did not want to bring children into a world that would shun their left-handedness. According to the McManus data, RxR parental pairings produced 3.1 children while LxL couples produced only 2.3 children, on average, in the years between 1880 and 1939. The drop in left-hander birth rate caused a decrease in the gene pool favoring left-handedness. This resulted in fewer left-handers in the mating population and fewer left-handers. By 1950, the situation rectified itself. RxR and LxL parental pairs produced children at the same rate of about 2.5 on average. The genes favoring left-handedness rose in frequency when left- and right-handed parents produced children in equal numbers. The percentage of left-handers in the population increased as a result. The graph shown here illustrates this trend.
The McManus theory assumes a strong genetic influence on the development of handedness. Reductions in the left-hander mating population and gene pool resulted in fewer left-handers. However, there was increased acceptance of left-handedness and a reduction in direct pressures to change left-handers to right-handers over the early years of the 20th century. These changes in societal attitudes also contributed to the rise in the numbers of left-handers from the 1930’s onward to the present day. The rates of left-handedness in the late 19th and early 20th centuries are estimated from family studies conducted at the time. These early researchers may have used stricter rules for defining left-handedness than those used today. Changing rates of left-handedness over time, from a low of 2% to a high of 12%, may be based partially on different methods used by researchers over the years to define who is a right- or a left-hander.
Both geography and historical era are factors that affect rates of left-handedness. Read additional details about these influences in Chapter 8 of my book Laterality: Exploring the enigma of left-handedness. The chapter is titled Geography, history and the left hand.