My last blog post discussed variations in rates of left-handedness across regions of the world. Left-handedness is most prevalent in Western Europe, North America, Australia and New Zealand (10%) while left-handers are a rarity in South America at 4% of the population.¹ Fluctuation in the rates of left-handedness within a country is an even more intriguing fact and one statistic that is harder to explain than international differences. Three countries, the United States of America, Italy and the United Kingdom, are good examples of these internal rate differences. The rates of left-handedness in Scotland and Wales are 2% lower than the 12% incidence in the rest of England.

There is a 4% difference in the rates of left-handedness in Italy with southern sections of Italy having a reported rate of 5% compared to the 9% rate in northern regions of the country.

The percentages of left-handers are lower in the mid-west of the United States as compared to the numbers of left-handers on the two coasts.

How do researchers account for these rate differences within rather than across borders? There are a variety of theories. First, these data often are based on population studies (some conducted over the internet) where respondents report on their own handedness. Left-handers in certain regions of a country may be more likely to participate in these large surveys, raising the rates of left-handedness in only specific sections of a country. Second, micro-cultures within a country may exist where left-handedness is not tolerated and more people are subjected to rightward conversion pressures. As an example, take the 2015 news report of a pre-school teacher in Oklahoma who forced a child to learn to write with the right hand because left-handedness is evil. Fortunately, the left-handed mother of this boy intervened to prevent the conversion. Such pressures lower rates of left-handedness in particular areas of a country. Third, left-handers may be more likely to migrate to areas of a country, raising the rate in some regions and lowering the rate in others. Finally, differences in the genetic pool within a country, where the genes favoring left-handedness are more prevalent in some regions as compared to others, can exist. This last explanation  appeals to those who claim an important genetic component to handedness formation.

There are inter- and intra-national differences in rates of left-handedness. Men are 20% more likely to be left-handed than women and adults under the age of 35 are more likely to be left-handed than adults over the age of 65. These combined facts make it difficult to estimate the overall percentage of left-handers worldwide. One fact is clear, however. No country, sex or age group has been found with a majority of left-handers. The right-handed majority is as persistent as the left-handed minority.

¹Left-handers around the world. August 8, 2017.