I was supprised to find that there are more in the US than some other countries:Oh, I know the practical reason for not making things hard(er) for left-handed soldiers, my question was a separate one: why are there so many left-handed people in the US?
Source
throughout history there was a great deal of bias and stigma associated with being left-handed and lefties were forced to use their right hands
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Only in recent years North America and Western Europe have taken a more liberal stance on handedness and permitted children to use their naturally dominant hand. The rates of lefties in these countries tend to be higher, approximately 12%. In Asian, Muslim and Latin American counties the rates tend to be lower. In countries where children are still forced to stop using their dominant left hand such as Korea (2%), Japan (3%) and Taiwan (5%) you will find the lowest percentage of lefties.