Generally speaking, left-handed people are those who are most comfortable and most inclined to use their left hand to write. Left-handedness isn't as simple as a physical malfunction or "just another way to be." Being left- or right-handed has to do with how the right and left sides of the brain relate to one another, and evidence supports the idea that there are clear differences between the way left-handed and right-handed people think.
According to Scientific American, about 15 percent of people are left-handed. Studies do vary on this statistic, partly because the number of left-handed people varies from culture to culture, and partly because of the lack of a true definition of left-handedness. Some scientists consider those who use their left hand to write but their right side for everything else--their right eye to look through a camera lens and their right foot to kick a ball.
The reasons why some people are left-handed are not entirely clear, but they seem to depend on a mixture of genetic and environmental factors. Left-handedness does seem to run in families, but it also appears in families where no immediate member is left-handed. Some theoretical environmental reasons for left-handedness are birth trauma, exposure to high levels of testosterone in the womb and physical conditioning.