What Causes Handedness?

About 10% of Americans are left handed. Since so many people are right handed, what accounts for lefthandedness? The fact is, we don’t know for sure. It might have something to do with language processing. Of 100 people, 90 would most likely be right handed and 86 of them would have their language areas in the left hemisphere. The remaining 10 people would be left handed and 7 of them would have language in the left hemisphere. This means that of 100 people, about 93 would have language in the left hemisphere, regardless of handedness. So, most righties process language on the left side of the brain. There’s more variability with lefties even though most of them process language on the life side as well. Okay, so what are the basic theories for handedness?

The first theory is the environmental theory. This theory suggests that hand preference is learned by children modelling adults and also through environmental pressure to use one hand over another. We know that some kids who would naturally become left handed are forced by their parents to use their right hands. That’s probably why some people have such bad penmanship. In Asian countries, only about 5% of the population is left handed because it is highly discouraged. Clearly, the environmental theory explains some aspects of why more people are right handed, but it does not account for how handedness develops in the first place. For that, we need to look at genetic theories.

There are three major genetic theories of handedness. The first is called the Right Shift Theory. This theory suggests that there is a factor which, if inherited leads to right handedness (and left brain language specialization). If this trait is not inherited, then there is more balance between right and left-handedness and environmental influences may have a stronger bearing on the development of handedness.

The second theory is called the McManus Model, named after the researcher who proposed it. This model asserts that genes do not pass on information about left or right, but instead determine whether an individual’s brain will be strongly lateralized or not. So, whether or not we are left or right handed is influenced by development in utero, which gives us a predisposition toward specific lateralization, and then by environmental influences after we are born. The third theory is related to brain pathology. This theory basically suggests that we were all meant to be right handed but that left handedness occurs due to some kind of brain injury that causes lateralization to occur in the right hemisphere. While it is true that the incidence of left handedness among people with autism and schizophrenia is much higher than in the population (about 30%), the vast majority of left handers do not have any brain pathology so this theory does not fully account for handedness.

What should we make of these explanations? Well, each theory seems to be partly correct. There are factors that determine which side of the brain controls language and motor processing. For some individuals, the tendency for specific lateralization is much stronger than for others (extremely right handed or extremely left handed). For others, lateralization of motor function is more mixed (e.g., someone who writes left handed but plays pool right handed, like this author for instance lol). Even if a child has a tendency to be left handed, environmental influences play an important role. Depending on how you count people, as much as 15% of the population may actually be left handed. This is probably because the stigma about lefthandedness in Western culture decreased during the 20th century. So, it may be that more people will be born lefthanded in the future but it depends how accepting society is about there being more lefthanders. In the meantime, my advice to those lefties out there is to never use a circular saw designed for a righthander. The result could be deadly!