The right shift theory of cerebral dominance is the idea that genetic markers or mutations exist that cause human beings to be primarily right-handed. According to M. Annett, a long time researcher on this subject, most mammals exhibit handedness (or pawness) in a 25%-50%-25% ratio. This means that 25% are right-handed, 25% are left-handed and 50% can be either. This does not follow through to humans, who are 65%-70% right-handed. This handedness seems to follow through among all human cultures, whether primitive or modern.
The reasons for this are still under study. Studies using twins and brain injured persons have not yielded simple solutions. Some known variations are the Scottish Kerr clan which built their towers and castles for left-handed defense since the majority of the family carried their swords on the left side. Other studies have shown the Kerr family has the same number of left-handed people as any other group. The British Royal family is also known for its left-handed members. 20% of the high scorers on the SAT are left-handed although there are only about 10% true left-handed people. A number of studies have shown right-handed people live longer than left-handed people, but the way of this is not certain or clear. Left-handed people are also strongly associated with artistic pursuits.
Another question is just what is handedness. We know there are true right-handed and left-handed people, but there are also ambidextrous people who use both hands naturally, and those who train themselves to use both hands. Trained left-handed people usually stand out for their poor handwriting often have poor reading skills when young, and these often include some kind of speech impediment. Perhaps their processing becomes at bit short circuited. But, there are plenty of natural leftys that don’t have these issues. With a social environment that encourages right-handedness including schools, fitting it is also a key player in human handedness.
One gene called LLMRT1was discovered in 2007 that is linked to left-handedness when present. A form of this gene is also linked to increased psychotic diseases including schizophrenia. However, if both parents are left-handed, on average only 26% of their children will be left-handed, showing again the inheritance factors are not simple.
One difference in humans is the ability to speak. This ability is usually found in the left hemisphere of the brain. The brain crosses over, so the left side influences the right side. In true left-handed people, the ability to generate speech is often found in both hemispheres of the brain. All left-handed people start out generating speech on the left side, some exclusively. Some time after birth, a number pick up the ability to generate speech on the right side as well. Right-handed people do not originate speech on the right side of the brain.
A shift to right dominance does exist among human beings and is being studied to learn how the brain handles speech, reading, writing and other functions. The reason for this is not simple, and most likely has more than one basis. There is much more to be learned about right side dominance in the brain.