Despite many years of research, the reason why one person turns out to be a left-hander and another does not remains a mystery. Handedness is partly heritable A characteristic that can be inherited, because genes affect the way it develops. (see Glossary): left-handed parents tend to have more left-handed children than right-handed parents do. Studies that compared identical twins to fraternal twins
Identical twins develop from the same fertilization and therefore share virtually all of their DNA. They differ from fraternal twins, who develop from different fertilizations and are only genetically related, like any other pair of siblings. (see Glossary) show that the heritability is around 25%,
which suggests a genetic contribution to hand preference. Note, however, that if handedness is only 25% heritable, then the majority of the factors contributing to left-handedness is not straightforwardly genetic. A much debated influence is the level of the hormone testosterone in utero
(in the womb) while an unborn child (fetus) is developing. Another possibility is that handedness is influenced by random (“chance”) processes that occur during the early development of an unborn child, when it is still very small. Regarding the heritable factors,
researchers have found several genes which may be involved in handedness. Interestingly, it seems that there is not one gene which contributes to determining whether people are left-handed or not. Most likely, different genetic influences are involved.
Of course, children learn from their caregivers, so one may expect that if one of the parents is left-handed, the child might become left-handed just by imitation. One reason why this cannot be the full story is that hand preference can already be observed before birth. Fetuses like to move around,
and one can predict a child’s hand preference reasonably well by looking at which arm and hand they prefer to move before birth, as can be seen with ultrasound scanning. This observation of fetal handedness is at odds with the finding that some children seem to switch their hand preference, at least up to the age of. Perhaps some people are left- or right-handed at birth, whereas others develop their preference later on, during the first years of life.
