Left-handedness is sometimes the expression of a genetic defect or an early developmental disturbance. In this issue of Deutsches Ärzteblatt International, Stefan Gutwinski and coauthors present the current scientific understanding of left- and right-handedness.
Left-handedness is found in all cultural groups. It arose early in the course of human evolution and played a key role in the development of higher cognitive functions. Human speech, for example, is thought to have arisen as an outgrowth of the unilateral cerebral control of manual communication by gesticulation. Likewise, it was only after handedness had become well established that very fine motor functions of the human hand could develop.
Most of the time, left-handedness is simply a naturally occurring, normal variant. In some cases, however, it is disadvantageous and may well reflect a genetic defect or early developmental disturbance. Thus, both left-handedness and extreme right-handedness seem to be more common among persons suffering from certain diseases.
This phenomenon can be observed, for example, in epilepsy, schizophrenia, and autism. Furthermore, current research suggests that diminished activity of the left cerebral hemisphere compared to the right may lead to depression, and the opposite imbalance to mania.