Summary

The left and right hemispheres of the brain process sensory information in different ways and function differently in controlling behavior. This lateralization of brain function, originally thought to be unique to humans, is now known to occur in a broad range of non-human vertebrates and even in invertebrates, indicating that it is an essential feature of both large and small brains. Some evidence indicates that lateralization of brain function improves cognitive capacity of the brain. Many, often unrelated, brain functions are lateralized. In humans, these include differential specialization of the hemispheres to process language and produce speech, express emotions, respond to faces, attend to spatial information, and control hand use. As now clear for handedness, both genetic expression and environmental influences are involved in complex ways. Since lateral division of function in the brains of non-human animals has sufficient similarity to that of humans, it is probable that at least some asymmetries were evolutionary precursors to language and speech.

It is notable that lateralization is in no species totally in a single direction. Some evidence from studies of animal species provides support for the hypothesis that alignment of laterality in most individuals in a species occurs only for control of behavior that requires one lateralized individual to interact with another lateralized individual (i.e., in social interaction). In humans, individuals with non-right-handedness (left-handed and mixed-handed) are more prone to a number of psychiatric conditions or other behavioral conditions, including autism, schizophrenia, and dyslexia.