lateralisation

Lateralisation, or lateralization, refers to the specialization of certain brain functions to one hemisphere or the other, reflecting structural and functional asymmetries observed in humans and many animal species.

In humans, language processing is typically left-lateralised, with regions such as Broca's and Wernicke's areas more commonly associated with language. Motor control is contralateral; the left hemisphere largely controls the right side of the body. Handedness is a common behavioural sign of cerebral lateralisation; about 85–90% of people are right-handed, which aligns with left-hemisphere language dominance, though there is substantial variation.

The brain exhibits anatomical asymmetries, such as a relatively larger planum temporale on the left in many individuals, and differences in interhemispheric connectivity. Lateralisation arises from a combination of genetic, developmental, and experiential factors, and is not absolute; many functions are distributed across both hemispheres and can reorganise after injury.

Clinically, atypical lateralisation patterns have been reported in conditions such as dyslexia, autism spectrum disorders, and schizophrenia, but findings remain complex and not diagnostic. Techniques to study lateralisation include functional neuroimaging (fMRI, PET) and the Wada test to determine language dominance.

In other species, lateralisation also occurs and can influence behaviour and ecology, sometimes conferring advantages in processing speed or predator evasion; patterns vary across taxa.