Aphasia, specifically the type that affects Broca’s or Wernicke’s area, is one of the prime examples used by linguists as evidence that linguistic ability is lateralized (located almost entirely in) in the left hemisphere of the brain. Damage to either region will result in speech disorder, losing syntax with lesions or injuries to the Broca region (frontal region) and lexicon when in the Wernicke area (posterior temporal and lower parietal regions). Damage to equivalent areas in the right hemisphere has no affect on lingual ability. (It must be mentioned, though, that the ability to comprehend semantics may be impaired with damage to the right hemisphere.) Studies of children with unilateral damage to the left hemisphere exhibited deficiency in acquiring language, while those with right hemisphere damage acquired language like normal children.

Perhaps the greatest piece of evidence, more final than studying brain damage, is the lack of advanced linguistic ability in left hemidecorticates. Hemidecorticates have only one hemisphere of the brain, either by surgical removal or lack since birth. Right hemidecorticates may still acquire language normally, those without a left hemisphere are severely impaired. Furthermore, children born as left hemidecorticates do not develop language normally, proving that language is not only left hemisphere lateralized but innate in human beings.