Age, sex, and aphasia type. Evidence for a regional cerebral growth process underlying lateralization.

The incidence of motor and sensory, (nonfluent and fluent) aphasia was studied in 389 patients with a left-sided vascular lesion. The mean age of patients with motor aphasia was 45.3 years, and the mean age of patients with sensory aphasia was 56.5 years. The age difference was also found in a separate analysis of 50 cases of traumatic aphasia. Nonfluent aphasias predominate in early life in both males and females, but are more common in females in the third decade. The relationship between aphasia type and age, and sex, is explained by a process of regional specification of the left language areas that proceeds more slowly in females than males. It is hypothesized that this process reflects the effects of the sex hormones on protein synthesis and brain growth.