Brain lesions and IQ: recovery versus decline depends on age of onset.

A growing literature suggests that early lesions are associated with poorer IQ outcome. Those studies covered a restricted age range in pediatric populations only and did not control for important moderator variables. The present investigation studied IQ change in brain-lesioned children and adults (age 0 to 84 years). Altogether, 725 cases with a documented unilateral focal lesion were gathered from hospital charts and from published cases in the literature, including 240 with repeated IQ testing. Multiple regression analyses isolated the contribution of age at lesion onset to IQ change. Important mediator variables included were lesion side, site, volume, etiology, and so on. An early lesion was significantly associated with poorer postlesion IQ in time and with decline of IQ in time. Later onset lesions were associated with better postlesion IQ and recovery in time. The so-called Kennard principle is refuted, with regard to IQ.