This chapter provides multidisciplinary investigation of development in individuals with neonatally ascertained sex chromosome abnormalities. Several studies have reported a second kind of behavioural anomaly in 47,XXY males. Despite difficulties in demonstrating this due to the absence of generally accepted methods of measurement, there is a significant body of data indicating that extra X males frequently present the signs of atypical interpersonal or temperamental development. The nature of the relationship indicates that extra X males with particularly pronounced right hemisphere specialization for nonverbal processing have the most severe language deficits. If disorders of hemispheric specialization underlie various aspects of behavioural abnormality in 47,XXY boys, it is appropriate to ask what is responsible for the neurological anomalies themselves. Identifying the factors that give rise to hemispheric specialization is complex, as is determining whether hemispheric specialization is relatively fixed early in life or, alternatively, is emergent and changing from birth to maturity.