A handful of magnetic resonance imaging (MRI) studies have examined the impact of an additional X chromosome in males (see Table 1). Warwick et al. [1999] followed up a cohort of individuals who had been identified prospectively during the Edinburgh study of growth and development of children [Ratcliffe et al., 1982] as having SCAs (10 XXY, 12 XXX, 10 XYY; findings relating to the other SCAs are described in the pertinent sections below). Individuals with SCAs were compared with healthy controls matched on age, parental socioeconomic status, and height (13 females, 26 males). Participants were between 16 and 28 years of age. The subjects in this study had been found in a previous investigation to have significantly lower IQ (47,XXY: mean NART IQ 86.1, SD 15.8; controls 105.1, SD 7.6, P < .001) and higher levels of schizoid/ schizotypal personality traits, although none met criteria for a major psychiatric disorder [Gotz 1996; Gotz et al., 1999]. The investigators therefore also included a structured psychiatric interview (the Structured Interview for Schizotype, SIS) [Kendler et al., 1989] to assess if these traits
