It has been proposed that the ratio of the second to fourth digits (2D:4D) may be a proxy of prenatal androgen exposure, such that low 2D:4D ratio is associated with high prenatal androgen exposure. The aim of the present study was to measure the 2D:4D ratio in 100 right- and non-right-handed individuals with intellectual disability of unknown idiopathic origin and compare them to a control group of 85 typically developing individuals. We also sought to determine whether sexually dimorphic traits, such as 2D:4D ratio, tend to be more pronounced on the right hand of these groups than on the left. Our results indicated that males had lower 2D:4D ratios than females in both groups, and individuals with intellectual disability had higher ratios only in their right hand compared to typically developing individuals. Further, right-handed individuals had lower ratios in both hands compared to non-right-handed individuals. Our results are discussed in relation to the "Geschwind-Behan-Galaburda theory of cerebral lateralisation" and Witelson's callosal hypothesis that differential levels of prenatal testosterone exposure will cause atypical cerebral laterality in individuals with intellectual disability, and the suggestion that this atypicality will become evident in the 2D:4D ratio (Manning, Scutt, Wilson, & Lewis-Jones, 1998).

Turner syndrome (TS) is a genetic disorder in females characterized by the complete or partial absence of one X chromosome. Its most consistent physical features include short stature and ovarian dysgenesis. TS individuals demonstrate a characteristic neurocognitive profile involving weaknesses in visuospatial processing. The hypothesis of defective right hemisphere specialization has been offered to explain the visuospatial deficits in TS. In contrast, an alternative explanation proposes a more uniform dysfunction of the left and right hemispheres, based on findings of symmetrical abnormalities. This article presents an overview of the two hypotheses, along with relevant findings on hemispheric specialization with respect to TS. The impact of the genetic and hormonal mechanisms on the neurocognitive profile of TS is also discussed and directions for further empirical research are identified.

Klinefelter syndrome (KS) is a genetic disorder in males characterized by the presence of an extra X chromosome. Its most consistent endocrinological manifestations include lower testosterone production and impaired spermatogenesis. KS individuals have a general typical appearance with taller stature, and they demonstrate a characteristic cognitive phenotype involving weaknesses in verbal processing. Anomalous cerebral lateralization involves the inverse or weak dominance of hand, language, and visuospatial abilities and has been associated with the cognitive deficits of KS individuals. This article summarizes the ongoing research in this field, discusses the main findings, and attempts to provide a thorough description of the cause of the observed functional and anatomical cerebral asymmetries associated with the syndrome. Nonetheless, efforts have been directed to incorporate evidence for and against theoretical accounts that explain the experimental findings, to discuss issues involving the implications of the chosen methodology, and present key research areas for future empirical research.