The aim of this review was two‐fold. The first aim was to review to what degree impairments in areas of global intellectual functioning, language development, social cognition, and EF have been studied in children with SCT, and identify possible gaps in research that future research should focus on. The second aim, was to establish clinical significance of these impairments and identify risk‐factors that should be closely monitored from early development onwards or that should be included in standard clinical neuropsychological screening to identify potential targets for support and intervention.
With regard to the first aim, the reviewed studies collectively gave the following results. On the domain of GIF, seven studies report outcomes in children between the ages of 4 and 18 years, with three studies focusing on children from the age of 4 years, and four studies studying school‐aged children. To our knowledge, there were no studies that examined GIF in children with SCT before the age of 4 years. On the domain of language development, five studies reported outcomes in children between the ages of 2 and 18 years. To our knowledge, there were no studies that examined language development in children with SCT before the age of 2 years. Of the seven studies, two studies used only parent reports, the other three studies used either a performance task or a combination of parent report and performance tasks.
Finally, one study used parent report, with the other four studies using performance‐based tasks or a combination of both. On the domain of social cognition, six studies reported outcomes in children between the ages of 4 and 18 years. To our knowledge, there are no studies to date that assess (precursors of) social cognition in children with SCT before the age of 4 years. In addition, until the age of 8 years, and in XXX and XXY groups only, social cognition has not been tested with performance‐based measures, but has solely been assessed with parent reports.
From a developmental perspective, it is important to monitor neuropsychological functioning of children with SCT at the start, or even before, the sensitive developmental period when these skills typically develop and identify precursors and early markers of developmental risk. Considering the increased prevalence of (characteristics of) behavioural and neurodevelopmental disorders, such as ADHD, autism spectrum disorders, anxiety, and depression in the SCT population,14, 34, 35 more knowledge of developmental neurocognitive risk markers could lead to more timely, preventive support, hopefully reducing the risk for these behavioural and neurodevelopmental disorders in the future. In addition, the results of this review call for more studies on early neurocognitive vulnerabilities, which are expected based on the impact of the extra chromosome on the development of the brain.
It is important to learn more about the involvement of genes on the sex chromosomes in order to identify how expression of these genes can lead to the behavioural phenotype of individuals with SCT and how different genes on different sex chromosomes can lead to the similarities and differences in the behavioural profile of children with XXX, XXY, and XYY. There is a specific need for more knowledge in areas in EF and social cognition, not only because more extensive research has shown these domains appear to be affected in adulthood,14 but also because these cognitive domains are crucial for behavioural and socio‐emotional development, adaptive functioning, and quality of life.
Also, the results of this review illustrate that more attention should be given to timely screening for cognitive vulnerabilities, that these should be monitored during relevant developmental stages, and that interventions should be tailored to these risk profiles.
It is also important to gain more insight in the karyotype‐specific profiles of neurocognitive functioning, as the presence of an extra X or Y may have similar ánd different effects on the development of brain areas involved in social cognition and language, and therefore could have an effect on neurocognitive development. This may help in understanding expected neurodevelopmental profiles and related, tailored, intervention options.
To conclude, this review of studies shows that the presence of an extra sex chromosome may have an impact on neurocognitive functioning of children with SCT, and identified that domains of language development, executive functioning, and social cognition should be closely monitored in these children. In addition, it is important to gain more insight in the early development of children with SCT population, especially before the age of 4 years on the domains of social cognition and executive functioning. Finally, it is important that social cognition and EF will be included in the standard screening and assessment methods, as this review showed that social cognition and EF in addition to language development, are domains that require close monitoring, and are targets for early support and intervention programs. With more knowledge about the development of young children with SCT, existing evidence‐based (preventive) intervention programs can be tailored to the SCT profile in hopes of reducing these difficulties, and by reducing these neurocognitive underpinnings of behaviour, could possibly prevent neurobehavioural problems in later life.