Most people have 23 pairs of chromosomes; one set from the mother and one from the father. However, nondisjunction errors during meiosis can lead to a case of trisomy, where there are three rather than two chromosomes. Although such events are not uncommon, they are usually lethal, and account for a high proportion of spontaneous abortions. There is surprisingly little research on sex chromosome trisomies: The explanation is largely due to the mild impact of the trisomy, which means that many people who have a sex chromosome trisomy would not be aware of their status. Most of the information about prevalence and consequences of sex chromosome trisomies comes from a set of studies carried out in the 1960-1970s in which newborn babies underwent chromosome screening. Findings from the newborn screening studies were summarized in a systematic review by Leggett, Jacobs, Nation, Scerif, and Bishop (2010), who noted a reduction in IQ in XXX, XXY and XYY groups, with both groups of males showing evidence of disproportionate verbal impairments. The report by Lee et al. makes a unique contribution by extending the study of the impact of supernumerary sex chromosomes to include rare cases of children with four or five sex chromosomes. They demonstrate a clear "dosage" effect, whereby the more chromosomes, the greater the negative impact on IQ and development. The study by Lee et al. (2012) has important clinical implications, as well as theoretical importance, because parental choices can be influenced by what they are told about the likely outcome of the child. It is important to emphasise that although there is an increased risk of both structural language problems and autistic features in children with additional sex chromosomes, there is wide individual variation. Some children with trisomies do not have any difficulties, and only a minority merit a diagnosis of autistic disorder (Bishop et al., 2011; Ross et al., 2012).