as “uneven” with weaknesses in auditory short‐term memory relative to visual short‐term memory and other aspects of cognition (Chapman, 2003), and with strengths in social functioning abilities (Kasari & Baunminger, 1998).
Language acquisition is a challenge for people with DS, although vocabulary level tends to be less impaired than grammatical abilities (Chapman, Schwartz, & Kay‐Raining Bird, 1991; Miller, 1996). Many children with DS do not acquire their first words before the age of two (Rondal, 2001), but early lexical development generally shows a positive linear relationship with mental age (Rondal & Edwards, 1997). Language performance tends to be lower than expected from their general level of cognitive development (Chapman & Hesketh, 2000; Fowler, Gelman, & Gleitman, 1994; Perovic, 2002; Vicari, Caselli, & Tonucci, 2000), though there is individual variation. In a study of 43 children with DS using the McArthur Child Development Inventory, Miller (1999) reported that 65% of the children scored below their mental age on vocabulary and only 35% of the children had vocabulary ranges that were consistent with their mental age. Compared with typically developing individuals matched for nonverbal mental age, children and adults with DS often have receptive vocabulary deficits (Jarrold, Baddeley, & Phillips, 2002), although some children with DS have similar expressive and receptive vocabulary to typically developing children of the same nonverbal mental age (Laws & Bishop, 2003). In adolescents and adults with DS, receptive vocabulary is usually reported as a relative strength (Abbeduto, Warren, & Conners, 2007) and generally in line with nonverbal mental age. Importantly, receptive vocabulary has sometimes been reported to exceed general nonverbal abilities (Abbeduto et al., 2007; Næss, Lyster, Hulme, & Melby‐Lervåg, 2011).
Only a handful of studies have investigated early language development in DS, hence knowledge on the early stages of language acquisition in DS is still limited. Cardoso‐Martins, Mervis, and Mervis (1985) followed longitudinally 6 children with DS aged 17–19 months at the start of the study, and compared them with 6 typically developing children aged 9 months at the start of the study, investigating the acquisition of object names. There was no difference between the two groups at the start, but the DS group (both comprehension and production) started to lag behind their general nonverbal cognitive skills. This suggests that vocabulary acquisition develops at a slower pace than general cognitive abilities from an early age. Similar findings were reported almost 30 years later by Zampini and D’Odorico (2013) who used the Italian version of the MacArthur–Bates Communicative Development Inventory with 18 children with DS aged between 2 and 3. Specifically, expressive vocabulary was reported to lag behind general cognitive development, with the main changes in vocabulary development occurring at 36 months of age, when individual differences become more prominent. However, different findings were reported in a longitudinal study of expressive vocabulary by Te Kaat‐van den Os, Volman, Jongmans, and Lauteslager (2017) who found that children’s expressive vocabulary skills were related to their nonverbal mental abilities. Similarly, a relationship between receptive vocabulary development and general nonverbal abilities was reported by Cuskelly, Povey, and Jobling (2016).
With regard to language abilities beyond the level of single words, the general picture suggests that morpho‐syntactic abilities may be more impaired than vocabulary skills (Chapman, 2006). Children with DS have also been compared to children with SLI on the accuracy of marking finiteness in spontaneous speech. The study found that the profiles of the groups of children were similar, suggesting that grammatical abilities in children with DS are comparable to those of children with known language impairments (Eadie, Fey, Douglas, & Parsons, 2002). Few studies have investigated early language development beyond single words in DS. Oliver and Buckley (1994), using parental report, showed that two‐word combinations emerged between 25 and 52 months (mean age of around 36 months) in English‐speaking children with DS. Two decades later, Levy and Eilam (2013) reported that Hebrew‐speaking children with DS started using word combinations at approximately 55 months of age. Levy and Eilam further argued for an atypical trajectory of language development in DS due to its delayed onset and slowed pace. The only study to our knowledge that has investigated longitudinally the early stages of vocabulary and general language acquisition, by Mason‐Apps, Stojanovik, Houston‐Price, Seager, and Buckley (2020), reports that expressive communication and expressive vocabulary developed at the same rate and level in children with DS aged between 18 and 35 months compared with typically developing (TD) controls aged between 9 and 23 months, who were matched to the DS group on gains in nonverbal mental ability over a period of 12 months. Interestingly, the infants with DS showed a receptive language advantage over the TD group; this group’s auditory comprehension and receptive vocabulary scores were superior to those of the TD group when nonverbal mental ability was accounted for, hence showing a receptive language advantage at this early stage of language development.
With regard to social communication skills, it has been shown that people with DS are very keen to engage in conversation and to keep the conversation going, but they often lack the appropriate language skills to do so (Rondal, 2001). Two studies described in Abbeduto and Murphy (2004) point to some strengths and weaknesses in the DS communication profile. For example, in a barrier task, individuals with DS were less likely than typically developing individuals matched for nonverbal mental age to provide listeners with referential frames to help the listener’s comprehension. Also, they were less likely than mental age‐matched peers to signal non‐comprehension, suggesting that perhaps they were unable to monitor their own comprehension, which can seriously disrupt the conversational interaction. On the other hand, individuals with DS were found to be appreciative of shared knowledge in conversation and they made appropriate shifts from indefinite descriptions (such as: “a house”) to definite descriptions (such as: “the house”). In a task that required the participants to describe a novel shape to their interlocutor, the individuals with DS were found to be very consistent when describing the same shape every time it occurred, suggesting awareness of the listener’s informational needs.
Research also shows that children with DS as young as 6 years old experience impaired pragmatic communication, showing a profile of relative strengths and weaknesses. Specifically, after controlling for nonverbal mental age, the children with DS scored at or below 1 SD of the TD norm on all subscales of the Norwegean adaptation of the Children’s Communication Checklist‐2 (Helland, Biringer, Helland, & Heimann, 2009) apart from the nonverbal communication scale. Nonverbal communication emerged as a relative strength, which is in line with other studies, such as Franco and Wishart (1995) and John and Mervis (2010).
In summary, research studies so far point to the fact that children with DS have difficulties with language, and these difficulties can sometimes be more severe than expected for the general level of cognitive abilities. However, findings are mixed and reflect differences in methodology, age of participants, and the heterogeneity of the DS population. There is evidence that language abilities in DS may develop atypically, but more research is needed to determine the characteristics of the atypical language trajectory.
5.4 Conclusions
Considering the studies reported above with regard to language and communication abilities for these two genetic syndromes, Williams and Down syndrome, points to uneven neurocognitive profiles and areas of relative strengths and weaknesses in each of the populations. There is no systematic and consistent evidence from the two disorders for clear dissociations between different cognitive skills, which may partially be due to heterogeneity within each of the syndromes, possibly small sample sizes, different methodologies, and other yet unknown factors. It is very important that inquiry into language and communication impairments in genetic syndromes continues if we are to understand the effects of genes on the development of cognitive profiles. As pointed out by Rice, Warren, and Betz (2005), although we have a lot of information with regard to individual diagnostic profiles of different syndromes, there are few comparative studies, and therefore there is much need for systematic comparisons across disorders if we are to fully understand what is common across conditions and what is syndrome‐specific, so that we can clarify the nature of language, communication, and other cognitive impairments. One way forward would definitely be longitudinal studies detailing how language is acquired in the context of cognitive development, and how development of