https://doi.org/10.1177/1362361318801617 Autism 1–13 © The Author(s) 2018 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/1362361318801617 journals.sagepub.com/home/aut Introduction Autism spectrum disorder (ASD) is characterised by dif- ficulties with social communication and repetitive and restricted behaviours and interests (American Psychiatric Association, 2013). Difficulty with pragmatic language (the way we use language to interact with others) is a uni- versal feature of ASD; yet there is substantial variability in children’s use of semantics, grammar and syntax (Tager- Flusberg, 2006). Some children with ASD have average or advanced vocabulary, grammar and sentence structure but around 15%–25% are nonverbal or minimally verbal and use less than five words (Norrelgen et al., 2014; Rose et al., 2016). There has been ongoing debate about whether there is a subgroup of children with ASD who have a phe- notype that closely resembles those with specific language impairment (Rapin et al., 2009; Tager-Flusberg, 2006) and whether children with ASD and specific language impair- ment have shared or separate aetiological underpinnings (Kjelgaard and Tager-Flusberg, 2001; Whitehouse et al., 2008). Receptive and expressive language difficulties are one of the most commonly co-occurring features of ASD (Carlsson et al., 2013), with just over 60% estimated to have some degree of language impairment (Carlsson et al., 2013; Levy et al., 2010). The ability to use language by 5 years of age is an important predictor of later outcomes in children with ASD (Pickett et al., 2009) and difficulties using language in children with ASD are associated with several adverse outcomes including behaviour difficulties Predictors and growth in receptive vocabulary from 4 to 8 years in children with and without autism spectrum disorder: A population-based study Amanda Brignell 1,2 , Tamara May 1,3 , Angela T Morgan 1,2 and Katrina Williams 1,2,4 Abstract Few studies have examined growth and predictors of receptive vocabulary in children with autism spectrum disorder. Here we aimed to compare receptive vocabulary from 4 to 8 years and identify predictors of receptive vocabulary, at 8 years, in children with and without autism spectrum disorder. Participants were drawn from a nationally representative population-based study with two cohorts recruited at birth (N = 4983) and kindergarten (N = 5107). Receptive vocabulary growth was compared for children with and without autism spectrum disorder at 4 (n = 188, n = 7136), 6 (n = 215, n = 7297) and 8 (n = 216, n = 7408) years. Predictors of receptive vocabulary were analysed. Estimated mean receptive vocabulary scores for children without autism spectrum disorder were 2.3 units higher than the autism spectrum disorder group across three time points. This difference was significant (p = 0.004; 95% confidence interval 0.769–3.927). Children with and without autism spectrum disorder progressed at a similar pace. There was no significant difference between the proportions of children with and without autism spectrum disorder who had stable, improving and declining trajectories. Age was the only significant predictor of greater receptive vocabulary growth in children with autism spectrum disorder. Baseline receptive language and nonverbal IQ were significant predictors of receptive vocabulary ability at 8 years. These findings inform prognostic advice given to families on language outcomes. Keywords autism spectrum disorder, communication, development, language, longitudinal, trajectory 1 Murdoch Children’s Research Institute, Australia 2 The University of Melbourne, Australia 3 Deakin University, Australia 4 Royal Children’s Hospital, Australia Corresponding author: Amanda Brignell, Murdoch Children’s Research Institute, 50 Flemington Rd., Parkville, VIC 3052, Australia. Email: amanda.brignell@mcri.edu.au 801617AUT 0 0 10.1177/1362361318801617AutismBrignell et al. research-article 2018 Original Article