Cognitive and Behavioural Differences in Toddlers with Autism Spectrum Disorder from Multiplex and Simplex Families

Background: Studies of ‘high-risk’ siblings have provided valuable information regarding the early Autism phenotype and our understanding of the early developmental trajectories of affected individuals later diagnosed with Autism Spectrum Disorder (ASD). However, it remains unclear whether these cases of affected siblings are representative of the wider population of children with ASD. To date there is only one published (Taylor et al., 2015) and one unpublished (Pandey, 2008) study on this topic, with inconsistent findings, and both failing to take account of birth order in determining whether children are from Simplex families.

Objectives: Our objective was to investigate whether the early development of toddlers with ASD from Multiplex (MPX) families, who have an affected older sibling, is similar to toddlers from Simplex (SPX) families where there is no affected sibling or family member. A further aim was to determine the pattern of association between ASD symptom severity and cognitive functioning within each group to help shed light on possible mechanisms for group similarities/differences.

Methods: Behavioural and cognitive assessment data, as well as demographic details, from an un-selected sample of 244 toddlers, diagnosed with ASD (183 males) at age 2-years, were utilised to address the study aims. The sample comprised 45 MPX children, 127 first-born (FB) SPX children and 72 later born (LB) SPX children. Children in each group, matched on age, gender and parental education, had been administered the Autism Diagnostic Observational Schedule (ADOS) and the Mullen Scales of Early Learning (MSEL) to assess their autism symptoms and cognition, respectively. Parents had been administered the Autism Diagnostic Interview (ADI-R). The test administrators were blind to the study aims.

Results: MPX children had significantly higher developmental quotients (DQ), with differences found on Verbal, Nonverbal, and Overall DQ between the MPX group and each of the FB SPX and LB SPX groups; the SPX groups were not different from each other. The three groups did not differ on overall symptom severity as measured on the ADOS, or on their Social Affect (SA) and Restricted and Repetitive Behaviour (RRB) scores. No group differences were found on the ADI-R Social Interaction or Communication domains, with the only difference being that the SPX LB children had lower scores on the RRB domain compared with the other groups. Although autism symptom severity (ADOS and ADI-R scores) and the MSEL DQ scores were negatively correlated in each group, the groups were similar in the pattern of relationship found between autism severity and cognitive ability.

Conclusions: The different cognitive profiles between MPX and SPX groups suggest that findings from high-risk sib studies may not generalise to other samples of children with ASD. It is important that future research examine biological and environmental factors that may explain the enhanced cognitive development of children with ASD from MXP families.