Characterization of the Broader Autism Phenotype in Later-Born Siblings

Background: Research has found that some family members of children with autism spectrum disorders (ASD) exhibit qualitatively similar, yet milder features of ASD described as the “Broader Autism Phenotype” (BAP).  Few have characterized this group from a developmental standpoint, by comparing characteristics of BAP in young siblings of children with ASD (Sibs-ASD) to children with ASD and typical development (TD) at different points in development.  BAP subgroups appear to exhibit similar levels of ability in autism related symptomatology such as responding to joint attention (RJA; Sullivan et al., 2006) and language (Gamliel et al., 2007) at 14 months.  However, when reassessed (at 24 and 54 months, respectively), these BAP subgroups demonstrate significant developmental gains, showing profiles more similar to TD children.

Objectives: The purpose of the current study was to broaden our understanding of early manifestations of BAP in Sibs-ASD by comparing their performance to Sibs-ASD with ASD outcomes and later-born siblings of TD children (Sibs-TD) on a variety of developmental domains related to autism symptomatology.

Methods: Participants:  Forty-nine (49) Sibs-ASD and 30 Sibs-TD participated. Children were initially seen at 12-23 months and were reassessed 17-23 months later (mean CAs=15 mos and 34 mos, respectively).  At T2, 6 Sibs-ASD (12%) were diagnosed with ASD (3 PDD-NOS; 3 Autistic Disorder) and 8 (16%) were classified as BAP according to two a priori criteria: ADOS social algorithm score exceeding cutoff, and clinician concern about the child’s social development. No Sibs-TD received a diagnosis. Measures: Autism severity was assessed using the Childhood Autism Rating Scale (CARS), cognitive abilities were assessed using the Mullen, and social-communication skills were assessed using measures of initiating joint attention (IJA), RJA, and the Social Behavior Checklist (SBC). 

Results: At T1, siblings later classified as BAP did not differ from those with ASD in any of the domains assessed, but showed significantly lower abilities than Sibs-TD in social-communicative skills (IJA, t(36)=-2.89, p=.006; RJA, t(36)=-4.88, p<.001) and cognitive development (Visual Reception, t(36)=-2.04, p=.049).  At T2, children with BAP differed significantly from those with ASD by showing less autism symptomatology (CARS, t(12)=-2.58, p=.024), higher social-communicative skills ( IJA, t(12)=4.82, p<.001; RJA, t(10)=2.70, p=.022), and higher cognitive skills (Fine Motor, t(12)=2.28, p=.041; Receptive Language, t(12)=3.38, p=.005; Expressive Language, t(12)=4.67, p=.001; ELC, t(12)=3.30, p=.006). Relative to Sibs-TD, those with BAP at T2 demonstrated significantly more autism symptomatology (CARS, t(36)=4.21, p=.003), lower social-communicative skills (RJA, t(34)=-2.72, p=.010; SBC, t(33)=-4.75, p<.001) and lower overall cognitive ability (ELC, t(36)=-2.06, p=.047).

Conclusions: These results replicate and extend those previously reported. In the second year of life, siblings later classified as BAP differed from Sib-TD controls in several developmental domains, but did not differ from siblings with ASD outcomes. When reassessed 18 months later, the BAP group showed weaker performance than the Sib-TD group, and also differed from the ASD group in all domains. These results suggest a developmental growth trajectory in the BAP group that exceeds that of the ASD group. Continued follow-up and replication with larger samples is needed to understand the implications of these findings.