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There have been several studies of the possible effects of birth order on IQ and ADI scores in multiplex autism families, without any clear pattern emerging. Lord (1992) and Spiker et al (2001) found decreasing Performance IQ (PIQ) with increasing birth order in 16 and 144 multiplex families respectively. Spiker et al. (2001) and Reichenberg et al. (2007) both found higher repetitive ADI domain scores in firstborns in 144 and 106 multiplex families respectively. Reichenberg et al. (2007) also found significantly more firstborns with functional and spontaneous phrase speech, but found no difference in social or non verbal communication scores in 106 families.
Objectives:
This study aims at clarifying the nature of the relationship between birth order and different domains of functioning in ASD. This research overcomes some previous methodological flaws (e.g. heterogeneous IQ tests; small sample sizes) and extends previous research to include verbal IQ (VIQ).
Methods:
Data was collected on 480 individuals with ASD from 240 multiplex families included in the International Molecular Genetics Study of Autism Consortium (IMGSAC). All individuals with ASD received an ADI-R (Lord, C., Rutter, M., & Le Couteur, A., 1994) and several IQ tests were administered but most were Raven’s Progressive Matrices (performance IQ) or Picture Vocabulary (Verbal IQ). The age at testing for the secondborns was significantly younger than firstborns at time of administration of the ADI-R and the IQ tests. The effect of age was controlled for on both performance and verbal IQ, by saving the unstandardized residuals of a regression analysis, with age as the predictor and IQ as the dependent variable. The saved unstandardized residuals of each computation were then used as variables in the paired sample t-test analysis.
Results:
There was no significant difference in PIQ between first and second born siblings when either all PIQ data were included (t=-.304, p=.761) or when the analysis was restricted to Raven’s Progressive Matrices scores (t=.022, 0=.983). Neither was there any significant difference in VIQ between first and second born siblings when all tests were included (t=-1.521, p=.132) or when only Picture Vocabulary data were analysed (t=.482, 0=.631).
Conclusions:
Once the effect of age of administration was controlled for, there was no significant difference in performance or verbal IQ between first and second born individuals in multiplex autism families.