Baby sibs

Early indicators of autism:

Currently, there are no accepted diagnostic tests for autism in children younger than approximately 3 years. However, studies of baby sibs has allowed researchers to follow high-risk infants from birth and led to the identification of early signs of autism.^1,2 In a pioneering study, Zwaigenbaum, Bryson and colleagues (2005) noted that in their first year, infants who later go on to develop autism exhibit specific behaviors, particularly in social settings, such as not reacting to social interaction, and not tracking others’ faces or following others’ eye gaze (known as “joint attention”), compared to their typically developing peers.³

Another study of baby sibs revealed that infants who later develop autism show atypically rapid brain growth in their first year of life.⁴ Overall, prospective studies that identify high-risk baby sibs have contributed greatly to the recognition that symptoms of autism may begin to emerge within the first year of life. Even in baby sibs who do not develop autism, studies are finding atypicalities, presumably because these infants nonetheless carry some of the genes associated with autism. For example, studies of 10-month-olds’ neurological responses to social and non-social stimuli (e.g., faces versus objects) demonstrated that high-risk baby sibs exhibit different brain responses relative to their typically developing peers from families without autism history.⁴ In sum, these results have broad implications for early detection and risk signs associated with autism.

Recurrence rate:

The recurrence rate refers to the probability that the younger sibling of a child with autism (called a ‘baby sib’) will be diagnosed with autism as well. Several laboratories across the U.S. and other countries have formed the Baby Siblings Research Consortium (BSRC, founded in 2003 by Autism Speaks and the National Institutes of Health), which tracks the development of baby sibs soon after birth and up until 3 years of age, when autism can be reliably diagnosed. To date, the research that has come out of this collaboration has added to the literature on recurrence rates of baby siblings, as well as identifying early signs of autism and developing interventions for high-risk infants.

Research has shown the recurrence rate to be between 8.6 percent and 18.7 percent.^1,5 The variability in this statistic is thought to come from problems such as stoppage – that many parents with one child diagnosed with autism choose not to have more children – and other biasing factors such as over-reporting and ascertainment bias (which would artificially inflate the recurrence rate). One large epidemiological study in the 1980s found that siblings born after an affected child had a recurrence risk of 8.6 percent.² Furthermore, they also found that if the affected older child (called a proband) was female, the baby sib was twice as likely to develop autism than baby sibs whose older affected sibling was male. This is consistent with a multifactorial threshold model of transmission, which demonstrates that the less common the disorder is for the proband, the higher the likelihood of transmission to relatives. This finding is controversial, however, as more recent studies have not found evidence for this threshold model.^1,3,8

Recent studies have adopted a prospective method, in which they follow the baby sibs from birth. This method overcomes the problems of stoppage, over-reporting and ascertainment bias by clinically assessing high- and low-risk infants longitudinally, and obtaining clinical assessments of autism at 36 months.³ Using this method, Ozonoff and colleagues (2011) found a recurrence rate of 18.7 percent among younger siblings of probands.¹ Gender of the proband did not significantly predict recurrence in this study. In addition, this study found that infants with two or more probands (called multiplex families) were twice as likely to be diagnosed with autism than infants in simplex families, with only one affected older sibling. Finally, this study evaluated the proband functioning level, such as IQ and social communicative skills, and found that this was not a good predictor of recurrence in younger siblings.