It is known that the chances of having an autistic child are higher for families who already have an autistic child. This is the “recurrence risk”. In the past, the recurrence risk has been estimated at between 3 and 10%. A study just out (and discussed somewhat yesterday) puts the risk at something closer to 20% (18.7%). This is a fairly large study, where they monitored the baby siblings in families with an autistic child. They were watching these baby sibs and testing them to see which ones had an autism spectrum disorder (ASD). This makes this study much more powerful in that they are much less likely to miss any ASD baby sibs.
OBJECTIVE: The recurrence risk of autism spectrum disorders (ASD) is estimated to be between 3% and 10%, but previous research was limited by small sample sizes and biases related to ascertainment, reporting, and stoppage factors. This study used prospective methods to obtain an updated estimate of sibling recurrence risk for ASD.
METHODS: A prospective longitudinal study of infants at risk for ASD was conducted by a multisite international network, the Baby Siblings Research Consortium. Infants (n=664) with an older biological sibling with ASD were followed from early in life to 36 months, when they were classified as having or not having ASD. An ASD classification required surpassing the cutoff of the Autism Diagnostic Observation Schedule and receiving a clinical diagnosis from an expert clinician.
RESULTS: A total of 18.7% of the infants developed ASD. Infant gender and the presence of >1 older affected sibling were significant predictors of ASD outcome, and there was an almost threefold increase in risk for male subjects and an additional twofold increase in risk if there was >1 older affected sibling. The age of the infant at study enrollment, the gender and functioning level of the infant’s older sibling, and other demographic factors did not predict ASD outcome.
CONCLUSIONS: The sibling recurrence rate of ASD is higher than suggested by previous estimates. The size of the current sample and prospective nature of data collection minimized many limitations of previous studies of sibling recurrence. Clinical implications, including genetic counseling, are discussed.
The full paper is free online.
Since this was already presented here and elsewhere, I’ll point out a few findings that I found interesting:
1) 41% of the baby sibs received autistic disorder diagnoses. The remaining 59% received PDD-NOS diagnoses.
2) The recurrence risk did not depend on the “functioning status” or “severity” of the autism for the first child. They used ADOS and IQ scores to measure “severity”.
3) the gender of the first autistic child does not increase the recurrence risk. So, if a family has an autistic daughter, the recurrence risk is the same as if the autistic child is a son. My recollection is that previous studies indicated a high recurrence risk if the older sibling was a daughter.
Left Brain Right Brain 
A study published less than a year ago involving 1,235 families replicated the 10% sibling recurrance rate:
http://www.ncbi.nlm.nih.gov/pubmed/20889652
The ADOS-R has been questioned for overlapping ASD especially with respect to the specific language impairment category. People diagnosed with a specific language disorder before the broadening of diagnostic criteria in DSM-IV (1994) and ICD-10 (1994) now routinely meet diagnostic criteria for a PDD diagnosis. There seems to be no distinctive boundary between childhood language disorders and autism spectrum disorders.
The Costantino et al 2010 study did note that that in unaffected siblings there was a high rate of language delay, not autism.
The new study reported an 18.7% rate of recurrance for autism but 60% of the recurrance rate of 18.7% were classified in the PDD/NOS category thus reducing the recurrance risk to less than 10% for traditionally defined autism (autistic disorder.)in agreement with the Costantino 2010 study.
The recurrence rate was about 10% in the study by Peter Bearman’s group as well.
They also found a different recurrence risk depending upon gender of the older sibling.