DOES AUTISM RUN IN FAMILIES?
Parents often ask whether it is likely a second child will have autism if the first born was diagnosed with that condition. They tend to overestimate the actual risk due to lack of accurate information (Whitelaw et.al. 2007). The short answer is “yes” it is more likely a second child will have autism than if the first-born did not have autism, but the increased risk is not extremely high.
The earliest evidence for autism running in families came from a study by Susan Folstein and Michael Rutter who studied 21 pairs of British twins. For 10 pairs of fraternal (dizygotic) twins none of the fraternal twins were diagnosed with autism, but if they were identical (monozygotic) twins the risk was 36%.
An 1989 study by Ed Ritvo in Utah who found the risk of a second child having autism if the first had autism was 8.6%. If the first child was a boy the risk of a second having autism was 7% but if the first was a girl, the risk was 14%.
A more recent study in Queensland Australia indicates 4-6% of second children will have autism if the first was so diagnosed, but if two previous children of a couple had been diagnosed with autism the risk greatly increased to 35%. In a recent review, Slatkin (2008) reported family recurrence rates of 4-10% and identical twins of 30-50%.
Risk of having a second child with autism depends on the type of autism. Judith Miles and colleagues at the Universty of Missouri have studied a large sample of children diagnosed with autism and finds they are of two types. One has unusual physical features and either unusually large or small head size. They have more intellectual, language and social limitations. She called this type Complex Autism. Children who had no different physical features and more closely resembled members of their family had higher IQ and stronger language and social skills (Essential Autism. Among children with Complex Autism 9% of brothers, sisters, parents and cousins have autism, while children with Essential Autism, 20% are diagnosed with autism.
In addition, it is increasingly clear that social and language limitation features of autism are often present in milder forms in siblings, parents and cousins of individuals with autism, though they are not diagnosable as having autism. This is called the Broad Autism Phenotype (Constantino et. al; Piven et.al).
References
Constantino, J. et. al. (2006) Am. J. Psychiatry, 163: 294-6
Folstein S. and Rutter M. (1977). J. Child Psychol. Psychiat. 18, 297-321
Losh, M and Piven, J. (2007) Child Psychol Psychiatr. 48:105-12
Losh, M. et.al. & Piven, J. (2008) Am. J. Med. Genetics B Neuropsychiatr. Genetics. 147B:424-33.
Miles, J.H. et. al. (2005) Am. J. Med Genetics A. 135:171-80.
Queensland Health Australia, http://www.genetics.com.au/pdf/factSheets/FS43.pdf
Ritvo, E.R. (1989). American Journal of Psychiatry, 146, 1032-36.
Slatkin, M. (2008) Genetics, 179: 2253–2261.
Whitelaw, C. et. al. (2007) J. Paediatric Child Health, 43:752-4.