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The notion that autism runs in families is as old as the diagnosis itself. In the landmark 1943 paper that included the first formal descriptions of children with autism, child psychologist Leo Kanner noted that some parents showed milder versions of the same features he saw in their children. Kanner wrote that these parents seemed to be overly focused on details and uninterested in social interactions. His observations suggested that the same genetic factors that led to the children’s autism may have also been responsible for the unusual features of the parents.
In the late 1970s, Kanner’s hunch found confirmation in studies of twins, which eventually established autism as among the most heritable psychiatric conditions. If one twin has autism, an identical twin has up to a 94 percent chance of sharing the diagnosis.
Intrigued by these observations, psychiatrists in the 1990s conducted lengthy interviews of parents and siblings of people with autism, probing their personalities and behaviors. They found that at least 20 percent of these relatives tend to be resistant to change, have problems with language and speech, and are hypersensitive to criticism, anxious and aloof. These features are so similar to the tendencies of many people with autism that researchers collectively labeled them the broad autism phenotype, or BAP. These psychiatrists ultimately developed various questionnaires and rating scales for probing BAP during family interviews.
But studying BAP has turned out to be tricky, in part because the field lacks evidence on exactly how to measure it or distinguish it from features that run in families with other psychiatric conditions, such as schizophrenia. “It’s very slippery in terms when you try to pin it down,” says Michael Rutter, professor of developmental psychopathology in the Social Genetic and Developmental Psychiatry Department at King’s College London. “There are various measures that have been put forward, but I don’t think any of them are entirely satisfactory.” (Rutter led the landmark 1977 study of twins with autism that established the condition’s heritability.)
Since those initial attempts to characterize BAP, some researchers have devised efficient tests to quantify aspects of it. For example, in 2000, Constantino developed the Social Responsiveness Scale (SRS), a 20-minute questionnaire designed to measure autism traits in yourself or someone else. The scale primarily gauges social and communication skills, such as how savvy a person is at reading facial expressions and how comfortable he is in social situations. It also looks for rigid behaviors and restricted interests.
Constantino first had parents and teachers use the scale to rate 158 children with confirmed psychiatric issues, including 19 with autism, and 287 children from the general population. The test scores fell on a continuum in both groups, some of which overlapped, suggesting that autism traits can be found to varying degrees across the population. In 2001, a group led by Simon Baron-Cohen at the University of Cambridge in the United Kingdom used the Autism Spectrum Quotient, a different measure of autism traits, and observed a similar trend. These studies suggest that the line between having and not having an autism diagnosis is arbitrary. “All you have to do is just nudge the line to the left a little bit and dramatically increase prevalence,” Constantino says.