New research examined the genetic relationship between autism spectrum disorders (ASD) and ASD-related traits in the wider population.

Researchers from the University of Bristol, the Broad Institute of Harvard, MIT and the Massachusetts General Hospital (MGH) found that genetic risks relating to ASD, including inherited variants and those not seen in an individual's parents, affect a variety of behavioral and developmental traits in the population. In other words, individuals diagnosed with ASD would represent a severe presentation of those traits.

"There has been a lot of strong but indirect evidence that has suggested these findings," said co-director of the Broad Institute's Medical and Population Genetics (MPG) Program and senior study author Mark Daly. "Once we had measurable genetic signals in hand - both polygenic risk and specific de novo mutations known to contribute to ASD - we were able to make an incontrovertible case that the genetic risk contributing to autism is genetic risk that exists in all of us, and influences our behavior and social communication."

The study is based on data from unaffected individuals who came from a general population cohort (the Bristol-based Avon Longitudinal Study of Parents and Children) and a nuclear family cohort (Simons Simplex Collection) of ASD cases and unaffected siblings. The ASD collections included several large, international autism genetic studies based on the Psychiatric Genomics Consortium Autism group, the iPSYCH autism project in Denmark, the SSC and the Autism Sequencing Consortium.

"Many traits that related to disease risk - like blood pressure or cholesterol levels - demonstrate a similar continuum of risk, with contributions from common and rare genetic variants, plus environmental and chance events," said professor George Davey Smith, co-author and scientific director of ALSPAC. "The present study demonstrates how this continuum applies to a condition generally thought of as either existing or not."

The researchers said they expect the approach to be used to explore the associations between genetic risk and behavioral traits in other neuropsychiatric disorders such as schizophrenia in the future.

Autism now affects 1 in 68 children and 1 in 42 boys, according to health organization Autism Speaks. They're characterized by social interaction difficulties, language impairments and struggles with proper communication. These core symptoms are central to the definition of an ASD diagnosis and also occur in varying degrees via affected individuals and from an underlying behavioral continuum.

The study was published today in the journal Nature Genetics.