The heritability of autism is the percentage of autism that can be explained by genetic variation; if the heritability of a condition is high, then the condition is considered to be mostly caused by genetics.
Several lines of evidence support a heritable component to ASD, although no particular ASD-predisposing gene has been confirmed to date.
The genetics of autism are complex and it is not known whether autism spectrum disorder (ASD) is explained more by multigene interactions or by rare mutations with major effects.
Early studies of twins estimated the heritability of autism to be more than 90%. However these figures are disputed due to low sample size, inconsistency in case definition, the role of de novo mutation, and the possibility that heritable causes of ASD in non-identical twins are affected by environmental factors.
A 2011 California Twin study looked at 192 twin pairs. A large and diverse study group is important when you consider that only about 1% of the population has autism and only 3.2% of the population are twins.
The study concluded that previous rates of genetic heritability were largely inflated and that susceptibility to ASD has moderate genetic heritability and a substantial shared twin environmental component.
The researchers believed that these over-inflated heritability rates from previous studies looking at only a small group had caused autism research to be largely focused on finding the underlying genetic causes, with less emphasis on potential environmental triggers or causes. The finding of significant influence of the shared environment, experiences that are common to both twin individuals, may be important for future research paradigms.
Nongenetic risk factors that may have environmental influences that the researchers pointed out include parental age, low birth weight, multiple births, and maternal infections during pregnancy.
However, a recent study by Dan Geschwind’s group at UCLA has gathered some support for the genetic mechanisms of transmission in ASD. The team looked at instances of autism recurrence in full siblings and half siblings using information from over five thousand families from the Interactive Autism Network (IAN). The recurrence risk is the chance that a sibling has autism if another sibling does.
The recurrence rate in full siblings was found to be approximately twice that among half siblings. While these findings point to a primarily genetic model of autism risk, other studies being released also highlight environmental influences. Without a specific problem gene being identified the studies above provide little more than one more piece to the mystery of autism.