The Heritability of Autism
Figure 1: The Heritability of Autism Spectrum Disorder. Researchers estimate that 80% of variation in autism risk is due to inherited genetic factors. This varies by country. In Israel, ASD is 51% heritable. In Finland, researchers estimate that it is 87% heritable. This data was obtained from an analysis of 2,001,631 individuals across 5 countries.
A new study shows that autism is 80% heritable. This is roughly in line with what scientists were expecting based on previous studies, except with much more precision.
Another key finding of the study was that heritability was different between countries. In Israel for example, just about 51% of the variation in autism risk was explained by genetics. Meanwhile, in Finland, Just about 87% of the variation was explained by genetics.
Autism Spectrum Disorder
The Key Deficits in Autism Spectrum Disorder
What does this mean?
Figure 2: Identical vs Fraternal Twins. Identical twins share 100% of their DNA while fraternal twins share about 50% of their DNA, like siblings. Identical twins arise from the same egg and sperm, while fraternal twins arise from separate eggs + sperm cells.
In broad terms, this means that around 20% of autism variability is explained by environmental factors, such as parenting style, schooling, and the neighborhood.
80% of variability comes from their parent's genes, though we don’t necessarily know what many of these genes are yet.
One of the best ways to illustrate this is with twin studies.
Twin studies look at identical twins and fraternal twins to see how often both siblings within the twin develop a certain disease.
Twin Studies in Autism
Figure 3: Twin Studies in Autism. Identical twins are far more likely to “share autism” than fraternal twins due to the contribution of genetics. If one identical twin has autism, the other identical twin has approximately a 60% chance of also developing autism. If one fraternal twin has autism, the other twin has around a 20-30% chance of developing autism. These data were obtained from a study that examined 40 identical twins and 45 fraternal twins.
In autism, we can see that if one identical twin has autism, the second identical twin has around a 60% chance of developing autism. This makes sense since twins share the same genetics and are also raised the same way.
However, if a fraternal twin has an autistic twin sibling, they only have a 20 to 30% chance of also developing autism.
The only significant difference between fraternal and identical twins is that identical twins share 100% of the DNA and fraternal twins only share 50%. They usually have the same upbringing, schools, teachers, and parents.
This statistic shows the substantial role that genetics plays in autism, compared to environmental factors.
If autism was purely a result of the environment and not genetics, we would expect fraternal twins and identical twins to have essentially the same odds of developing autism.
Twin Concordance for Autism Spectrum Disorder
Figure 4: Twin Concordance for Autism Spectrum Disorder. Identical twins are far more likely to “share autism” than fraternal twins due to the contribution of genetics. If one identical twin is on the autism spectrum, their twin sibling would have a 50-80% chance of also developing autism spectrum disorder. Meanwhile, if one fraternal twin is on the autism spectrum, their twin sibling would have a 30-40% chance of developing autism spectrum disorder.
Autism spectrum disorder has a slightly higher concordance rate. This means that when one member of a set of twins is on the autism spectrum, their twin siblings would have a 50-80% chance of also being on the autism spectrum.
Keys to Health
This research confirms a significant role that genetics play in autism.
It is important to note that this doesn’t mean predestination or fate when it comes to autism. There is no autism gene that causes someone to necessarily become autistic.
Even among identical twins, a twin only has a 60% chance of developing autism if their sibling is autistic, despite sharing the same environment and genetics.
Much of what drives autism is either random chance or remains unknown. The most important insight to take away from this is that we cannot blame anyone or any parent for autism. Autism development is heavily driven by factors completely outside of our control.
There is no genetic test for autism and we cannot control or change our genes even if we could test for it.