The Heritability of Autism

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.

Evidence Score:

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Endpoints - Researchers used health registries for autism diagnoses, which was the most feasible endpoint for this research.

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Study Size - This study was one of the largest cohorts studied for nearly any disease. Researchers used all live singleton live births in 5 countries within the specified study period.

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Strength of the findings - Research obtained a precise estimate of the heritability of autism, which was the aim of this study.

Top Questions and Answers

What does this mean?

Identical vs Fraternal Twins

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

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

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.

Expert Opinions

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National Institutes of Health

ASD has a tendency to run in families, but the inheritance pattern is usually unknown. People with gene changes associated with ASD generally inherit an increased risk of developing the condition, rather than the condition itself. When ASD is a feature of another genetic syndrome, it can be passed on according to the inheritance pattern of that syndrome.

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Mayo Clinic

Families who have one child with autism spectrum disorder have an increased risk of having another child with the disorder. It's also not uncommon for parents or relatives of a child with autism spectrum disorder to have minor problems with social or communication skills themselves or to engage in certain behaviors typical of the disorder.

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American Academy of Pediatrics

The AAP recommends that all children be screened for autism at 18 and 24 months of age. The sooner autism is identified, the sooner an intervention program can start. If you are concerned about how your child plays, learns, speaks or acts, talk with your pediatrician. At the same time, call your state’s public early childhood system to request a free evaluation to find out if your child qualifies for intervention services. You do not need to wait for a doctor’s referral or medical diagnosis. Research shows that early intervention services can greatly improve a child’s development.

Clearvue Health is not affiliated with above organizations. The information above is provided to highlight and link to useful further reading.