Causes of autism: what research knows

What “heritability” means

“Heritability” describes what share of the differences between people in a population is due to genetic differences (an explanation of the term by the editorial team, not a statement from any of the sources). It says nothing about how autism came about in an individual person. Nor does it mean that autism is “only inherited from the parents” or that environmental factors play no role.

What twin and family studies show

• A meta-analysis of seven twin studies (literature search up to 2015) estimated heritability, depending on the assumed prevalence of autism, at 64 to 91 %. The authors conclude that autism rests on strong genetic influences. ^[1]
• A Swedish study with data on millions of siblings and twins estimated heritability at 83 %. The shared family environment contributed only minimally. ^[2]
• A study (Bai et al. 2019, as described by Thapar and Rutter) with data from five countries found a median heritability of about 81 %; the values ranged from about 51 % (Finland) to 87 % (Israel). The share of shared environmental influences was negligible. ^[3]

All three analyses see a high but not complete genetic share; the estimates vary by population and method. In the model, a remainder is left that is not explained by shared genes or a shared environment. What lies behind it is open.

Vaccinations: no link

The claim that vaccinations cause autism goes back to a publication from 1998, which the journal retracted in 2010. ^[5] Since then the question has been investigated very thoroughly:

• A meta-analysis of case-control and cohort studies found no link between the vaccinations examined and autism, including for thiomersal, mercury or the combined MMR vaccine. ^[4]
• A Danish study of 657,461 children found no increased share of autism in MMR-vaccinated children, including in children at increased risk, for example because a sibling was already on the autism spectrum. ^[6]

More on this in the article Do vaccinations cause autism?.

Parenting: not a cause

In the middle of the 20th century, the idea spread that “cool” mothers (“refrigerator mothers”) caused autism. The very first twin study in 1977, by contrast, showed a predominantly genetic contribution. ^[3] There is no scientific basis for the assumption that parenting causes autism. The studies find a negligible influence of the shared family environment. ^{[3; 2]}

Sources and literature

1. Tick B, Bolton P, Happé F, Rutter M, Rijsdijk F. Heritability of autism spectrum disorders: a meta-analysis of twin studies. Journal of Child Psychology and Psychiatry 2016;57(5):585–595. https://doi.org/10.1111/jcpp.12499 – Meta-analysis
2. Sandin S, Lichtenstein P, Kuja-Halkola R, et al. The heritability of autism spectrum disorder. JAMA 2017;318(12):1182–1184 (Research Letter). https://doi.org/10.1001/jama.2017.12141 – Register study
3. Thapar A, Rutter M. Genetic advances in autism. Journal of Autism and Developmental Disorders 2021;51(12):4321–4332, Abschnitt „The Heritability of Autism: From Early to Modern Twin and Family Studies“ (enthält Bai et al. 2019, 5 Länder). https://doi.org/10.1007/s10803-020-04685-z – Review article
4. Taylor LE, Swerdfeger AL, Eslick GD. Vaccines are not associated with autism: an evidence-based meta-analysis of case-control and cohort studies. Vaccine 2014;32(29):3623–3629. https://doi.org/10.1016/j.vaccine.2014.04.085 – Meta-analysis
5. Retraction – Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. The Lancet 2010;375(9713):445. https://doi.org/10.1016/S0140-6736(10)60175-4 – Retraction notice by the journal (on the 1998 publication: The Lancet 1998;351(9103):637–641, marked there as “RETRACTED”)
6. Hviid A, Hansen JV, Frisch M, Melbye M. Measles, mumps, rubella vaccination and autism: a nationwide cohort study. Annals of Internal Medicine 2019;170(8):513–520. https://doi.org/10.7326/M18-2101 – Cohort study