Science has known for a while now that there is a strong genetic component in autism spectrum disorders (ASD), although those opposed to vaccination tend to deny this. (David Gorski points this out in his commentary on this new 2019 paper.)
In this paper, Bai et al. cite data from a meta-analysis of twin studies that estimated the heritability of ASD to be between 64 & 91%. and a set of three Swedish studies with a range of 66 – 83%. These values suggest that much to most of the variability in ASD between populations is the result of genetic differences.
Bail & his colleagues also note that it’s also possible for ‘maternal effects’ (especially maternal phenotype) and shared environmental effects to make some contribution, and so their just-published study was designed to allow them to “estimate the heritability** together with maternal effects and shared and nonshared environmental effects of ASD.”
To do this they used data on the medical histories of people from 5 countries (Israel, Australia, Denmark, Finland, & Sweden), in what they believe to be “the largest-ever dataset for population-based epidemiologic autism research to date”. This gave them information for a total of 2,001,631 individuals from 680,502 families, with ~51% of their sample being male. Individuals were followed from birth for up to 16 years, and included 22,156 people who received a diagnosis of ASD in that period. The study didn’t include twins, due to a lack of information on whether they were fraternal or identical.
After analysing how verious genetic and non-genetic factors contributed to the risk of ASD, the researchers concluded that
[the] current study results provide the strongest evidence … to date that the majority of risk for ASD is from genetic factors.
They also found that the impact of maternal factors (eg things like weight, or whether a birth was vaginal or caesarian) was minimal-to-zero. However, the actual genetic component could be higher: it’s possible that de novo mutations could also contribute (any given individual will have a small but measurable number of new mutations, not shared with either parent), especially when we consider the large ‘target’ for mutations provided by the large number of genes implicated in ASD. For example, this 2014 paper suggests that de novo mutations contribute to a relatively large number of ASD diagnoses.
As Dr Gorski sums things up, in his post at Science-Based Medicine:
[T]his study was not a gene association study, but rather the largest study to date to estimate how much of the risk of autism is genetic. As such, it produced an estimate that is in line with previous estimates and strengthens the scientific conclusion that autism is mainly heritable, with an effect on risk due to environment that is much smaller and, however large it actually is, not due to vaccines.
** Readers wanting to find out more about the concept of heritability could find this Khan Academy video interesting.
D.Bai, B.H.K.Yip, G.C.Windham, A.Sourander, R.Francis, R.Yoffe, E.Glasson, B.Mahjani, A.Suominen, H.Leonard, M.Gissler, J.D.Buxbaum, K.Wong, D.Schendel, A.Kodesh, M.Breshnahan, S.Z.Levine, E.T.Parner, S.N.Hansen, C.Hultman, A.Reichenberg & S.Sandin (2019) Association of Genetic and Environmental Factors with Autism in a 5-Country Cohort. JAMA Psychiatry doi: 10.1001/jamapsychiatry.2019.1411