All in the genes

There’s a story at the  Huffington Post, based on a new research paper, saying

Genes account for about 80% of a child’s risk of developing autism, a massive new study finds.

That link is not from the HuffPost story (they don’t link), but via Spectrum News, whose story is similar except they actually use the word “heritability” in a few places.

The abstract of the research paper says

Based on population data from 5 countries, the heritability of ASD was estimated to be approximately 80%, indicating that the variation in ASD occurrence in the population is mostly owing to inherited genetic influences, with no support for contribution from maternal effects.

so this is a case where the claims come from the scientists — but where the claims aren’t as strong before you translate them from Science to English.

If you think about it for a few minutes, it obviously can’t be true in a straightforward sense that 80% of the risk of developing autism is genetic(or, more precisely of being diagnosed with it, since that’s what can be measured).  Autism (diagnosis) is much more common than it used to be. For example, the US Centers for Disease Control report that from 2000 to 2014, “ASD prevalence estimates increased from 6.7 to 16.8 per 1,000 children aged 8 years, an increase of approximately 150%.” Absolutely none of this change is genetic: fourteen years isn’t long enough for population genetic changes. In the US, over that period, something non-genetic that varies with time was responsible for the majority of the autism (diagnosis) risk.  Improvements in diagnosis are probably one contributing factor.

Heritability is an important technical measure that sounds more interesting than it actually is.  It’s related to whether things are genetically determined, but it isn’t the same. For example, in humans, “number of legs” has very low heritability, but the reason nearly everyone has two legs is genetic.

More seriously, suppose that a condition requires both that you have a particular genetic variant and that you experience a particular environmental exposure.  Whether the condition ‘looks’ genetic or environmental will depend on who you compare.  If you compare among people of northern European ancestry living in NZ, melanoma skin cancer looks to be mostly environmental: it’s caused by sunburn.  If you compare among people living in South Africa, melanoma looks substantially genetic: it’s caused by a lack of melanin in the skin, and the genetic contributions to skin colour are moderately well understood.