I’ve been pondering some of the points Robert Plomin has made in Blueprint, and I find myself thinking about some things that he doesn’t say, but should have. All this in the context of a book that is generally sensible.

Imagine a certain kind of personality variation: some people are aggressive, some not. The aggressive guys tend to prevail over the peaceful, but get in trouble from fighting each other when common: so you get an equilibrium, where both have the same average fitness. Hawk-dove, a la George Price and Maynard Smith. You might like or dislike hawks or doves, but you couldn’t say that either kind of personality was a disease ( A Darwinian disease, the only reasonably definition of disease). Now if those traits were influenced by many genes, there would be continuous variation, but that can be a stable state too, an ESS. Maybe some people would go too far, be too far out on the curve of aggressiveness or way too passive – say four sigma out – they might well have lower fitness, and maybe you could call that a disease – but it’s really just the far edge of a strategy.

Another case: tertiary syphilis, which used to be a common cause of mental illness, with whole wings of mental hospitals dedicated to it. Clearly a Darwinian disease.

Again: you find that having more mutational load, more deleterious mutations, increased your chance of schizophrenia, or autism, or low IQ: that strongly suggests schizophrenia, autism , and low IQ are not the far edge of some strategy. Note: people talking about shamans and schiz: you’re probably wrong. Same for autism – not a strategy.

It is not so easy to for us to determine just how deleterious a mutation is by looking at the sequence, but natural selection gives a strong hint: if they’re bad, they’re rare. And,on the whole, if they’re rare, they’re bad.

Plomin notes that “there is an excess of rare mutations in individuals with schizophrenia, autism and intellectual disability”, and also says that “individuals of extremely high intelligence have fewer of these rare mutations”, suggesting that “rare mutations are not good for you.”

But we knew that before we looked: theory, population genetics, says that deleterious variants will be rare. Selection happens. Now maybe Plomin is simplifying things for the average reader, but I get the nagging feeling that he doesn’t know much about evolutionary game theory, mutational load, mutation-selection balance, truncation selection. I don’t think that most behavioral geneticists ever took any courses covering such things – or am I wrong? Most started as psychologists, or so I’m told.

There are interesting results that make sense if (and only if) you’re thinking in terms of mutational load: intelligence is extremely polygenic, and if you have fewer bad mutations that influence IQ (true on average for people with high IQ) – you have fewer total bad mutations. Lower mutational load, which should result in better health and increased lifespan – and apparently does.

He also says “that the abnormal is normal, meaning that there are no qualitative disorders, just quantitative dimensions. The many DNA differences that are associated with what we call a disorder affect people throughout the distribution.” I don’t think that’s the clearest way of looking at things: better to say that we all have some deleterious mutations, while some people have more than average. But there really is a qualitative difference between what works and what doesn’t.