There are two kinds of people in this world: those who are deeply suspicious of twin studies, and those who welcome their confirmation that that their identity is fixed and heritable. I’m in the first group. I always have been. Maybe it’s something in my genes.
I first encountered the popular accounts of the Minnesota twin studies when I was a teenager, seeing the scientist and some of the twins doing the rounds of the afternoon talk shows — I think I saw them on the Mike Douglas Show (I’ve dated myself now). I remember them going on and on about the amazing similarities between the twins who had been raised apart. They both married women with the same name! They drank the same brand of beer! They were both volunteer firemen! They gave their dogs the same name! But while there were some recognizable similarities in the pairs, at the same time the obsession with superficial trivia wrecked the credibility of the stories. What? You’re trying to argue that my pet’s names are somehow encoded in my genome? It seemed to me that what we were seeing is echoes of similar culture in their upbringing (later confirmed: most of the twins weren’t really ‘separated’, but were raised by different relatives).
I also saw psychological tropes that ought to have been recognized. These were people who were rewarded for finding coincidences, and they avidly complied, and the scientists were readily accepting of coincidences as evidence of fundamental causal similarity. I was exposed to this pop genetics at the same time I was reading Fate magazine with a critical eye, and the stories were similar. I’d see stories that claimed to confirm the fact of reincarnation, for instance, by compiling lists of similarities between the contemporary claimant and their past life incarnation. They have the same birthday! Note the resemblances in this old-timey photograph! He lived in the Civil War era, now he is a Civil War re-enactor! He died in a fire, and now he’s afraid of fire!
It was exactly the same. That bugged me. And to this day I still see people touting the old twin studies as conclusively demonstrating the genetic basis of personality and intelligence, declaring that it has been positively confirmed that the heritability (a word they often don’t understand — genetically, it has a very narrow and precise meaning that isn’t exactly what they think it is) of intelligence is exactly 50%, meaning that half your IQ is determined by your genes (again, that’s not what it means), and therefore we should be more concerned with breeding intelligent people than teaching people. I also see this fandom coupled with other ugly associations — racists love it, as do Libertarians and simple-minded techno-fetishists. There are definitely genetic contributions to brain development and behavior, but human twin studies are deeply flawed and prone to exaggeration.
Stephen Hsu is a member of the gullible second group. He has posted a reply to my criticisms of his claim that we can readily ramp up human intelligence to reach an IQ of 1000 because hey, intelligence is obviously heritable. The twin studies say so.