A Doppelganger is a person who is the image of another person. Doppelgangers have been featured in a number of films and novels. Possibly the best is Akira Kurosawa's film "Kagemusha" in which a samurai period king is killed, and his advisors substitute a peasant who looks like the now-dead king, and then begins to display his personality. I had a doppelganger once. I have never met him but some mutual friends did mistake one of us for the other. We exchanged photographs, and indeed there was a remarkable physical likeness. Now let's look at what that implies.
Physical characteristics are substantially but not entirely determined by the genes. Eye color is almost completely determined genetically. So is male pattern baldness. In fact, these are pretty good examples. There are lots of blue-eyed, bald headed men who never met each other, have no known relatives, but must share some common genes. My doppelganger and I have very different ancestories. He is apparently descended from Middle Europeans, while I'm genetically a Celtic-Anglo Saxon mix. Nevertheless, we would not have looked so much alike if we did not share some genes. (Things are not perfect. I'm told that he has gained weight as we both aged. I have not.)
Granted, we can have genetically determined physical characteristics. Might unrelated individuals share genetically determined personality characteristics, including intelligence? An important article, by 32 different authors, most affiliated with Prof. Ian Deary's laboratory at the University of Edinburgh, has shown that unrelated individuals do share similar values on cognitive tests...intelligence tests and related evaluations. (Davies et al., Molecular Psychology, 2011, pgs. 1-10). Thanks to modern, cheaper methods for determining genomes, Davies and his many colleagues obtained samples from 3500 people who had participated in a longitudinal studies conducted in Scotland and Norway.
Because studies had included a variety of cognitive tests Davies et al. were able to construct patterns of similarities between both the genetics and the patterns of intelligence displayed by unrelated people. After complicated statistical analyses, they Davies et al. were able to show that somewhere around 40 to 50% of the variation in cognitive test scores was associated with genetic variation.
This is a very important study, but it is not the last word on genetics and intelligence...nor do the authors claim that it is. There are two important limits on the conclusions. First, the 40-50% figure applies to the particular populations that were studied. Scots and Norwegians are not the same, but Scotland and Norway are both first-world, industrially developed countries in Northern Europe. This limits the environmental variation (which, if it goes up, the 40-50% figure would go down) and, because Scots and Norwegians do share some common history, the genetic variation is less than if would be if, say, the study were to be repeated in Africa, where genetic variation is high. Increasing the genetic variation would drive the 40-50% figure upwards.
Also, because the longitudinal studies began in the 1930s, most of the participants were "senior citizens." We know that the heritability of intelligence increases in advanced age. All these reservations say is that the study ought to be repeated in other populations. You can say that about a lot of studies. You can also say that the 40-50% heritability figure had already been found, using more conventional genetic analyses. (See my book Human Intelligence for a discussion of these studies.) That is carping; confirming findings obtained with one method by the use of another, entirely different, method is an important step in scientific progress.
The Davies et al. paper is a substantial step forward in understanding the relationship between genetic composition and variations in human cognitive behavior. The study could be extended to other aspects of personality. I hope it will be.
Do we have any behavioral evidence about me and my Doppelganger? Well, we do. Remember, we are unrelated and never met; we just look alike. As young men we both enlisted in the same branch of the military services. We then both decided to follow academic careers, and we both became interested in the scientific side of psychology. We both obtained Ph.D.'s, and when we learned about each other we both were employed by major research universities. (This suggests that our intelligence test scores would not be too different.) The match is not complete. We followed different military specialties and have followed different sub-specialties within scientific psychology. But it does make you think, doesn't it?
In a later blog, I am going to make some comments about the larger meaning of new technologies such as the one used by Davies et al.