How should we think about IQ?
Many Minds
Unraveling the Genetics-Environment Debate in Twin Studies
This chapter explores the contributions of genetics and environment to complex behaviors, particularly through the lens of twin studies. It discusses findings on heritability of traits such as IQ and personality, revealing a more intricate relationship than previously understood. The chapter emphasizes advancements in genetic research techniques and the complexities surrounding the influence of shared family environments on intelligence.
IQ is, to say the least, a fraught concept. Psychologists have studied IQ—or g for “general cognitive ability”—maybe more than any other psychological construct. And they’ve learned some interesting things about it. That it's remarkably stable over the lifespan. That it really is general: people who ace one test of intellectual ability tend to ace others. And that IQs have risen markedly over the last century. At the same time, IQ seems to be met with increasing squeamishness, if not outright disdain, in many circles. It's often seen as crude, misguided, reductive—maybe a whole lot worse. There's no question, after all, that IQ has been misused—that it still gets misused—for all kinds of racist, classist, colonialist purposes. As if this wasn't all thorny enough, the study of IQ is also intimately bound up with the study of genetics. It's right there in the roiling center of debates about how genes and environment make us who we are. So, yeah, what to make of all this? How should we be thinking about IQ?
My guest today is Dr. Eric Turkheimer. Eric is Professor of Psychology at the University of Virginia. He has studied intelligence and many other complex human traits for decades, and he's a major figure in the field of “behavior genetics.” Eric also has a new book out this fall—which I highly recommend—titled Understanding the Nature-Nurture Debate. In a field that has sometimes been accused of rampant optimism, Eric is—as you'll hear—a bit more measured.
In this conversation, Eric and I focus on intelligence and its putatively genetic basis. We talk about why Eric doubts that we are anywhere close to an account of the biology of IQ. We discuss what makes intelligence such a formidable construct in psychology and why essentialist understandings of it are so intuitive. We talk about Francis Galton and the long shadow he’s cast on the study of human behavior. We discuss the classic era of Twin Studies—an era in which researchers started to derive quantitative estimates of the heritability of complex traits. We talk about how the main takeaway from that era was that genes are quite important indeed, and about how more recent genetic techniques suggest that takeaway may have been a bit simplistic. Along the way, Eric and I touch on spelling ability, child prodigies, the chemical composition of money, the shared quirks of twins reared apart, the Flynn Effect, the Reverse Flynn Effect, birth order, the genetics of height, the problem of missing heritability, whether we should still be using IQ scores, and the role of behavior genetics in the broader social sciences.
Alright folks, lots in here—let's just get to it. On to my conversation with Dr. Eric Turkheimer. Enjoy!
A transcript of this episode is available here.
Notes and links
3:30 – The 1994 book The Bell Curve, by Richard Herrnstein a Charles Murray, dealt largely with the putative social implications of IQ research. It was extremely controversial and widely discussed. For an overview of the book and controversy, see the Wikipedia article here.
6:00 – For discussion of the “all parents are environmentalists…” quip, see here.
12:00 – The notion of “multiple intelligences” was popularized by the psychologist Howard Gardner—see here for an overview. See here for an attempt to test the claims of the “multiple intelligences” framework using some of the methods of traditional IQ research. For work on EQ (or Emotional Intelligence) see here.
19:00 – Dr. Turkheimer has also laid out his spelling test analogy in a Substack post.
22:30 – Dr. Turkheimer’s 1998 paper, “Heritability and Biological Explanation.”
24:30 – For an in-passing treatment of the processing efficiency idea, see p. 195 of Daniel Nettle’s book Personality. See also Richard Haier’s book, The Neuroscience of Intelligence.
26:00 – The original study on the relationship between pupil size and intelligence. A more recent study that fails to replicate those findings.
31:00 – For an argument that child prodigies constitute an argument for “nature,” see here. For a memorable narrative account of one child prodigy, see here.
32:00 – A meta-analysis of the Flynn effect. We have previously discussed the Flynn Effect in an episode with Michael Muthukrishna.
37:00 – James Flynn’s book, What is Intelligence? On the reversal of the Flynn Effect, see here.
40:00 – The phrase “nature-nurture” originally comes from Shakespeare and was picked up by Francis Galton. In The Tempest, Prospero describes Caliban as “a born devil on whose nature/ Nurture can never stick.”
41:00 – For a biography of Galton, see here. For an article-length account of Galton’s role in the birth of eugenics, see here.
50:00 – For an account of R.A. Fisher’s 1918 paper and its continuing influence, see here.
55:00 – See Dr. Turkheimer’s paper on the “nonshared environment”—E in the ACE model.
57:00 – A study coming out of the Minnesota Study of Twins reared apart. A New York Times article recounting some of the interesting anecdata in the Minnesota Study.
1:00:00 – See Dr. Turkheimer’s 2000 paper on the “three laws of behavior genetics.” Note that this is not, in fact, Dr. Turkheimer’s most cited paper (though it is very well cited).
1:03:00 – For another view of the state of behavior genetics in the postgenomic era, see here.
1:11:00 – For Dr. Turkheimer’s work on poverty, heritability, and IQ, see here.
1:13:00 – A recent large-scale analysis of birth order effects on personality.
1:16:00 – For Dr. Turkheimer’s take on the missing heritability problem, see here and here.
1:19:00 – A recent study on the missing heritability problem in the case of height.
1:30:00 – On the dark side of IQ, see Chapter 9 of Dr. Turkheimer’s book. See also Radiolab’s series on g.
1:31:00 – See Dr. Turkheimer’s Substack, The Gloomy Prospect.
Recommendations
The Genetic Lottery, Kathryn Paige Harden
Intelligence, Stuart Ritchie
Intelligence and How to Get It, Richard Nisbett
‘Why our IQ levels are higher than our grandparents’’ (Ted talk), James Flynn
Many Minds is a project of the Diverse Intelligences Summer Institute, which is made possible by a generous grant from the John Templeton Foundation to Indiana University. The show is hosted and produced by Kensy Cooperrider, with help from Assistant Producer Urte Laukaityte and with creative support from DISI Directors Erica Cartmill and Jacob Foster. Our artwork is by Ben Oldroyd. Our transcripts are created by Sarah Dopierala.
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