Not Everything Is An InteractionShare
Why is it wrong to say that all development is the product of interactions between genes and environment?
Albert Einstein was a brilliant man. Whether his famous equation of E=mc2 means much to you or not, I think we can all concur on the intellectual prowess—and stunning hair—of Einstein. But where did his brilliance come from? Environment? Perhaps his parents fed him lots of fish (it’s supposed to be brain food, after all). Genetics? Surely Albert hit some sort of genetic lottery—oh that we should all be so lucky. Or does the answer reside in some combination of the two? How very enlightened: both genes and environment interact and intertwine to yield everything from the genius of Einstein to the comedic talent of Lewis Black. Surely, you cannot tease their impact apart; DNA and experience are hopelessly interlocked. Except, they’re not. Believing that they are is wrong; it’s a misleading mental shortcut that has largely sown confusion in the public about human development, and thus it needs to be retired.
Despite strong genetic influences on IQ (and there are strong genetic influences on IQ), we can’t calculate the proportion of credit for Einstein’s intellect that is owed solely to his genes. He’s just one person, and this prevents us from knowing that, say, 70 per cent of his genius was scrawled in his DNA. Though true for given individuals, this reality has lead some to claim that under no circumstances can we ever talk meaningfully about genetic effects on development separately from environmental effects. Trying to split the two would be a bit like trying to calculate the area of a rectangle using only its height. Your middle school teacher would be ashamed, as everyone knows you need both height and width. As the great psychologist Robert Plomin noted, however, “f we ask not about a single rectangle but about a population of rectangles, the variance in areas could be due entirely to length, entirely to width, or both.” As with rectangles, so with humans. Differences observed in a population of humans can be described as the result entirely of genetic differences, environmental differences, or some combination of the two.
The studies that can split genetic from environmental influences nearly always demonstrate that both matter to some extent. Why is it wrong, then, to say that all development is the product of interactions between genes and environment? The wrongness has to do with understanding what the word “interaction” actually means. The term is commonly used to refer to the fact that practically every complex human outcome is influenced by both genetic and environmental factors. The more technical definition of “interaction”, however, refers to instances where the magnitude of genetic effects on some trait either increases or decreases depending on the environment. Whether genes and the environment interact in this narrow sense is a mathematical issue, a different ball of wax entirely from parochial ideas about interaction. Most importantly, genes and environments might interact in the parochial sense—meaning that both might matter for development—while in the mathematical sense there is no interaction to be found.
Consider an example of a “technical interaction.” Prior research has suggested that, in the United States, the relative influence of genes on differences in general intelligence appears to depend on socioeconomic status. Genetic effects either explain more, or less, of individual differences in intelligence depending on whether one is raised in relative wealth versus poverty. Technical interactions remain a bugaboo for researchers. They are notoriously difficult to detect, prone to certain methodological problems, and like many things in science, they often do not replicate. The SES-IQ interaction gives the appearance of being isolated to the United States. A large recent review by the psychologists Elliot Tucker-Drob and Timothy Bates didn’t really find the effect in samples from other countries. This, by the way, doesn’t mean that the studies finding interactions were wrong. Not at all. But it does illustrate the fact that detecting an interaction does not guarantee that it exists everywhere, and in all places. When testing for interactions using specific genes measured with specific environments, the replication record becomes poorer still (for a variety of reasons that the behavior geneticists Laramie Duncan and Matthew Keller outline here). In short, the research landscape is not replete with confirmed interactions.
I hail from the southern part of the United States and so I speak with a lovely southern inflection (so I’ve been told!). Had I been born in Australia (or France or Britain), I would have an even lovelier accent. Accent variation is wholly environmental; it depends on where you’re born. Saying that accent is “an interaction between environment and genes”, is false. Alternatively, there are outcomes where individual differences are more fully explained solely by genetic differences. Most traits are the product of genetic and environmental influence, but the fact that both genes and environment matter does not mean that they interact with one another. Don’t be lured by the appeal of “interactions.” Important as they might be from time to time, and from trait to trait, not everything is an interaction. In fact, many things likely are not.
This article originally appeared at Quilette.