Not Everything Is An Interaction


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Synopsis

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

Brian
Boutwell
 is an Associate Professor of
Criminology and Criminal Justice at Saint Louis University.
Follow him on Twitter @fsnole1

Tags: brian
boutwell
, genetic
correlations
, gene–environment
interaction
, indyvidual differences,
technical interaction

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