For
a few decades now, we science writers have been unwitting victims of a
scientific muddle called "heritability." Now we have a chance to wipe
the slime off and do our jobs.
The popular confusion started in 1979, when University of Minnesota psychologist Thomas Bouchard became fascinated with a particular pair of long-separated identical twins, and adopted what he thought was a method to distinguish genetic influences from environmental influences -- to statistically separate nature from nurture. The approach was to compare the ratio of similarities/differences in separated-identical-twins with the same ratio in separated-fraternal-twins. Since identical twins were thought to share 100% of their DNA and fraternal twins share, on average, 50% of their genetic material (like any ordinary siblings), comparing these two unusual groups allowed for a very tidy statistical calculation.
Bouchard and colleagues used the
words "heritable" and "heritability" to describe their
results.
There were just two problems with
this approach. First, these terms were possibly the most misleading in
scientific history. Second, it turns out that genetic influence cannot be separated from environmental
influences. Nature is inextricably intertwined with nurture.
***
Strangely, "heritability"
and "heritable" were actually never intended by behavior geneticists
to mean what they sound like -- "inherited." What they called
"heritability" was defined as "that portion of trait variation
caused by genes." In a quick glance, that might seem awfully similar to
"the portion of a trait caused by genes." But the difference is as
great as Mt. Everest and the anthill in front your home.
This led to quite the muddle when
Bouchard and others published twin-study data that seemed to demonstrate that
intelligence was 60%-70% "heritable." What was that actually supposed to mean?
It
did not mean that 60-70% of every
person's intelligence comes from genes.
Nor
did it mean that 30-40% of every
person's intelligence comes from the environment.
Nor
did it mean that 60-70% of every
person's intelligence is fixed, while only 30-40% can be shaped.
What
Bouchard et al intended it to mean was this (read v e r y
slowly): on average, the detectable portion of genetic influence on the
variation in -- not the cause of -- intelligence among specific groups of
people at fixed moments in time was around 60-70%.
If
that sounds confusing, that's because you are a human being.
"Heritability" is so confusing that most of the people who use it professionally
don't really understand it. Let's pick it apart:
On
average.
Heritability,
explains author Matt Ridley in his book Nature via Nurture "is a population average, meaningless for any
individual person: you cannot say that Hermia has more heritable intelligence
than Helena. When somebody says that heritability of height is 90 percent, he
does not and cannot mean than 90 percent of my inches come from genes and 10
percent from my food. He means that variation in a particular sample is
attributable to 90 percent genes and 10 percent environment. There is no
heritability in height for the individual."
"Cause
of variation" is not remotely the same as "cause of trait."
In
discussing "heritability" in the media, scientists have allowed the
public to confuse "causes of variation" with "causes of
traits." Heritability studies do not, and cannot, measure causes of
traits. They can only attempt to measure causes of differences (or variation)
in traits.
So,
for example, a heritability study cannot even attempt to measure the cause of
plant height. It cannot purport to tell you that some percent of plant height
is caused by genes.
What
it can attempt to do is measure the percentage influence that genes have on the
differences in height in a
particular group of plants. But the
percentage would only apply to that particular group.
Fixed
moments.
Heritability
derives from a fixed moment in time. It can only report on how life is, at that
moment, for the specific group studied. It cannot offer any guidance whatever
about the extent to which a trait can be modified over time, or project how
life can be for any other group or individual enjoying different resources or
values.
This
means that is these studies don't even pretend to say anything about individual
capability, or potential.
Finally,
many scientists now think that twin-study heritability estimates are sorely
compromised by a basic flawed supposition. "[They] rest on the
extraordinary assumption that genetic and environmental influences are
independent of one another and do not interact," explains Cambridge biologist Patrick Bateson
"That assumption is clearly wrong."
Now you'll have a sense how much salt to ingest when you come across silly phrases like this in the news:
"Since personality is heritable. . ." (The New York Times)
"Forty percent of infidelity [can] be blamed on genes" (The Daily Telegraph)
"Men's Fidelity Controlled By 'Cheating Genetics'" (The Drudge Report)
In the end, by parroting a strict "nature vs. nurture" sensibility, heritability estimates are statistical phantoms; they purport to represent something in populations that simply does not exist in actual biology. It's as if someone tried to determine what percentage of the brilliance of "King Lear" comes from adjectives. Just because there are fancy methods available for determining distinct numbers doesn't mean that those numbers actually have any meaning.
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