24 Nature vs. Nurture
Description
This article is from the Psychology FAQ, by Rolf Marvin Bøe Lindgren roffe@tag.uio.no with numerous contributions by others.
24 Nature vs. Nurture
This section is courtesy Frank Fujita
One of the big debates that occupies a lot of many people's time is
the Nature/Nurture controversy. It is also sometimes known as the
genetic/environmental controversy. A lot of this controversy has been
recently sparked by The Bell Curve, but in some form or another goes
back to antiquity. In this controversy, we ask if the behavior of
people is due to their Nature (or genetics) or to their Nurture (or
environment). This is a philosophical question which science can only
answer at the sidelines.
Let me spend some time discussing some trivially true examples, so
that we can limit the future discussion. If we take a baby human, and
a baby monkey and give them both the best environment that we can
imagine, the child will be radically different from the monkey and the
differences will be almost totally caused by genetic differences. At
the extreme, the proponents of the Nature side of the controversy
would have us believe that the differences in human behavior are like
the difference between human behavior and monkey behavior, just less
extremely.
Similarly, if we take identical twins, and give one the best
environment possible, and put the other one in closet for eighteen
years the differences will be profound, and caused totally by
environmental differences between the two children. At the extreme the
proponents of the Nurture side of the controversy would have us
believe that the differences in human behavior are like the
differences in the twin's behavior, just less extremely.
Thus, we can create a scientific environment that will produce an
almost totally genetic answer to our question, or a scientific
environment that will produce an almost totally environmental answer
to our question. The answer we get will be determined by the situation
that we set up. This point cannot be overemphasized, any scientist can
create the answer to the Nature/Nurture question that she desires.
Some scientists produce results, and other scientists say that the
scientific environmental environment was not a good one (too much like
the monkey or twin environment above). Usually, it is the scientists
that favor the Nurture side of the argument that actually set up the
studies, and the scientists on the Nature side of the argument that
criticize the studies. This leads to name calling. The Nurture
scientists say in effect, produce data or be quiet. The Nature
scientists say in effect, if you can't produce quality data, be quiet.
The heritability coefficient sounds like it answers the Nature/Nurture
question, but it only does so at the edges.
Family, adoption, and twin studies and combinations of these designs
can be used to estimate the magnitude of genetic effects as well as
their statistical significance. This is the descriptive statistic
called heritability. Heritability is an estimate of effect size
given an particular mix of existing genetic and environmental
factors in a particular population at a particular time. It is a
descriptive statistic that estimates the proportion of phenotypic
variance (i.e., individual difference in a population, not behavior
of a single individual) that can be accounted for by genetic
variance. It describes «what is» rather tan predicting «what could
be» or «what should be.» Heritability does not imply genetic
determinism-it refers to probabilistic propensities, not
predetermined programming.
Consider height. Correlations for first-degree relatives are about
.45 on average, whether relatives are reared together or adopted
apart. Identical and fraternal twin correlations are .90 and .45,
respectively, regardless of whether they are reared together or
adopted apart. These results indicate significant genetic effects.
For these height data, heritability is estimated as 90%. This
estimate of effect size indicates that, of the difference among
individuals in height in the population sampled, most of the
differences are due to genetic rather than environmental differences
among individuals. (Robert Plomin, Genetics and Experience: The
Interplay Between Nature and Nurture, p 43-44)
Certainly we can change height by adding growth hormone, restricting
vitamin intake, or any of many other environmental interventions.
However, in the sample of people measured, at the time measured, the
individual differences in the sample were mostly genetically caused.
There are some questions about the heritability coefficient. First, in
some studies, the computed heritability coefficient is greater than
1.0, that is in some samples the identical twins are more than twice
as similar as fraternal twins. Secondly, adoption studies show that
siblings are even less alike than we would expect (based on the
heritability coefficient). This may be because of nonadditive genetic
variance. That is that a configuration of genes may produce a certain
trait (say Extraversion) rather than a simple additive scenario where
the more genes you have to be extraverted, the more extraverted you
are. One estimate of variance in personality traits (Dunn & Plomin,
Separate Lives: Why Siblings are so Different, p. 50) is that 40% is
genetic, 5% is shared environmental, 35% is non-shared environmental,
and 20% is error variance.
Shared environmental differences are differences that can be given to
everybody, say living in a big home is a shared environmental
difference, as is a nice school, a good library, kind parents, etc.
Non-shared environmental differences are differences that are specific
to a child (within a family). Environmental influences in individual
development are specific to each child rather than general to an
entire family.
One myth that I would like to dispel is that the Nature- genetic
differences are difficult to change, but Nurture- environmental
differences are easy to change. Many genetic defects are very easy to
correct, and many environmental deficiencies are impossible to change.
For example, poor eyesight is genetic, and yet there are many ways to
correct it, eyeglasses, contact lenses, and surgery. On the other
hand, a childhood with little protein makes one short, and once grown,
we cannot raise the individual's height to what we might want to think
of as her genetic potential.
In short, the answer to the question «Is X caused by the environment
or is it genetic?» is usually «Both.» Even heritability coefficients
of 1.0 do not mean that the environment cannot (under different
circumstances than ones in which the study was performed) affect the
trait in question, and even coefficients of 0.0 do not mean that in
some other sample of people that there will not be a genetic influence
on the trait. The genetic influence may be additive (as in height) or
non-additive (as in extraversion). The environmental influences may be
things that the parents (or government) can change, or they may be
non-shared environmental differences that cannot be fine-tuned with
our present, crude understanding of the influences of the environment
on behavior. Lastly, whether the influences are genetic or
environmental do not inform us as to whether the trait will be easy to
change or not - some genetic traits will be easy to fix (like
eyesight) some environmental traits will be hard to fix.
Nature vs. nurture
{Further Reading]
[Anastasi, Anna] (1958) Psychological Asessment 3rd ed.
This is the classic tome which outlines the current position as
regards the nature/nurture controversy.
[Brand, Christofer] (1996) The g Factor London: Wiley
This one will probably become a classic. The most sober and
comprehensive discussion of intelligence and intelligence research
that I have seen.
[Herrnstein, R., & Murray, C.] The Bell Curve New York: Free Press
A very controversial book, in several senses. Even as its statistics
and psychology are sound, the authors seem to believe that it is
possible to infer political conclusion directly on basis of scientific
evidence, a technique of argumentation which is in itself as
controversial as the research which is presented.
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