Evidence
for ‘Nature’
In this section, our goal is to mention
some studies which provide
evidence in favor of the ‘hereditary’ nature of intelligence. Recall
that the
‘nature’ side of the debate argues that a person maintains his mental
ability
only based on what he is born with genetically. Moreover, defending
this side
of the debate exclusively would be establishing that a person’s
environment
plays no role in determining his mental ability.
Scientists
have known for years that traits such as eye color and hair color are
determined by specific genes encoded in each human cell. In a way, the
‘nature’
theory takes things a step further to say that more abstract traits,
including
intelligence, personality, aggression, and sexual orientation, are also
encoded
in an individual's DNA. There are some very obvious reasons for an
individual
to be convinced that genetics play a large part in a person’s
intelligence.
When considering the biology of heredity, it is obvious that genes
provide
humans with their own physical equipment, which is in essence, their
basis.
Genes and chromosomes are passed on from each generation to the next.
Therefore, without heredity, humans would have nothing to pass on
biologically
to their offspring; hence the environmentalist idea of genetics being
purposeless is clearly incorrect.
 www.micro.utexas.edu/.../
genetics/genetics.html
The
hypothesis about the hereditary nature of any trait, including
intelligence, can be tested thanks to certain events that occur
naturally, such
as the birth of monozygotic/identical twins, that is, twins born from
the same
cell (ovule). It was mentioned earlier that Francis Galton primarily
used twin
studies to collect evidence when hypothesizing his hereditarian
explanation of
the phenomenon of intelligence. First used
by Galton, twin studies have always been the major source of collecting
evidence by the investigators of the ‘nature’ camp. These studies are
rendered
on sets of twins; these include both identical twins and fraternal
twins. As
noted earlier, they are conducted to determine the comparative
influence of
heritability and environment. In fact, there are largely three types of
studies
(including twin studies) that you will regularly see in the research
area of
hereditarians:
- Family
studies. Children share 50% their genes
with each of their parents.
- Twin
Studies. As we saw this proves an
interesting area of research, as there is a tendency to compare
variables among Monozygotic
(identical) twins who share 100 percent of their genes, and dizogotic
(fraternal) twins who share 50 percent of their genes (the same
percentage as non-twin siblings).
- Adoption
Studies. The influence of the
environment and genes is often compared in adoption studies. Therefore
variables are often compared between
siblings or twins reared apart, to examine the relative influence of
genes and the environment. For example, if two twins show similar
behaviors, despite being raised in different environments, this may
suggest that genes may be important in that behavior.
The motivation
of studies involving fraternal twins is; if genetics didn't play a
part, then
fraternal twins, reared under the same conditions, would be alike by
means of
their intelligence and other traits, regardless of differences in their
genes.
But, while studies making use of IQ tests show they do more closely
resemble
each other than do non-twin brothers and sisters, they also show these
same
striking similarities when reared apart. This shows that genetics play
a major
role in human intelligence. A
similar
idea/logic can be seen in studies involving identical twins: In the
case of
identical twins, both twins have exactly the same hereditary load, so
that
differences between them can only be attributed to environmental
factors. In
general, in the case of intelligence tests, it can be stated that, as
indicated
by several researches, genetic inheritance is responsible of about 50%
of a
child’s variation with respect to the average, but this value increases
with
age. Identical twins show a higher degree of correlation in their
scores, even
when they are reared apart. It can then be stated, therefore, that
heredity has a great influence in the
intelligence of a person
As noted earlier, adoption studies form another set of research tool
for
the investigators of the ‘nature’ camp; yet, they are also used by the
‘nurture’ camp. Child adoptions are beneficial for the investigators of
the
‘nature’ camp as they give the chance to see the effects of bringing
up, in the same
family, children with different inheritance, as well as bringing up
children
with the same inheritance in different families. Of particular interest
to the
study of hereditary traits are the cases of monozygotic twins that are
reared
apart. Even identical twins that are reared apart have more similar
IQ's than
fraternal twins that are reared together. Hereditarians claimed this as
powerful support for genetic influences on intelligence.
Another piece of evidence in favor of hereditarian view of intelligence
comes
from cases where the intelligence levels of adopted children and
biological
sons of the same family are compared. On the contrary of what was found
in the
case of identical twins that are reared apart, there is no significance
relation between the intelligence of adopted children and that of the
biological
sons of the same family, although all of these kids grew up in the same
environment (i.e. with same education and economic condition).
According to this
last finding, it can then be stated, again, that not environment but heredity
has a great influence in the intelligence of a person.
Below is the figure which summarizes the
results/conclusions given above, and much more. The data represented in
the figure is taken from a study conducted by Jensen (1969): ‘Concept
of
Heritability’, as an argument for the heritability of intelligence.
In general, in behavior genetic research,
researchers
would use the data to compute heritability estimates (h2),
which are estimates of the proportion of variance accounted for by
genetic
factors. Jensen, in this study, found that 80% variance between IQ and
individuals can be explained by genetics. The math/statistics part of
the research
process is left out for the sake of simplicity here. However, it is
important to
mention that the two sources of variation in a trait can be separated
into
independent genetic (G) and environmental (E) components that add to
l00% of the
variance that is accounted for. On the basis of this fact, the
heritability
coefficient (h2) can be subtracted from 100% to yield an
estimate of the
environmental contribution to variance. Therefore, if we computed that
if genetics accounted for 25% of the variance for a personality trait,
we
would estimate that the environmental factors account for 75% of the
variance.
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