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\title{How Genetics and Environment Affect Intelligence}
\author{Marc Horowitz}
\date{29 September 1995}
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It is obvious to laymen and professionals alike that children tend to
be like their parents intellectually.  Since the turn of the century,
psychologists have discussed and researched this phenomenon.  Is this
similarity due to similarities in genes, or in the environment?  Given
measurements of the parents' abilities, or of the environment in which
children are raised, can one predict the abilities of their children?

The outcome of these psychological questions can have a profound
sociological effect (Hoffmann, 1985).  It is a fundamental belief of
our society that if someone works towards something hard enough, that
they can achieve it.  On the other hand, a person's intelligence has a
significant effect on his or her life.  A purely genetic model of
intelligence would fix at conception what a person was capable of
doing.  On the other hand, a purely environmental model of
intelligence would fix a person's capabilities to that of his or her
family.

In order to study intelligence, the first requirement is to establish
measurable quantities.  The key measurement of intelligence is the
intelligence quotient, or IQ.  There are a number of tests which
measure IQ, all of which attempt to place an individual on a normal
distribution of ability with a mean of 100 and a standard deviation of
15.

The source of the effect of genetics on IQ, if any, is clear.  Parents
and children share approximately 50\% of their genetic material; any
genetic influence on intelligence derives from this.  Of course, a
child is the biological offspring of a parent, or adopted.  There is
no ``in between''.

The effect of environment on intelligence is less clear.  What sort of
measurements can we make of a child's environment?\footnote{There was
an unwritten assumption in all my sources that all intellectual
development occurs during childhood.  This seems intuitive, but nobody
ever addressed this point directly.}  The most common measure of
environment as it affects intellectual development seems to be
socioeconomic status, or SES.  SES is also measured on a normal
distribution.  This is not a perfect indicator, but it is easy to
measure.

In order to measure and compare the contributions of genetics and
environment to intelligence, the effects of both need to be measured
separately.  Unfortunately, this is not simple at first glance, since
parents affect both the genes and environment of their children.  To
avoid this problem, most studies have involved the use of adopted
children.

Two kinds of studies appear most often in the literature.  The first
kind measures intelligence scores of children adopted near birth,
their biological parents, and their adoptive parents.  The scores of
the children and biological parents are correlated to determine the
effect of genetics on intelligence.  The scores of the children and
adoptive parents are correlated to determine the effect of environment
on intelligence.

The second kind of study compares the scores of adopted children and
their adoptive parents.  A group of demographically similar biological
families is measured as a control for environmental effects.  The
resulting difference is presumed to be due to genetic
effects. (Hoffman, 1985).

One instance of the first kind of study was conducted by Horn (1983,
as cited by Hoffman, 1985).  In this study, IQ scores were collected
from the adopted children, their biological mothers (from adoption
records), both adoptive parents, and any siblings (biological or
adopted) living with the family.

Three correlations were computed from this data.  The first was a
correlation of the IQs of the adopted children with their biological
parents (with whom they had never had any interaction).  The second
was a correlation of the IQs of the biological parents and the
children they had raised themselves.  The third was a correlation
between the scores of the adoptive parents and their adopted children.

The first and second correlations are almost exactly the same, which
is a very strong argument for a genetic influence on intelligence of
children.  The third correlation is somewhat smaller than the first
two.  Hoffman argues that this is a result of the adoption placement
process, implying that this study shows almost a purely genetic
influence on intelligence.  This process seems like a contributing
factor, but I do not think her arguments explain the entire
correlation for adoptive families.

In addition, there is other evidence in the study which seems to
contradict a purely genetic theory.  There were two more correlations
computed.  The fourth correlation matched the IQs of unrelated adopted
children reared in the same family.  The fifth was a correlation
between the IQs of biological siblings raised with their biological
parents.  These correlations were almost equal.  This evidence would
support an environmental factor contributing to intelligence.

One instance of the second kind of study was conducted by Scarr and
Weinberg (1978).  In this study, two groups of families were examined.
The first group of families consisted of parents and their
adolescent-age adopted children.  The second group of families were
biological families with similar socioeconomic backgrounds to the
first set of families.  In this study, too, there is more correlation
between the scores of biological parents and children than adoptive
parents and children.  However, the study also measures a difference
which seems to be attributable to environment.  The biological mothers
of the adopted children are an almost perfect random sample, with a
mean IQ of 100.00 and a standard deviation of 15.4.  Yet the average
IQ of the adopted children was 106.  Even if you account for adoption
placement biases, there is still a 3-5 point discrepancy; the best
explanation for the remaining difference is environmental influences.

Based on these experiments and others like them, it is clear that
intelligence is a complicated composite of genetics and environment.
This leads to many interesting conclusions.  On the other hand, none
of the correlations are strong enough to predict anything meaningful
about the intelligence of a child, even given ``perfect'' information
about the parents, environment, etc.

One simple application of these results is to Skinner's assertion that
given any dozen children and complete control over their environment,
he could turn them into anything he wanted.  Perhaps this is true to
some extent, but the genetic basis for intelligence would seem to
limit Skinner somewhat.  A child of average or below-average parents
would not, generally speaking, be able to become a university
professor, no matter what the environmental influence.

It is also important to realize how closely tied genetics and
environment are.  As stated earlier, parents tend to pass on both
genes and environment to their offspring.  But the environment which
the parents create is partially a function of their own intelligence.
Children of wealthy parents will participate in certain kinds of
expensive activities such as travel or theater more than parents who
are less well-off.  But the money to provide this sort of environment
comes from the parents, who probably have this money because their
intelligence, which is strongly genetic, allows them to hold a more
financially lucrative job than average.

In a broader sense, having a more detailed knowledge of the
psychological background of intelligence (even knowing that there
exists a psychological background) enables us to deal with
intelligence in a more productive fashion.  For instance, most parents
would like their children to have the most effective education
possible.  It would be useful to study what environmental conditions
seem to have the most advantageous effect on children.  Since genetics
and environment are so closely tied, it is quite possible that
environments which are seemingly beneficial to some children will not
be beneficial to others, or will be found to have no impact at all on
the intelligence of the children later in life.

On the other hand, it has been demonstrated that an environment which
is beneficial for upper-middle-class children can also be beneficial
to children who might have normally been raised in a very different
environment.  In a study of black and interracial children adopted by
upper-middle-class families, Scarr and Weinberg (1976, as cited by
Weinberg, 1989) showed that the adopted children performed
significantly above average on IQ tests and other school achievement
measures.  Similar children not raised in such an environment did not
perform nearly as well.  Although this might indicate biases in the
tests, a more likely possibility is that something in the
upper-middle-class environment is causing this change.  Finding out
whatever it is, and educating and assisting all parents in providing
this to their parents will result in a better educated population,
which is clearly beneficial to everyone.  Most importantly, this means
that children of underprivileged parents, regardless of their
intelligence, are not fated to being below average if a decent
environment can be created for them.

In conclusion, it is evident from many studies (Munsinger, 1975) that
genetics is a stronger influence on intelligence than environment.
Nevertheless, the psychological effects of environment on intelligence
cannot be discounted, as they can and do have a profound effect on the
result.  ``There is a myth that if a behavior or characteristic is
genetic, it cannot be changed.  Genes do not fix behavior.  Rather,
they establish a range of possible reactions to the range of possible
experiences that environments can provide.'' (Weinberg, 1989)
Determining what these reactions are seems to be the essence of the
study of psychology.

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\bf\Large References
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Hoffman, L.W., (1985). The Changing Genetics/Socialization Balance.
{\em Journal of Social Issues, Vol. 41, No. 1,} pp. 127-148.

Horn, J.M., (1983). The Texas adoption project: Adopted children and
their intellectual resemblance to biological and adoptive parents.
{\em Child Development, Vol. 54,} pp. 268-275.

Munsinger, H. (September 1975). The Adopted Child's IQ: A Critical
Review. {\em Psychological Bulletin, Vol. 82, No. 5,} pp. 623-659.

Scarr, S. and Weinberg, R.A., (1976). IQ test performance of black
children adopted by white families. {\em American Psychologist, 31,}
726-739.

Scarr, S. and Weinberg, R.A, (October 1978). The Influence of ``Family
Background'' on Intellectual Attainment. {\em American Sociological
Review, Vol. 43,} pp. 674-692.

Weinberg, R.A., (1989). Intelligence and IQ, Landmark Issues and Great
Debates. {\em American Psychologist, Vol 44, No 2,} pp. 98-104.
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