I. The Issue
Showed that there is significant genetic variation for IQ within homogeneous
(e.g. middle-class whites).
Concluded that the differences between whites and blacks in average IQ score
This conclusion led him to conclude that it was unlikely that educational
be successful in "bringing black intelligence up to white levels" (not his quotation)--
essentially, if a trait is genetically determined, it is immutable and not subject to alteration
by environmental intervention.
These arguments have been attacked repeatedly on a number of fronts, including
IQ tests are culturally biased, and therefore give false readings of intelligence
among ethnic groups.
The data used for many of the analyses described by Jensen and by Herrnstein
and Murray is
riddled with ambiguity, and in some cases appears to be fabricated. (See "Behind the Curve"
from Scientific American--Required Reading)
As evolutionary biologists, then we should be able to evaluate validity of
both the premises and the
inferences in Jensen's and Herrnstein and Murray's arguments.
We shall consider the evidence, as well as the logic of these author's underlying
deductions, and will
come to three conclusions:
Even if it were true that racial differences in IQ were due in part to underlying
differences, it is not true that environmental intervention could not eliminate this difference,
and even raise everyone's IQ, assuming that high average IQ is a desirable goal.
Example: The National Longitudinal Survey of Labor Market Experience of Youth
for short, the "NLSY").
f. Attempts have been made to "correct" for such biases. For example,
and Murray present a graph showing that factoring out parental socio-economic
status (SES) still shows that blacks have a lower IQ than "equivalent" whites:
Example: comparison of monozygotic and dizygotic twins.
c. When monozygotic and dizygotic twins are compared, the contribution
environment to the correlation should be similar.
d. By contrast, when the genetic contribution to the correlation is
this contribution to be greater for monozygotic twins because they are genetically
e. Hence, the difference between the correlations for monozygotic and
twins should reflect the degree of genetic control over the trait:
f. Data summarized by Cavalli Sforza and Bodmer indicate that Monozygous
(MZ) tend to have higher intraclass correlations for IQ than dizygous twins (DZ):
Newman et al. 0.886 0.6 31
Shields 0.76 0.51
g. This data indicates a fairly strong genetic component to differences
(non-twin) individuals in IQ.
b. By contrast, the contribution of the environment to the correlation
smaller for twins raised apart, since the the environment experienced by separated
twins is probably more different than the environment experienced by twins
c. This means that we expect the intraclass correlation coefficient
to be sustantially
smaller for twins reared apart if home environment has a large effect in IQ, relative
to the effects of genetics.
d. There is such an effect, as revealed by several studies (Data
from the Minnesota
Twin Project; Bouchard, T. J. et al. 1990. Science 250: 223-228 below):
intraclass correlation, r
Test MZ apart MZ together
WAIS IQ--verbal 0.64 0.88
WAIS IQ--performance 0.71 0.79
Raven, Mill-Hill Comp. 0.78 0.76
e. However, the effect is rather small, suggesting that only a small
portion of the
variation among individuals (non-twins) in IQ is explainable by differences in
home environment; this result is consistent with the notion that genetic variation
contributes importantly to IQ variation.
Unfortunately, experimentation on humans, even relatively benign common garden
experiments, are prohibited for moral reasons, which means that we do not have this
tool available to answer the basic question.
Consequently, some people are led to using logical reasoning to deduce whether
differe genetically for a character.
In most cases this approach will not work.
2. Next, let us choose two samples of 1000 seeds randomly from the same bag.
Let us plant one of the samples out in a field that is plowed but not fertilized,
us plant the other sample in a field that is plowed and fertilized.
4. Then, we go out on the 4th of July and measure the height of every plant in both fields.
First, consider what we will see in the unfertilized field, if we plot a
frequency histogram of
number of individuals in a given height category:
Next, consider what we will see in the fertilized field:
We have thus set up a situation in which the following statements are true:
In other words, we have just shown that it is possible to have genetic variation
for a trait (e.g
IQ) within populations, differences in the mean of the trait between populations (races),
without there being differences in genetic composition between the populations (races).
This demonstration proves that Jensen's and Herrnstein and Murray's conclusion
implied by their premises.
Suppose we start with two different bags of seeds.
Now, we take the seeds from both bags and plant them in the same field, with
positions randomized (why is this important?), keeping track of which bag each
seed, and the resulting plant, is from.
On the 4th of July we measure the height of every plant in the field and construct
frequency histogram of plant height for the two populations:
Implication with respect to IQ: There is no good reason to believe that
racial differences in mean IQ
score are caused by underlying racial differences in intelligence.
In particular, one would like to know whether it is hopeless to elevate the
of a racial group were racial differences in IQ to prove genetically based.
Again, J's and H&M's conclusion that it is hopeless is based on a fundamental
of what it means for differences to be genetically based, and in particular on failure to
understand or appreciate the common occurrance of genotype x environment interactions.
The simplest manifestation of this principle is that the environment affects
e.g. fertilizing a plant will, on average, increase height, so that a given genotype can
produces different height phenotypes, depending on the fertility of the soil in which an
In general, most genotypes respond to fertilizer in a similar way, by increasing
4. However, this pattern is not always the case for other characters.
Example: metal tolerance in plants.
A concrete example: Bristle number in Drosophila (Gupta and Lewontin.
The importance of this is that just because genetic differences causing differences
in IQ are
expressed in one environment, doesn't mean that they will be expressed in other
other environments, or even that the relative ranking of IQ scores won't be reversed in
It is thus quite possible that educational, or other environmental, intervention
IQ scores of blacks up to or beyond that of whites.
In no way, then, can evidence for a genetic basis to racial differences in
IQ under current
environmental conditions be taken to imply that intervention can not eliminate those
In fact, a story that appeared quite a few years ago suggests one possible
environmental intervention might boost IQ's of both blacks and whites up to more or
less equal levels.