A reader asks:
To clarify, while I believe natural selection explains a lot I have
caveats about IQ as a tool for testing intelligence. If you can’t
measure the coast of France with a single number how can you do it
with human intelligence?
Easily. Human intelligence is a great deal less complex than the
coast of France. :-)
It’s fashionable nowadays to believe that intelligence is some
complicated multifactor thing that can’t be captured in one number.
However, one of the best-established facts in psychometry (the science
of measuring mind) is that it is quite difficult to write a test of
mental ability that is not at least 50% correlated with all other such
tests. Or, to put it another way, no matter how you design ten tests for
mental ability, at least about half the variance in the scores for any one
of them statistically appears to be due to a “general intelligence”
that shows up on the other nine tests as well.
Psychometricians call this general intelligence measure “g”. It
turns out to predict important real-world success measures quite well
— not just performance in school but income and job success as
well. The fundamental weakness in multiple-factor theories of intelligence
is that measures of intelligence other than g appear to predict
very little about real-world outcomes. So you can call a lot of other
things “intelligence” if you want to make people feel warm and fuzzy,
but doing so simply isn’t very useful in the real world.
Some multifactor theorists, for example, like to describe accurate
proprioception (an acute sense of body position and balance) as a kind
of intelligence. Let’s say we call this “p”. The trouble with this
is that there are very few situations in which a combination of high p
and low g is actually useful — people need to be able to balance
checkbooks more often than they need to walk high wires. Furthermore,
g is easier to substitute for p than the other way around; a person
with high g but low p can think up a way to not have to walk a high
wire far better than a person with low g but high p can think up a way
not to have to balance a checkbook. So g is in a strict functional
sense more powerful than p. Similar arguments apply to most of the
other kinds of specialized non-g ‘intelligence’ that have been
Once you know about g, you can rank mental-capability tests by
how well their score correlates with g. IQ is valuable because a
well-composed IQ test measures g quite effectively. For purposes
of non-technical discussion, g and IQ can be considered the same, and
pychometricians now accept that an IQ test which does not closely track
g is defective.
A lot of ink has been spent by people who aren’t psychometricians
on insisting that g is a meaningless statistical artifact. The most
famous polemic on this topic was Stephen Jay Gould’s 1981 book
The Mismeasure of Man, a book which was muddled,
wrong, and in some respects rather dishonest. Gould was a
believing Marxist; his detestation of g was part of what he perceived
as a vitally important left-versus right kulturkampf. It is
very unfortunate that he was such a persuasive writer.
Unfortunately for Gould, g is no statistical phantom. Recently g
and IQ have been shown to correlate with measurable physiological
variables such as the level of trace zinc in your hair and performance
on various sorts of reaction-time tests. There are hints in the
recent literature that g may be largely a measure of the default level
of a particular neurotransmitter associated with states of mental
alertness and speed of thought; it appears that calling people of
subnormal intelligence “slow” may not be just a metaphor!
IQ is one of several large science-related issues on which
political bias in the dominant media culture has lead it to present as
fact a distorted or even reversed version of the actual science. In
1994, after Murray and Herrnstein’s The Bell Curve got a
thoroughly undeserved trashing, fifty leading psychometricians and
psychologists co-signed a summary of mainstream
science on intelligence. It makes eye-opening reading.
The reasons many popular and journalistic accounts continue to
insist that IQ testing is at best meaningless and at worst a sinister
plot are twofold. First, this belief flatters half of the population.
“My IQ may be below average, but that doesn’t matter because IQ is
meaningless and I have high emotional intelligence!” is,
understandably, a favorite evasion maneuver among dimwits. But that
isn’t the worst of it. The real dynamite is not in
individual differences but rather that the distribution of IQ (and
hence of g) varies considerably across groups in ways that are
Men vs. women is the least of it. With other variables controlled,
men and women in a population have the same mean IQ, but the
dispersion differs. The female bell curve is slightly narrower, so
women have fewer idiots and fewer geniuses among them. Where this
gets touchy is that it may do a better job than cultural sexism of
explaining why most of the highest achievers in most fields are male
rather than female. Equal opportunity does not guarantee equal
results, and lot of feminist theory goes out the window.
But male/female differences are insignificant compared to the real
hot potato: differences in the mean IQ of racial and ethnic groups.
These differences are real and they are large enough to have severe
impact in the real world. In previous blog entries I’ve mentioned the
one-standard-deviation advantage of Ashkenazic Jews over gentile
whites; that’s roughly fifteen points of IQ. Pacific-rim Asians
(Chinese, Japanese, Koreans etc.) are also brighter on average by a
comparable margin. So, oddly enough, are ethnic Scots — though
not their close kin the Irish. Go figure…
And the part that, if you are a decent human being and not a racist
bigot, you have been dreading: American blacks average a standard
deviation lower in IQ than American whites at about 85. And
it gets worse: the average IQ of African blacks is lower
still, not far above what is considered the threshold of mental
retardation in the U.S. And yes, it’s genetic; g seems to be about
85% heritable, and recent studies of effects like regression towards
the mean suggest strongly that most of the heritability is DNA rather
than nurturance effects.
For anyone who believe that racial equality is an important goal,
this is absolutely horrible news. Which is why a lot of
well-intentioned people refuse to look at these facts, and will
attempt to shout down anyone who speaks them in public. There have
been several occasions on which leading psychometricians have had
their books canceled or withdrawn by publishers who found the actual
scientific evidence about IQ so appalling that they refused to print
Unfortunately, denial of the facts doesn’t make them go away. Far from
being meaningless, IQ may be the single most important statistic about
human beings, in the precise sense that differences in g probably drive
individual and social outcomes more than any other single measurable
attribute of human beings.
Mean IQ differences do not justify making assumptions about any individual.
There are African black geniuses and Ashkenazic Jewish morons; humanity and
ethics demand that we meet each individual human being as an individual,
without prejudice. At the same time, group differences have a significance
too great to ignore. In the U.S., blacks are 12% of the population but
commit 50% of violent crimes; can anyone honestly think this is
unconnected to the fact that they average 15 points of IQ lower than the
general population? That stupid people are more violent is a fact
independent of skin color.
And that is actually a valuable hint about how to get beyond
racism. A black man with an IQ of 85 and a white man with an IQ of 85
are about equally likely to have the character traits of poor impulse
control and violent behavior associated with criminality — and
both are far more likely to have them than a white or black man with
an IQ of 110. If we could stop being afraid of IQ and face up to it,
that would give us an objective standard that would banish racism per
se. IQ matters so much more than skin color that if we started paying
serious attention to the former, we might be able to stop paying
attention to the latter.
UPDATE: An excellent summary of science relating to g