What good is IQ?
<p>A reader asks:</p>
<blockquote>
<p>To clarify, while I believe natural selection explains a lot I have<br />
caveats about IQ as a tool for testing intelligence. If you can&#8217;t<br />
measure the coast of France with a single number how can you do it<br />
with human intelligence?</p>
</blockquote>
<p>Easily.  Human intelligence is a great deal less complex than the<br />
coast of France. :-)</p>
<p>It&#8217;s fashionable nowadays to believe that intelligence is some<br />
complicated multifactor thing that can&#8217;t be captured in one number.<br />
However, one of the best-established facts in psychometry (the science<br />
of measuring mind) is that it is quite difficult to write a test of<br />
mental ability that is not at least 50% correlated with all other such<br />
tests.  Or, to put it another way, no matter how you design ten tests for<br />
mental ability, at least about half the variance in the scores for any one<br />
of them statistically appears to be due to a &#8220;general intelligence&#8221;<br />
that shows up on the other nine tests as well.</p>
<p>Psychometricians call this general intelligence measure &#8220;g&#8221;.  It<br />
turns out to predict important real-world success measures quite well<br />
&mdash; not just performance in school but income and job success as<br />
well.  The fundamental weakness in multiple-factor theories of intelligence<br />
is that measures of intelligence <em>other</em> than g appear to predict<br />
very little about real-world outcomes.  So you can call a lot of other<br />
things &#8220;intelligence&#8221; if you want to make people feel warm and fuzzy,<br />
but doing so simply isn&#8217;t very useful in the real world.</p>
<p>Some multifactor theorists, for example, like to describe accurate<br />
proprioception (an acute sense of body position and balance) as a kind<br />
of intelligence.  Let&#8217;s say we call this &#8220;p&#8221;.  The trouble with this<br />
is that there are very few situations in which a combination of high p<br />
and low g is actually useful &mdash; people need to be able to balance<br />
checkbooks more often than they need to walk high wires.  Furthermore,<br />
g is easier to substitute for p than the other way around; a person<br />
with high g but low p can think up a way to not have to walk a high<br />
wire far better than a person with low g but high p can think up a way<br />
not to have to balance a checkbook. So g is in a strict functional<br />
sense more powerful than p. Similar arguments apply to most of the<br />
other kinds of specialized non-g &#8216;intelligence&#8217; that have been<br />
proposed.</p>
<p>Once you know about g, you can rank mental-capability tests by<br />
how well their score correlates with g.  IQ is valuable because a<br />
well-composed IQ test measures g quite effectively.  For purposes<br />
of non-technical discussion, g and IQ can be considered the same, and<br />
pychometricians now accept that an IQ test which does not closely track<br />
g is defective.</p>
<p>A lot of ink has been spent by people who aren&#8217;t psychometricians<br />
on insisting that g is a meaningless statistical artifact.  The most<br />
famous polemic on this topic was Stephen Jay Gould&#8217;s 1981 book<br />
<cite>The Mismeasure of Man</cite>, a book which was <a href='http://www.mugu.com/cgi-bin/Upstream/Issues/psychology/IQ/carroll-gould.html'>muddled,<br />
wrong</a>, and in some respects rather dishonest.  Gould was a<br />
believing Marxist; his detestation of g was part of what he perceived<br />
as a vitally important left-versus right <em>kulturkampf</em>.  It is<br />
very unfortunate that he was such a persuasive writer.</p>
<p>Unfortunately for Gould, g is no statistical phantom.  Recently g<br />
and IQ have been shown to correlate with measurable physiological<br />
variables such as the level of trace zinc in your hair and performance<br />
on various sorts of reaction-time tests.  There are hints in the<br />
recent literature that g may be largely a measure of the default level<br />
of a particular neurotransmitter associated with states of mental<br />
alertness and speed of thought; it appears that calling people of<br />
subnormal intelligence &#8220;slow&#8221; may not be just a metaphor!</p>
<p>IQ is one of several large science-related issues on which<br />
political bias in the dominant media culture has lead it to present as<br />
fact a distorted or even reversed version of the actual science.  In<br />
1994, after Murray and Herrnstein&#8217;s <cite>The Bell Curve</cite> got a<br />
thoroughly undeserved trashing, fifty leading psychometricians and<br />
psychologists co-signed a summary of <a href='http://www.mugu.com/cgi-bin/Upstream/Issues/bell-curve/support-bell-curve.html'>mainstream<br />
science on intelligence</a>.  It makes eye-opening reading.</p>
<p>The reasons many popular and journalistic accounts continue to<br />
insist that IQ testing is at best meaningless and at worst a sinister<br />
plot are twofold.  First, this belief flatters half of the population.<br />
&#8220;My IQ may be below average, but that doesn&#8217;t matter because IQ is<br />
meaningless and I have high emotional intelligence!&#8221; is,<br />
understandably, a favorite evasion maneuver among dimwits.  But that<br />
isn&#8217;t the worst of it.  The <em>real</em> dynamite is not in<br />
individual differences but rather that the distribution of IQ (and<br />
hence of g) varies considerably across groups in ways that are<br />
politically explosive.</p>
<p>Men vs. women is the least of it.  With other variables controlled,<br />
men and women in a population have the same mean IQ, but the<br />
dispersion differs.  The female bell curve is slightly narrower, so<br />
women have fewer idiots and fewer geniuses among them.  Where this<br />
gets touchy is that it may do a better job than cultural sexism of<br />
explaining why most of the highest achievers in most fields are male<br />
rather than female. Equal opportunity does not guarantee equal<br />
results, and lot of feminist theory goes out the window.</p>
<p>But male/female differences are insignificant compared to the real<br />
hot potato: differences in the mean IQ of racial and ethnic groups.<br />
These differences are real and they are large enough to have severe<br />
impact in the real world.  In previous blog entries I&#8217;ve mentioned the<br />
one-standard-deviation advantage of Ashkenazic Jews over gentile<br />
whites; that&#8217;s roughly fifteen points of IQ.  Pacific-rim Asians<br />
(Chinese, Japanese, Koreans etc.) are also brighter on average by a<br />
comparable margin.  So, oddly enough, are ethnic Scots &mdash; though<br />
not their close kin the Irish.  Go figure&#8230;</p>
<p>And the part that, if you are a decent human being and not a racist<br />
bigot, you have been dreading: American blacks average a standard<br />
deviation <em>lower</em> in IQ than American whites at about 85.  And<br />
it gets worse: the average IQ of <em>African</em> blacks is lower<br />
still, not far above what is considered the threshold of mental<br />
retardation in the U.S.  And yes, it&#8217;s genetic; g seems to be about<br />
85% heritable, and recent studies of effects like regression towards<br />
the mean suggest strongly that most of the heritability is DNA rather<br />
than nurturance effects.</p>
<p>For anyone who believe that racial equality is an important goal,<br />
this is absolutely horrible news.  Which is why a lot of<br />
well-intentioned people refuse to look at these facts, and will<br />
attempt to shout down anyone who speaks them in public.  There have<br />
been several occasions on which leading psychometricians have had<br />
their books canceled or withdrawn by publishers who found the actual<br />
scientific evidence about IQ so appalling that they refused to print<br />
it.</p>
<p>Unfortunately, denial of the facts doesn&#8217;t make them go away.  Far from<br />
being meaningless, IQ may be the single most important statistic about<br />
human beings, in the precise sense that differences in g probably drive<br />
individual and social outcomes more than any other single measurable<br />
attribute of human beings.</p>
<p>Mean IQ differences do not justify making assumptions about any individual.<br />
There are African black geniuses and Ashkenazic Jewish morons; humanity and<br />
ethics demand that we meet each individual human being as an individual,<br />
without prejudice.  At the same time, group differences have a significance<br />
too great to ignore.  In the U.S., blacks are 12% of the population but<br />
commit 50% of violent crimes; can anyone honestly think this is<br />
unconnected to the fact that they average 15 points of IQ lower than the<br />
general population?  That stupid people are more violent is a fact<br />
independent of skin color.</p>
<p>And that is actually a valuable hint about how to get beyond<br />
racism.  A black man with an IQ of 85 and a white man with an IQ of 85<br />
are about equally likely to have the character traits of poor impulse<br />
control and violent behavior associated with criminality &mdash; and<br />
both are far more likely to have them than a white or black man with<br />
an IQ of 110.  If we could stop being afraid of IQ and face up to it,<br />
that would give us an objective standard that would banish racism per<br />
se.  IQ matters so much more than skin color that if we started paying<br />
serious attention to the former, we might be able to stop paying<br />
attention to the latter.</p>
<p>UPDATE: An excellent summary of science relating to g<br />
is <a href='http://psycprints.ecs.soton.ac.uk/archive/00000658/'>here</a></p>
