[Paleopsych] Economist: The evolution of intelligence: Natural genius?
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Fri Jun 3 19:37:39 UTC 2005
The evolution of intelligence: Natural genius?
The high intelligence of Ashkenazi Jews may be a result of their persecuted
THE idea that some ethnic groups may, on average, be more intelligent than
others is one of those hypotheses that dare not speak its name. But Gregory
Cochran, a noted scientific iconoclast, is prepared to say it anyway. He is
that rare bird, a scientist who works independently of any institution. He
helped popularise the idea that some diseases not previously thought to have a
bacterial cause were actually infections, which ruffled many scientific
feathers when it was first suggested. And more controversially still, he has
suggested that homosexuality is caused by an infection.
Even he, however, might tremble at the thought of what he is about to do.
Together with Jason Hardy and Henry Harpending, of the University of Utah, he
is publishing, in a forthcoming edition of the Journal of Biosocial Science, a
paper which not only suggests that one group of humanity is more intelligent
than the others, but explains the process that has brought this about. The
group in question are Ashkenazi Jews. The process is natural selection.
History before science
Ashkenazim generally do well in IQ tests, scoring 12-15 points above the mean
value of 100, and have contributed disproportionately to the intellectual and
cultural life of the West, as the careers of Freud, Einstein and Mahler,
pictured above, affirm. They also suffer more often than most people from a
number of nasty genetic diseases, such as Tay-Sachs and breast cancer. These
facts, however, have previously been thought unrelated. The former has been put
down to social effects, such as a strong tradition of valuing education. The
latter was seen as a consequence of genetic isolation. Even now, Ashkenazim
tend to marry among themselves. In the past they did so almost exclusively.
Dr Cochran, however, suspects that the intelligence and the diseases are
intimately linked. His argument is that the unusual history of the Ashkenazim
has subjected them to unique evolutionary pressures that have resulted in this
paradoxical state of affairs.
Ashkenazi history begins with the Jewish rebellion against Roman rule in the
first century AD. When this was crushed, Jewish refugees fled in all
directions. The descendants of those who fled to Europe became known as
In the Middle Ages, European Jews were subjected to legal discrimination, one
effect of which was to drive them into money-related professions such as
banking and tax farming which were often disdained by, or forbidden to,
Christians. This, along with the low level of intermarriage with their gentile
neighbours (which modern genetic analysis confirms was the case), is Dr
Cochran's starting point.
He argues that the professions occupied by European Jews were all ones that put
a premium on intelligence. Of course, it is hard to prove that this
intelligence premium existed in the Middle Ages, but it is certainly true that
it exists in the modern versions of those occupations. Several studies have
shown that intelligence, as measured by IQ tests, is highly correlated with
income in jobs such as banking.
What can, however, be shown from the historical records is that European Jews
at the top of their professions in the Middle Ages raised more children to
adulthood than those at the bottom. Of course, that was true of successful
gentiles as well. But in the Middle Ages, success in Christian society tended
to be violently aristocratic (warfare and land), rather than peacefully
meritocratic (banking and trade).
Put these two things together-a correlation of intelligence and success, and a
correlation of success and fecundity-and you have circumstances that favour the
spread of genes that enhance intelligence. The questions are, do such genes
exist, and what are they if they do? Dr Cochran thinks they do exist, and that
they are exactly the genes that cause the inherited diseases which afflict
That small, reproductively isolated groups of people are susceptible to genetic
disease is well known. Constant mating with even distant relatives reduces
genetic diversity, and some disease genes will thus, randomly, become more
common. But the very randomness of this process means there should be no
discernible pattern about which disease genes increase in frequency. In the
case of Ashkenazim, Dr Cochran argues, this is not the case. Most of the dozen
or so disease genes that are common in them belong to one of two types: they
are involved either in the storage in nerve cells of special fats called
sphingolipids, which form part of the insulating outer sheaths that allow nerve
cells to transmit electrical signals, or in DNA repair. The former genes cause
neurological diseases, such as Tay-Sachs, Gaucher's and Niemann-Pick. The
latter cause cancer.
That does not look random. And what is even less random is that in several
cases the genes for particular diseases come in different varieties, each the
result of an independent original mutation. This really does suggest the
mutated genes are being preserved by natural selection. But it does not answer
the question of how evolution can favour genetic diseases. However, in certain
circumstances, evolution can.
West Africans, and people of West African descent, are susceptible to a disease
called sickle-cell anaemia that is virtually unknown elsewhere. The anaemia
develops in those whose red blood cells contain a particular type of
haemoglobin, the protein that carries oxygen. But the disease occurs only in
those who have two copies of the gene for the disease-causing haemoglobin (one
copy from each parent). Those who have only one copy have no symptoms. They
are, however, protected against malaria, one of the biggest killers in that
part of the world. Thus, the theory goes, the pressure to keep the sickle-cell
gene in the population because of its malaria-protective effects balances the
pressure to drive it out because of its anaemia-causing effects. It therefore
persists without becoming ubiquitous.
Dr Cochran argues that something similar happened to the Ashkenazim. Genes that
promote intelligence in an individual when present as a single copy create
disease when present as a double copy. His thesis is not as strong as the
sickle-cell/malaria theory, because he has not proved that any of his disease
genes do actually affect intelligence. But the area of operation of some of
them suggests that they might.
The sphingolipid-storage diseases, Tay-Sachs, Gaucher's and Niemann-Pick, all
involve extra growth and branching of the protuberances that connect nerve
cells together. Too much of this (as caused in those with double copies) is
clearly pathological. But it may be that those with single copies experience a
more limited, but still enhanced, protuberance growth. That would yield better
linkage between brain cells, and might thus lead to increased intelligence.
Indeed, in the case of Gaucher's disease, the only one of the three in which
people routinely live to adulthood, there is evidence that those with full
symptoms are more intelligent than the average. An Israeli clinic devoted to
treating people with Gaucher's has vastly more engineers, scientists,
accountants and lawyers on its books than would be expected by chance.
Why a failure of the DNA-repair system should boost intelligence is unclear-and
is, perhaps, the weakest part of the thesis, although evidence is emerging that
one of the genes in question is involved in regulating the early growth of the
brain. But the thesis also has a strong point: it makes a clear and testable
prediction. This is that people with a single copy of the gene for Tay-Sachs,
or that for Gaucher's, or that for Niemann-Pick should be more intelligent than
average. Dr Cochran and his colleagues predict they will be so by about five IQ
points. If that turns out to be the case, it will strengthen the idea that,
albeit unwillingly, Ashkenazi Jews have been part of an accidental experiment
in eugenics. It has brought them some advantages. But, like the deliberate
eugenics experiments of the 20th century, it has also exacted a terrible price.
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