[Paleopsych] Economist: The evolution of intelligence: Natural genius?

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The evolution of intelligence: Natural genius?
http://www.economist.com/displaystory.cfm?story_id=4032638
5.6.2

The high intelligence of Ashkenazi Jews may be a result of their persecuted 
past

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 
Ashkenazim.

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 
Ashkenazi society.

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.