Beliavsky wrote:

http://www.eugenics.net/papers/murray.html
IQ Will Put You In Your Place
By Charles Murray

From the Sunday Times, UK, May 25 1997.

An arcane debate about statistical methods ensued.

Ummm, that was not an "arcane" debate. That was
Methodology 101.

Then several
American academics began using a powerful, simple way of testing who
was right: instead of comparing individual children from different
households, they compared sibling pairs with different IQs. How would
brothers and sisters who were nurtured by the same parents, grew up in
the same household and lived in the same neighbourhood, but had
markedly different IQs, get on in life?

This throws up red flags right away. Siblings tend
to have similar IQs. When there are marked IQ differences
among siblings, right away it raises the issue of whether
there was something else going on along with the IQ differences.

A very large database in the United States contains information about
several thousand sibling pairs who have been followed since 1979. To
make the analysis as unambiguous as possible, I have limited my sample
to brothers and sisters whose parents are in the top 75 per cent of
American earners, with a family income in 1978 averaging £40,000 (in
today's money).

? First of all, this commits a major methodological
flaw of assuming your conclusion in drawing your sample. Bad,
bad researcher!

Families living in poverty, or even close to it, have been excluded.
The parents in my sample also stayed together for at least the first
seven years of the younger sibling's life.

Again, skewing the sample based on assuming the
anticipated results hold true. You're supposed to *test*
these things, not build them into your research design..

Each pair consists of one sibling with an IQ in the normal range of
90-110 ,a range that includes 50% of the population. I will call this
group the normals. The second sibling in each pair had an IQ either
higher than 110, putting him in the top quartile of intelligence (the
bright) or lower than 90, putting him in the bottom quartile (the
dull). These constraints produced a sample of 710 pairs.

It would be very interesting to see the profile of
the groups, at this point. I suspect we'd see some interesting
anomalies.

How much difference did IQ make? Earned income is a good place to
begin. In 1993, when we took our most recent look at them, members of
the sample were aged 28-36. That year, the bright siblings earned
almost double the average of the dull: £22,400 compared to £11,800.
The normals were in the middle, averaging £16,800.

And did they take birth order into effect? Quite a
few studies now seem to show sizeable birth order effects
on earnings, type of occupation, and risk tolerance (along
with small IQ differences). Clearly a confound, and one
that might explain a decent chunk of the results independently
of IQ.
Put together birth order issues and the odds that
a child with normal or above IQ parents and siblings would
have low IQ without any other disability that might also
affect earnings or success, and you've got some holes you
could drive a truck through.

Best wishes,
Ericka