"IQ will put you in your place" Comment

The above is the title of a paper by Charles Murray, which purported to show that IQ matters when it comes to income independent of upbringing and environmental factors.

The famed recent Zagorsky study didnt deny this, so this has nothing to do with that (apart from being a different more superior form of controlling for environmental factors). But nonetheless if youre interested in the zagorsky study click here (and read the comments for a link too)

In Murray's study, he, rather ingeniously, controlled for environmental effects by looking only at people within the same family. You can google it for the text or just click here for it. (Do read it or you wont understand what Im saying).

What I would like to point out here is this study underestimates the differentials (not earthshattering but what the heck).

The reason is, if One of the siblings had an IQ in the normal range, the sibling outside the normal range would tend to be close to the higher or lower 'normal range' cutoff . This is simply because there is less variation in a family in Iq terms than at the population at large. (I believe Daniel Seligman put the family S.D at 12, while a random choosing of two individuals in the population at large would show an S.D of 17 points).

Now the Earnings figure for the normal range would tend to be accurate, as it is presumably the average of siblings with an IQ closer to 110 and of siblings close to 90 (the two cutoffs for the normal range). But the earnings for the LOWER and HIGHER IQ comparision group will be overstated and understated respectively because the sample individuals used will tend to be closer to the 90 and 110 cutoff because we are dealing with families, not random people.

Just a quibble, but one Ive wanted to scratch for a long time.