September 17, 2009

The Half-Full glass

Scandinavians have been keeping careful records on themselves for many generations, which has been a boon to social scientists. Here's a new paper on IQ and Family Background that uses IQ data from the IQ test given to Swedish conscripts. Sample sizes are ample (over 20,000 father-son pairs, and hundreds of thousands of brother pairs):
We use a large representative sample of Swedish men to examine both intergenerational [father-son] and sibling [brother] correlations in IQ. Since siblings share both parental factors and neighbourhood influences, the sibling correlation is a broader measure of the importance of family background than the intergenerational correlation. We use IQ data from the Swedish military enlistment tests. The correlation in IQ between fathers (born 1951-1956) and sons (born 1966-1980) is estimated to 0.347. The corresponding estimate for brothers (born 1951-1968) is 0.473, suggesting that family background explains approximately 50% of a person’s IQ. Estimating sibling correlations in IQ we thus find that family background has a substantially larger impact on IQ than has been indicated by previous studies examining only intergenerational correlations in IQ. ...

What is it then that brothers share and is important for their IQ but is uncorrelated with their father’s IQ? An obvious candidate is the mother’s IQ.

Although spouses’ IQ are most likely positively correlated and thus partly capture the same background factors, the combination of father’s and mother’s IQ is likely to raise the explanatory power in an intergenerational equation. Indeed, in a summary of previous estimates based on small and non-representative samples, Bowles & Gintis (2002) report the highest correlation from a study that applies the average of the two parents’ IQ. We doubt though that simply adding mother’s IQ would bring the explanatory power close to what the sibling similarity suggests. For example, attempts to account for the sibling similarity in long-run earnings by means of the education of both parents do not appear to capture much of the sibling similarity (Bj√∂rklund et al. 2008). We hypothesize that very detailed information about parental aspirations, attitudes and parenting practices is needed to account for the large gap between what sibling studies and intergenerational studies suggest about the role of family background factors.

Of course, peer groups are also likely to play an environmental role as well as parents. Brothers are part of the peer group because they tend to interact a lot with each other, especially if they are close in age. Brothers born within 5 years of each other have IQs that correlate at about the 0.50 level, versus about 0.44 for brothers born more than five years apart. The researchers call that difference "marginal," and assert "permanent family and community characteristics are more likely determinants," but they seem pretty interesting to me since they are one factor we can tell (lacking maternal IQ scores) that isn't genetic.

Keep in mind that the higher correlation among brothers closer in age is not just measuring the influence of brothers on each other but of broader environments. Two brothers who are only two years apart are more likely to have, say, both been taken care of by the same grandmother, played with the same cousin, gone to the same school, watched the same TV shows, were supported by similar levels of parental income, and so forth than two brothers who are eight years apart.

Brothers born on the same day (i.e., twins) correlate at 0.65, with that made up of a mixture of identical and fraternal twins.

My published articles are archived at iSteve.com -- Steve Sailer

22 comments:

TH said...

What's the correlation in IQ between mothers and children? I've seen it suggested that it is higher than that between fathers and children, is that correct?

Ady Pose said...

So another study that leaves us none the wiser.

Anonymous said...

When I was in college in the mid-80's, a sociology professor gave my class the Standford-Binet IQ test, apparently in some effort to make a point, which I've since forgotten. At any rate, when I reported my score to my older sister, she told me that not only was my score identical to hers (132), but that hers (ours) was identical to our father's, who took his test in the military in the 1950's. Just an anecdote.

Lennie Small said...

So how does this mesh with some claims that IQ is upto 70% genetic?

What is the most reliable figure for the genetic component of IQ based upon the most convincing body of scientific evidence?

Anonymous said...

"The researchers call that difference "marginal," and assert "permanent family and community characteristics are more likely determinants," but they seem pretty interesting to me since they are one factor we can tell (lacking maternal IQ scores) that isn't genetic."

But you can't rule out it being to a substantial degree biological. Siblings who occupied the same womb 5 years apart may not have had the same biological environment to develop in, as the mother is more than 5 years older at the birth of the latter child and I doubt that aging has no effect on the quality of the womb environment.

Anonymous said...

Maybe brothers born only a few years apart share a more similar womb which accounts for their greater similarity in intelligence.

PD said...

It may not be entirely accurate to attribute the dirrerence in correllation between the IQs of brother close in age to those further apart in age soley to environmental factors such as peer groups. A brother much younger in age will be born to an older pair of parents who may not be as fit, as far as reproducing goes, as they were 5 or more years earlier. Numerous studies have shown that children born to older mothers and fathers are at a higher risk of a number of serious neurological problems. The ones who escape these obvious problems might still not come out of the oven as perfectly baked as their younger siblings and might have a lower IQ as a result of that.

PD said...

I might have mis-typed. I meant the younger siblings might not come out of the oven as well baked as their older siblings.

James said...

Ah yes, The Half Full Glass, my favorite Bergman movie.

Anonymous said...

"What's the correlation in IQ between mothers and children? I've seen it suggested that it is higher than that between fathers and children, is that correct?"

I'd like to know this too.

tanabear said...

"What's the correlation in IQ between mothers and children? I've seen it suggested that it is higher than that between fathers and children, is that correct?"

I just found this in a book called "Intelligence" by Nathan Brody.

"They also obtained IQ data for mothers and children in their sample. They found that mother's IQ predicted child's IQ controlling for differences in their measure of home environment. The partial correlation between the mother's IQ(averaged over two measures, the Raven and the Peabody) and the child's IQ was .33."

Parsi said...

Even assuming a persistent marriage, brothers may not share the same biological father. I would guess that the larger the gap between their birth dates, the greater the chance that different biological fathers were responsible. You wouldn't expect the effect to be large (even in Sweden in the 1960s and 1970s) but it may be contributing to the increasing IQ divergence as the gap between dates of birth widens.

Anonymous said...

So the parent-child IQ correlation is quite weak?

I'm surprised.

tanabear said...

"Higgins et al. (1962) collected data on 1016 mothers, 1016 father, and 2039 children. They were selected from a large pool of relatives of 300 patients of the Minnesota State School and Hospital for the mentally retarded. IQ's were collected for as many persons as possible, using the testing programs in various school systems. If IQ-Values of both parents and of at least one child were known, then the family was included. The father child correlation found was .43 +_ .02. the mother-child correlation .45 +_ .02"
Correlation between Raven Progressive Matrices test scores of fathers and sons

"This study compared adoptive children and matched, biological children to estimate the genetic and environmental effect of years of mothers' and fathers' education on children's verbal intelligence (VIQ), as assessed by knowledge of vocabulary words....The mother-child and father-child correlations in biological families were .41 and .36, respectively, vs .16 and .18 in adoptive families."
Parental education and child's verbal IQ in adoptive and biological families in the National Longitudinal Study of Adolescent Health

Anonymous said...

TH: What's the correlation in IQ between mothers and children? I've seen it suggested that it is higher than that between fathers and children, is that correct?

Are there any modern studies which use DNA to rule out cuckoldry and the cuckoo's egg?

[Although I wouldn't doubt the fundamental premise - I have long felt that children inherit their smarts from their mothers and their looks from their fathers - as counterintuitive as that might seem.]

Anonymous said...

Numerous studies have shown that children born to older... fathers are at a higher risk of a number of serious neurological problems. The ones who escape these obvious problems might still not come out of the oven as perfectly baked as their younger siblings and might have a lower IQ as a result of that.

Yeah, but there's a flip side to that coin:

Leading geneticist Steve Jones says human evolution is over
Julia Belluz
October 7, 2008
timesonline.co.uk

Human evolution is grinding to a halt because of a shortage of older fathers in the West, according to a leading genetics expert....

tanabear said...

"So the parent-child IQ correlation is quite weak?"

"People who share genes tend to have similar IQs. The correlation between parent and child is 0.42. If you average both parents’ IQ scores, then the correlation rises to 0.72, indicating that children tend toward the average intelligence of their two parents."
What's Going on in There? : How the Brain and Mind Develop in the First Five Years of Life by Lise Eliot, Ph.D.

jody said...

steve, i'm surprised no post about norman borlaug, the son of norwegian immigrants to the US, and the person perhaps most directly responsible for why there are now 4 billion third world people on earth.

http://en.wikipedia.org/wiki/Norman_Borlaug

Too Tall Jones said...

[b]steve, i'm surprised no post about norman borlaug, the son of norwegian immigrants to the US, and the person perhaps most directly responsible for why there are now 4 billion third world people on earth.

http://en.wikipedia.org/wiki/Norman_Borlaug[/b]

Ironically, Borlaug's "Green revolution" agricultural research has been widely criticized by liberals and environmentalists.

mark said...

I recall a study out of Australia in the Lancet a few years back in which they stated that they thought that IQ is on the X chromosome meaning that the IQ of a boy is determined more or less solely by the mother (who contributes the X, the father contributing the Y).

The author of the article also suggested this might explain the wider distribution in male IQ as the female IQ is balanced out a bit by the second X chromosome.

The popular press picked this study up with the them: men, if you want smart sons, pick a smart wife.

If true this might explain why there are not more famous father-son dynasties in arts/science - there are some but not as many as you would expect.

It would be interesting to see if the correlation in IQ between a mother and son was greater than that between mother and daughter and whether the correlation between a father and daughter is greater than father-son.

Anonymous said...

'Numerous studies have shown that children born to older... fathers are at a higher risk of a number of serious neurological problems. The ones who escape these obvious problems might still not come out of the oven as perfectly baked as their younger siblings and might have a lower IQ as a result of that. '

Men should freeze their sperm. Problem Solved.

Half Sigma said...

My own investigations into earnings and IQ show that earnings have a much weaker correlation with IQ or anything else than what most HBDers think.

I would expect parents' IQ to do a much better job of predicting children's IQ then I would expect it to predict their earnings.