Greg Mankiw Hearts Genetic Determinism

by John Sides on August 28, 2009 · 18 comments

in Uncategorized

sat scores by income.jpg

Greg Mankiw posts the above graph (courtesy of Economix) and opines:

…kids from higher income families get higher average SAT scores. Of course! But so what? This fact tells us nothing about the causal impact of income on test scores. (Economix does not advance a causal interpretation, but nor does it warn readers against it.)
This graph is a good example of omitted variable bias, a statistical issue discussed in Chapter 2 of my favorite textbook. The key omitted variable here is parents’ IQ. Smart parents make more money and pass those good genes on to their offspring.

I’ll just leave aside problems with the concept of IQ and with the notion that genes communicate intelligence. (See Cosma Shalizi.)

But you might think Mankiw would at least nod his head in the direction of omitted variables that are correlated with income and with SAT scores, but have nothing to do with genes. Like, I dunno, the quality of primary and secondary education, which could—and let me go out on a limb here—be a bit more sucky in poor neighborhoods than rich neighborhoods. Or, maybe, and again, this is just crazy speculation, rich parents do more to prepare their children for standardized tests.

That’s what I would do if I were writing a blithe little blog post. You know, hedge my bets. If I wanted to work hard, I would type “adopted children sat scores family income” into Google, and then I would learn just how difficult it is to separate out the effects of genes and the environment, even when you look at adopted children:

Chapter 4…tries to estimate the effect of specific family characteristics on young children’s test scores. This is not easy. Hundreds of different family characteristics correlate with children’s test performance. Disentangling their effects is a statistical nightmare. Almost any family characteristic can also serve as a proxy for a child’s genes. We know, for example, that a mother’s genes affect her test scores and that her test scores affect her educational attainment. Thus when we compare children whose mothers finished college to children whose mothers only finished high school, the two groups’ vocabulary scores can differ for genetic as well as environmental reasons. Even when a child is adopted, moreover, the way the adoptive parents treat the child may depend on the child’s genes. Parents read more to children who seem to enjoy it, for example, and whether children enjoy being read to may well depend partly on their genes.
The best solution to such problems is to conduct experiments. In the 1970s, for example, the federal government conducted a series of “negative income tax” experiments that increased the cash income of randomly selected low-income families. These experiments did not last very long, the samples getting any given “treatment” were small, and the results were poorly reported, so it is hard to know exactly what happened. Short-term income increases did not have statistically reliable effects on low-income children’s test scores, but that does not mean their true effect was zero. As far as we know, these are the only randomized experiments that have altered families’ socioeconomic characteristics and then measured the effect on children’s test scores.
In theory, we can also separate the effects of parents’ socioeconomic status from the effects of their genes by studying adopted children. But because adoption agencies try to screen out “unsuitable” parents, the range of environments in adoptive homes is usually restricted. The adoptive samples for which we have data are also small. Thus while parental SES does not predict adopted children’s IQ scores as well as it predicts natural children’s IQ scores, the data on adopted children are not likely to persuade skeptics.

And if I truly wanted to sacrifice my blood, sweat, and tears for the Glorious Enterprise of Blogging, I would Google “adopted children sat scores socioeconomic status” and—hey, look!—the first hit is a potentially relevant article in some backwater journal called the “American Economic Review.” It’s called “The Nature and Nurture of Economic Outcomes.” Let’s see…

In this paper, I use data on adoptees to measure causal effects on children’s outcomes from being raised in a high-education or high-SES family. My key identifying assumption is the random assignment of adoptees to families. I find that being raised in a high-SES family (or in a high-income town) greatly increases the probability that a child will attend college and increases the selectivity of the college attended…In the NCDS data, the effect of the nurturing parent’s SES on the child’s college attendance is similar for adoptees and non-adoptees. In results reported here and in Sacerdote (2000), there is some evidence that the effect of family environment may be greater on educational attainment than for test scores. These findings support the notion that environment can be incredibly potent in determining children’s outcomes and that environment’s potency may vary with the outcome considered.

And then I might really hedge my bets.

UPDATE: A friend sends along this piece on the intergenerational transmission of wealth. The authors conclude:

The results are somewhat surprising: wealth, race and schooling are important to the inheritance of economic status, but IQ is not a major contributor and, as we have seen above, the genetic transmission of IQ is even less important.

UPDATE: More from Brad DeLong.

{ 16 comments }

That Donkey Benjamin August 28, 2009 at 2:52 pm

Restate Cosma’s criticism, in your own words, please.

LFC August 28, 2009 at 3:55 pm

I find it surprising, to use a mild word, that Mankiw, or anyone else, would write that “smart parents make more money and pass those good genes on to their offspring.”

Smart parents make more money? Not, IIRC, according to the study Inequality, published way back in the 1970s, in which Christopher Jencks et al. presented a good deal of evidence that there is only a weak correlation between “cognitive ability” (I believe that’s their phrase) and earnings.

Also, Mankiw is assuming that the SAT measures IQ. No one who knows anything about this has ever claimed that that is the case, as far as I’m aware.

LFC August 28, 2009 at 4:08 pm

B. DeLong, ‘off the top of his head’, writes that SAT scores have a “high but not perfect (.7) correlation with IQ.”
Assuming that’s right, my point still stands: the SAT is not designed to measure IQ and does not measure it. There may be a high correlation w/ IQ (assuming DeLong is correct), but that’s not the same thing.

illuminatus? August 28, 2009 at 4:10 pm

To discuss the issue as regards to nature versus nurture. Social conservatives hold that nature is innate, 100 %. Socialists hold everything can be changed by nurture 100 %.

Classic liberals such as Charles Murray believes that ability is some 75 % heritable but conclude that massive intervention could make a difference. He is however a pessimist. The last 50 years of progressive social school programs, the enormous amount of monies spent on inner city schools, the massive redistribution and social welfare programs instituted have resultate in what? Close to nothing! Murrays conclusion that it is wasted money and a total overhaul of the education and welfare system is necessary. He also concludes that the most important factor is cultural change among the lower socioeconomic classes.

Murray has disproven that more money makes any difference once and for all. Redistribution is an extremely inefficeint tool.

A sharp critic of Murray, the very left liberal leading cognitive psychologist Richard E Nesbit has written a book refuting Murray and hereditarians, read the review of his book “Intelligence and how to get it, Why schools and culture count” in New York Times article Get Smart

[Nesbitt} grants that I.Q. tests — which gauge both “fluid” intelligence (abstract reasoning skills) and “crystallized” intelligence (knowledge) — measure something real.

They also measure something important: even within the same family, higher-I.Q. children go on to make more money than their less-bright siblings.

Heritability of I.Q. is higher for upper-class families than for lower-class families, because lower-class families provide a wider range of cognitive environments, from terrible to pretty good.

Nesbitt found that heridatiblity of IQ for the children of the professional class is close to 80 %, the top 10 %, but he showed that the heriditabilyt for lower socieoeconomic groups children is less than 40 %. Murray is according to Nesbitt both right and wrong.

Nesbitt also shows that funding for inner city schools does not matter. He gives an example of a Kansas inner city school that built a world class campus, every student got a computer and unlimited resources. No change in results.

I believe myself in early interventions but are much more skeptic than Nesbitt. If we over 50 years have not been able to get results, how can we expect to overcome the educational establishments mistrust as to reforms and new methods, especially politicians and the school unions?

Still, there are limits even to Nisbett’s optimism. Social policy can get rid of ethnic I.Q. gaps, he thinks, but “the social-class gap” in I.Q. “is never going to be closed”

Murray and Nesbitt both bleive that culture counts, that a radical change in lower socioeconomics parents culture is necessary.

Geoff August 29, 2009 at 8:38 am

Social psychologist Richard Nisbett takes on the genes and intelligence argument in a recent book “Intelligence and How to Get It: Why Schools and Cultures Count.” You can tell by the title where he comes down on the debate.

eric August 29, 2009 at 8:50 am

Also, the Flynn effect shows that environment matters. Mankiw lacks the intellectual honesty gene.

William August 29, 2009 at 1:03 pm

“In a paper titled “The Nature and Nurture of Economic Outcomes,” the economist Bruce Sacerdote addressed the nature-nurture debate by taking a long-term quantitative look at the effects of parenting. He used three adoption studies, two American and one British, each of them containing in-depth data about the adopted children, their adoptive parents, and their biological parents. Sacerdote found that parents who adopt children are typically smarter, better educated, and more highly paid than the baby’s biological parents. But the adoptive parents’ advantages had little bearing on the child’s school performances. As also seen in the ECLS data, adopted children test relatively poorly in school; any in?uence the adoptive parents might exert is seemingly outweighed by the force of genetics. But, Sacerdote found, the parents were not powerless forever. By the time the adopted children became adults, they had veered sharply from the destiny that IQ alone might have predicted. Compared to similar children who were not put up for adoption, the adoptees were far more likely to attend college, to have a well-paid job, and to wait until they were out of their teens before getting married. It was the in?uence of the adoptive parents, Sacerdote concluded, that made the difference.”

Eric August 30, 2009 at 12:06 am

How can anybody take Mankiw seriously?

Josh August 30, 2009 at 2:12 am

Intelligence, like other behavioural traits is significantly heritable.

“Data from more than 8000 parent-offspring pairs, 25,000 sibling pairs, 10,000 twin pairs and adoption studies provide evidence that genetic factors play a substantial role in the variation of general intelligence, with heritability estimates ranging from 40 to 80%” –Burdick et al, Cognitive variation in DTNBP1 influence general cognitive ability. Human Molecular Genetics, 2006, Vol 15, No. 10.

“Heritability estimtes for intelligence quotient (IQ) range from 0.50 to 0.80. This makes IQ a suitable target for attempts to identify the specific genes involved.” Chorney et al, Role of the cholinergic muscarinic 2 receptor (CHRM@) gene in cognition. Molecular Psychiatry (2003) 8. 10-13.

“A substantial body of literature from twin, family and adoption studies documents significant genetic effects on human intelligence. Heritability estimates range from 40 to 80% and meta-analyses suggest an overall heritability of around 50%” Dick et al,

(2006) “Association of CHRM2 with IQ: Converging Evidence for Genes Influencing Intelligence.” Behavioral Genetics.

“Multivariate genetic analyses indicate that general intelligence is highly heritable, and that the overlap in the cognitive processes is twice as great as the overall phenotypic overlap, with genetic correlations averaging around .80.”

Plomin et al (2004) “A functional polymorphism in the succinate-semialdehyde dehydrogenase genes is associated with cognitive ability,” Molecular Psychology 9, 582-586.

Michael Stein August 30, 2009 at 2:17 am

“I’ll just leave aside problems with the concept of IQ and with the notion that genes communicate intelligence. (See Cosma Shalizi.)”

Unfortunately for Shalizi, the brains of more intelligent people are different in terms of cortical thickness and myelination (affects processing speed – Einstein had a larger number of glial cells which produces this). This is significantly heredible:

“It is clear that intelligence is at least partly genetically determined. This was supported by the discovery in 2001 that the volume of the brain’s grey matter, made up of “processor” cells, is heritable and correlates with certain elements of IQ (Nature Neuroscience, DOI: 10.1038/nn758). The amount of white matter, which provides the connections between these processors, has since been shown to be heritable too (Journal of Neuroscience, vol 26, p 10235).

Now it seems that the quality of these connections, which is governed by the integrity of the protective myelin sheath that encases them, is also largely genetic, and correlates with IQ.”

http://www.newscientist.com/article/mg20126993.300-highspeed-brains-are-in-the-genes.html

“The UCLA researchers took the study a step further by comparing the white matter architecture of identical twins, who share almost all their DNA, and fraternal twins, who share only half. Results showed that the quality of the white matter is highly genetically determined, although the influence of genetics varies by brain area. According to the findings, about 85 percent of the variation in white matter in the parietal lobe, which is involved in mathematics, logic, and visual-spatial skills, can be attributed to genetics. But only about 45 percent of the variation in the temporal lobe, which plays a central role in learning and memory, appears to be inherited.”

http://www.technologyreview.com/biomedicine/22333/

Michael Stein August 30, 2009 at 2:19 am

“I’ll just leave aside problems with the concept of IQ and with the notion that genes communicate intelligence. (See Cosma Shalizi.)”

Unfortunately for Shalizi, the brains of more intelligent people are different in terms of cortical thickness and myelination (affects processing speed – Einstein had a larger number of glial cells which produces this). This is significantly heredible:

“The UCLA researchers took the study a step further by comparing the white matter architecture of identical twins, who share almost all their DNA, and fraternal twins, who share only half. Results showed that the quality of the white matter is highly genetically determined, although the influence of genetics varies by brain area. According to the findings, about 85 percent of the variation in white matter in the parietal lobe, which is involved in mathematics, logic, and visual-spatial skills, can be attributed to genetics. But only about 45 percent of the variation in the temporal lobe, which plays a central role in learning and memory, appears to be inherited.”

(see New Scientist 9 March 2009 ‘High Speed Brains in the Genes’ and MIT Technology Review, March 24, 2009)

Murph August 30, 2009 at 11:52 pm

Asians from low income families score higher on the SAT than blacks from high income families.

http://en.wikipedia.org/wiki/File:1995-SAT-Income.png

Blacks from homes where at least one parent has a graduate degree score lower than whites and asians where the most educated parent has only a high school diploma.

http://www.swivel.com/data_sets/spreadsheet/1002854

Anon August 31, 2009 at 6:31 pm

Thanks for those lovely oranges, Murph! Next time, please bring some apples, since that’s what John’s talking about above.

LFC September 1, 2009 at 12:19 am

Thanks, Illuminatus, for pointing to the NYT review of Richard Nisbett; unfortunately the link doesn’t work (there’s an extra “http” thrown in). If one takes that out, the URL does work.

silviosilver September 1, 2009 at 7:12 am

The fact is too many don’t want hereditary to be true so they find any excuse they can to believe it’s not true. Unfortunately for them the evidence very much substantiates the hereditarian position. Furthermore, there is no reason to privilege environmentalism by assuming it is true and demanding hereditarians show it is false. The intellectually honest approach would be to start from a neutral position.

I believe that hereditarianism is true and that society is amassing terrible social costs by insisting that it is false. With respect to such social costs I have had this to say: The social costs incurred by acting on the assumption that heredity is true are outweighed by acting on the assumption that heredity is false, whether or not heredity is in fact true. If heredity is true, acting on the assumption that it is true is good. If heredity is false, acting on the assumption that it is true is less bad than heredity being true and acting on the assumption that it is false.

Environmentalists who find hereditarianism too ghastly to contemplate can then continue to insist that they disbelieve hereditarianism while advocating policies based on its truth as a form social insurance.

Josh September 1, 2009 at 6:05 pm

Nisbett’s book overlooks a startling number of studies which go against his environmentalist position.

http://psychology.uwo.ca/faculty/rushtonpdfs/Intelligence%20and%20How%20to%20Get%20It%20(Working%20Paper).pdf

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