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Saturday, April 02, 2011

Earnings effects of personality, education and IQ for the gifted

Thanks to a reader for pointing me to this recent paper by Heckman and collaborators, which makes use of data from the Terman study of gifted individuals (minimum IQ of 135 on the Stanford-Binet).

Of the personality factors, Conscientiousness and Extraversion had the largest (positive) effect on lifetime earnings: the most conscientious or extraverted individuals earned, on average, about 50% more than the least (see figures below). See here for more on Big 5 personality factors and a link to a personality test.

The Effects of Education, Personality, and IQ on Earnings of High-Ability Men

This paper estimates the internal rate of return (IRR) to education for men and women of the Terman sample, a 70-year long prospective cohort study of high-ability individuals. The Terman data is unique in that it not only provides full working-life earnings histories of the participants, but it also includes detailed profiles of each subject, including IQ and measures of latent personality traits. Having information on latent personality traits is significant as it allows us to measure the importance of personality on educational attainment and lifetime earnings.

Our analysis addresses two problems of the literature on returns to education: First, we establish causality of the treatment effect of education on earnings by implementing generalized matching on a full set of observable individual characteristics and unobserved personality traits. Second, since we observe lifetime earnings data, our estimates of the IRR are direct and do not depend on the assumptions that are usually made in order to justify the interpretation of regression coefficients as rates of return.

For the males, the returns to education beyond high school are sizeable. For example, the IRR for obtaining a bachelor's degree over a high school diploma is 11.1%, and for a doctoral degree over a bachelor's degree it is 6.7%. These results are unique because they highlight the returns to high-ability and high-education individuals, who are not well-represented in regular data sets.

Our results highlight the importance of personality and intelligence on our outcome variables. We find that personality traits similar to the Big Five personality traits are significant factors that help determine educational attainment and lifetime earnings. Even holding the level of education constant, measures of personality traits have significant effects on earnings. Similarly, IQ is rewarded in the labor market, independently of education. Most of the effect of personality and IQ on life-time earnings arise late in life, during the prime working years. Therefore, estimates from samples with shorter durations underestimate the treatment effects.

Here are a couple of interesting excerpts from the paper:

... Our third contribution is to show how the effect of personality on earnings varies through-out the men’s working lives. We find that without access to long follow-up data, the estimated effect would be understated. Note that even though the Terman sample has a restricted range of IQ, there is substantial variation in personality. In fact, the Terman men do not differ from the general population in terms of personality.

... note that even when controlling for rich background variables, IQ maintains a statistically significant effect on lifetime earnings. Even though the effect is slightly diminished from the un-controlled association of the first column, it is still sizable. Malcolm Gladwell claims rather generally in his book Outliers that for the Terman men, IQ did not matter once family background and other observable personal characteristics were taken into account. While we do not want to argue that IQ has a larger role for the difference between 50 and 100, for example, than for the difference between 150 and 200, we do want to point out that even at the high end of the ability distribution, IQ has meaningful consequences. [The syntax of this last sentence is strange. Presumably the impact of IQ variation from 50 to 100 (from severely handicapped to average) is larger than for 150 to 200, even though their results show a significant effect even in the very high range.]

Below are some nice figures (click for larger versions). Note the personality factor distribution among Termites was similar to that of the overall population, whereas the IQ range was restricted due to selection. Typical lifetime earnings for this group of exceptionally able men ranged from $2 to $3 million in 2008 dollars.

Compare the bottom right IQ graph with SMPY results which show the impact of ability (SAT-M measured before age 13) on publication and patent rates. Ability in the SMPY graph varies between 99th and 99.99th percentile in quartiles Q1-Q4. The variation in IQ between the bottom and top deciles of the Terman study covers a similar range. The Terman super-smarties (i.e., +4 SD) only earned slightly more (say, 15-20% over a lifetime) than the ordinary smarties (i.e., +2.5 SD), but the probability of earning a patent (SMPY) went up by about 4x over the corresponding ability range.

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