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Stature and Status: Height, Ability, and Labor Market Outcomes« Back to Volume 23, Issue 2, June 2007 - Table of Contents One justification for growth hormone (GH) therapy to increase adult height is found in studies showing relationships between taller stature and higher occupational status and income. Researchers have emphasized factors such as self-esteem, social dominance, and discrimination as mediating this relationship. In secondary analyses of epidemiologic data sets from the US and the UK, Case and Paxson suggest an alternative explanation: on average, taller people earn more because they possessed stronger cognitive abilities throughout their lives. Investigators used data from the National Child Development Study (NCDS) which followed all children born in England, Scotland, and Wales in the week of March 3, 1958 from birth to age 42. Follow-up surveys on health and economic well-being were conducted at ages 7, 11, 16, 23, 33, and 42. The 1970 British Cohort Study (BCS) followed all individuals living in Great Britain born in the week of April 5, 1970 and conducted follow-up surveys at ages 5, 10, 16, 26, and 30. The Panel Study of Income Dynamics (PSID), a subset of the much larger US National Health Interview Survey, was used to analyze the association between height and occupational choice in the US. This data set provided important family background variable of parents’ education and a measure of whether respondents were poor in childhood. The PSID sample comprised men, ages between 25 and 65, observed during the 1988 to 1997 waves of the study. Case and Paxson obtained support for the hypothesis that superior cognitive ability (which covaries with the timing of the adolescent growth spurt and adult height) through a series of statistical analyses. In the first set, using the NCDS data, they demonstrated children whose fathers were of lower social class were shorter (on average, a full inch) throughout their lives than those whose fathers were professionals. Important for subsequent analyses, they demonstrated that men from lower social class households gained more height from age 16 to 33 than did men from higher social classes; this finding was interpreted as evidence of poorer health and nutritional factors among those from lower social strata. Because the timing of girls’ adolescent growth spurt is earlier than that for boys, the height differentials by social class at age 16 were nearly identical to those at age 7. Next, using data from the NCDS and BCS, investigators provided evidence for a large and statistically significant association between height and cognitive test scores for children ages 5 and 10 in the BCS and for children ages 7 and 11 in the NCDS. Results from multiple regression analyses demonstrated that an increase of one height SD at age 7 was associated with 0.1 SD increase in reading score at age 7, and in verbal language score at age 11. To put this in perspective, they noted that this effect was as large as that predicted by a 2 SD increase in (log) household income for these children. The authors considered this association as unlikely the consequence of taller children being afforded greater cognitive stimulation at school because other studies have shown that increased body length of 5- to 12-month-old infants is associated with measures of information processing speed. The next step in support of their model came from analyses showing a positive association between height and adult earnings; for both men and women in the BCS data set, each additional inch of height was associated with a 1% to 2% increase in earnings. However, once cognitive test scores obtained at much earlier in life were included in the analyses, results showed that adult height no longer accounted for differences in earnings. Adding information about parental family background, presence of low birth weight and maternal smoking during pregnancy further reduced the influence of height for men and women. Finally, the investigators used their data sets to revisit a study by Persico et al1 demonstrating that the association between height at age 16 and adult income is statistically significant whereas it is not so at age 33. Case and Paxson first replicated this analysis, then expanded upon it by including additional cognitive test scores at ages 7 and 11. Inclusion of test scores reduced the age 16 height premium by over 50%. The inclusion of additional family background variables to the regression model reduced the significance of height even further. The investigators reasoned that height at 16 years depends on the timing of the adolescent growth spurt, which depends on health and nutrition in infancy and childhood, so that those who were healthier and better nourished were taller at age 16. In the case of girls, however, negative associations were detected between change in height from 11 to 16 years and cognitive test scores (whereas these associations were positive for boys). This interaction with gender was explained by noting that girls initiate their adolescent growth spurt earlier than boys. By age 16, growth velocity has typically slowed markedly; in cases where it has not, negative (pre- and postnatal) health and nutritional factors are posited. These factors are also predicted to negatively affect cognitive development. The final set of analyses demonstrated that taller adults select into occupations that have higher cognitive skill requirements and lower physical skill demands. Editor’s CommentCase and Paxson’s study caused a predictable uproar.2,3 Public response is captured by the following quotes: “I find your hypothesis insulting, prejudicial, inflammatory and bigoted,” and “I want to thank you and your colleague for perpetuating the crusade against short people.” In response to the negative reactions to their publication, Case and Paxson’s response emphasized that a person genetically programmed to be 6 feet 4 inches who reaches only 6 feet 2 inches because of poor nutrition is not necessarily smarter than someone who with optimal nutrition has reached his full height potential of 5 feet 4 inches. Stephen Hall,2 author of “Size Matters”4 , reemphasized the point that many studies have shown that adult height is under the conjoint influences of genetic endowment and the environment, the latter encompassing prenatal environment, early postnatal nutrition, and even the family’s socioeconomic status. Even so, Case and Paxson could have avoided offering the inflammatory sound bite “on average, taller people earn more because they are smarter.” This study demonstrated substantial occupational and economic benefits of height in the US and UK labor markets. For both men and women, an increase of 4 inches height was associated with a 10% increase in earnings. Further, US males 6 feet 2 inches tall were 3% more likely to be an executive and 2% more likely to be a professional than a man 5 feet 10 inches. But more to the point, Case and Paxson convincingly demonstrated that height was only a proxy measure for an attribute of far greater importance in predicting occupational status and income: cognitive ability. This study is in line with reports from Barker’s group suggesting that pre- and early postnatal developmental factors exert lasting effects on health and psychosocial outcomes, including income and marriage status in adulthood.6,7 David E. Sandberg, PhD References - (linked to
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Volume 25 Number 1
June 2009
www.GGHjournal.com
ISSN 1932-9032
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