In 1973, a massive study of almost 400,000 Dutch men appeared to confirm what anecdotal evidence and even some scientific research had led scholars to suspect: The first-born child in a family tends to be the most intelligent. The researchers, Lillian Belmont and Francis Marolla, found that within a given family size, earlier-born children tended to have slightly higher IQs than later-born children, even after controlling for social class. Their study pool was the entire population of 19-year-old men in the Netherlands.
Since then, researchers have developed all sorts of models to try to explain why it is that first-born children have higher IQs. But recently, a few studies have begun to question the methods Belmont and Marolla used to analyze the Dutch data. Their study was cross-sectional: they weren’t actually studying children from the same family; they simply took all the 19-year-olds and analyzed their IQ data in conjunction with information about their family and demographics.
But same-family research also suffers from problems. It’s difficult to compare the IQ of a 7-year-old to a 14-year-old: typically children are compared with other kids their same age, which by definition would preclude comparing siblings based on birth order. Even if you measure IQs of the children in a family as they pass an age milestone, that family’s economic status may have changed, so economic status becomes more difficult to control for.
A team led by Aaron Wichman has devised a method of data analysis that attempts to combine the advantages of cross-sectional research with same-family research. They looked at the children of women who were originally studied in the National Longitudinal Survey of Youth beginning in 1979: 6,283 women who had 10,918 children by 1998. These children had been given the Peabody Individual Achievement test at two-year intervals, and Wichman’s team looked at those scores at age 7 to 8, and again at age 13 to 14.
Their first analysis of the data found the familiar effect: test scores were higher for first-born children than for later-born children. But the team didn’t stop there: they performed a secondary analysis that took into account the mother’s age when her first child was born. While this analysis isn’t equivalent to controlling for economic status, it does serve as an approximation of that status, since women who have their children earlier tend to be have lower economic status. In this way, the researchers avoided the problem of assessing changing economic status as parents age. Here are the results for one of the tests, math achievement:
Once maternal age was controlled, the effect of birth order effectively vanished. There was one subject area where a significant correlation between birth order and intelligence remained: small differences in reading comprehension, but otherwise, they found no relationship between birth order and intelligence.
Wichman et al. argue that any of these small differences in intelligence which appear to be due to birth order differences are more likely to be due to differences between families: smarter, richer parents are more likely to have smart kids.
Wichman, A.L., Rodgers, J.L., & MacCallum, R.A. (2006). A multilevel approach to the relationship between birth order and intelligence. Personality and Social Psychology Bulletin, 31(1), 117-127.