Don't believe your big siblings when they tell you they're naturally smarter

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.

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I am a youngest child of three, and I have to say I do believe in birth order effects. However, I feel as though they are entirely based on the environment of the family, and that under the right circumstances they will probably disappear once that family environment is disturbed (like going off to college, or older siblings doing so.) What's more, I think that IQ is the wrong measure; I think that there are differences in school acheivement, but they are based not on "intellegence" per se, but rather learning styles that relate to school. I think that older children are more responsible (because they have to be) than the younger ones, and they also then become more disciplined. So, the school environment suits them well.

I'm sure I'm basing my hypothesis on my family as well as the other families I know well. In my family, I was always an underacheiver in school--especially compared to my older sisters--until the end of high school. Once I got to college, I excelled academically, even more than they had. I also became responsible in other areas of my life.

In talking about birth order effects with my oldest sister a few weeks ago, she brought up another interesting observation. She noticed that in the families she's familiar with (including our own), older siblings are better at memorizing, whereas younger siblings usually can't memorize something unless they know everything there is to know about the underlying concept, or WHY they have to memorize. This could go back to the hypothesis that older siblings are responsible and intrinsically motivated to acheive...younger siblings might need more in order to learn. I think this difference is also highlighted in comparing elementary/middle/high school education to college education. Usually in college they hold you accountable for the underlying concept; hence the stubborn younger siblings will be able to use that strength.

If I were to do one of these studies I would measure school acheivement but compare it to some alternative types of learning and problem solving (I'm not aware of the literature enough to figure out what these would be) in order to figure out where the younger siblings excel. Then I would also look to see if any of the birth order effects wore off in adulthood.

I still think there is some effect of birth order. I don't think it's valid to control for mother's age at birth, because that is of course where the IQ effect comes from, the mother's age!

I guess there's a difference between applied psychology and basic psychology. Basic psychologists want to know what the real causes of high IQ are. Applied psychologists can stick with correlations and don't really care about third variables. The fact remains that if you are looking to hire two people alike in all qualifications other than birth order, you should hire the one with better birth order. Unless his mother was very old when he was born =)

By James Gambrell (not verified) on 08 Feb 2006 #permalink

If the IQ effect comes from the mother's age, isn't that different from birth order? Then it's not a birth order effect, it's a mother's age effect.

Does this mean your kids will be smarter if you have them at a younger age? And that the trend for having children later is not doing them any favors?

Jennifer -- these are all correlations, not causal relationships. This study can't tell us that that early maternity causes lower ability. I'd say it's more likely that early maternity is a proxy for economic status: mothers who have babies earlier tend to be a lower economic level, since they've had less opportunity for education, and have less work experience.

Economic status, in turn, is a predictor of IQ. But does low economic status cause low IQ, or is it the other way around?

I thought there was an established effect where IQ decreases the older the mother, with an especially pronounced drop for especially old mothers. I think it is primarily due to the incidence of mental retardation resulting from chromosomal abnormalities like down syndrome which rises sharply for mothers over 35. I'll try to find an article. Here is a chart from this professor's website (

mother's age = risk of mental retardation (defined as IQ below 70)
16-20 = 1 in 2,000
21-25 = 1 in 1,500
26-30 = 1 in 1,000
31-35 = 1 in 750
36-44 = 1 in 37
45-up = 1 in 12

This effect actually serves to counter the economic effect of low-IQ mothers belonging to low economic classes but having babies earlier.

By James Gambrell (not verified) on 09 Feb 2006 #permalink


The oldest mothers in this study were 21 years old in 1979. The data goes up to 1998, when they would have been 40. The youngest children measured were 7 years old, so the oldest their mothers could have been at childbirth was 33. Looking at your table, it appears that the most significant mental retardation effects occur at older maternal ages. I think that explains why we could have these two apparently contradictory findings.

"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."

This doesn't make any sense to me. You wrote:

"women who have their children earlier tend to be have lower economic status"

And the data seems to show that women who have their children earlier (e.g. first born) tend to have smarter children, so don't it follow that poorer - not richer - parents are more likely to have smart kids?

"And the data seems to show that women who have their children earlier (e.g. first born) tend to have smarter children, so don't it follow that poorer - not richer - parents are more likely to have smart kids?"

Adjusted for birth age ?

I once red (I don't remember where anymore), that parents with the same age (per ex. mother 26 and father 26), tend to have smarter children!

Is this a valid theory still?

By Jean-Pierre (not verified) on 23 Feb 2006 #permalink