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Bafflement over birth-order effects

I noticed this report of a new study on birth order and IQ:

The eldest children in families tend to develop higher I.Q.’s than their siblings, researchers are reporting today, in a large study that could settle more than a half-century of scientific debate about the relationship between I.Q. and birth order.

The average difference in I.Q. was slight — three points higher in the eldest child than in the closest sibling — but significant, the researchers said. And they said the results made it clear that it was due to family dynamics, not to biological factors like prenatal environment.

Here’s more from Petter Kristensen, coauthor of the study:

In the Science paper we [Kristensen and Bjerkedal] compared IQ test results among male military draft boardees who had experienced the early loss of an elder sibling (sibling death in infancy or stillbirth) and those who had not this experience. In this way, we were able to identify men who had first rank in social terms but second or third rank in biological terms and men who had second social rank in the family but were third born. Altogether, these men counted 4627. In the comparison we included 236 683 men of birth order one to three who had not experienced the early loss of an elder sibling. We examined for a social rank or biological rank effect on IQ adjusting for other factors of potential influence. We found that increasing social rank was associated with decreasing IQ whereas biological rank had no effect once social rank had been accounted for. The size of the rank effect was approximately 2 IQ units per rank unit.

The Science study is supported by another paper with the same objectives, in press in Intelligence journal where we compared brothers within families. The results of this study was more or less identical with the Science paper results. Both studies included approximately a quarter of a million young men from approx. 175 000 families who had been tested between 1985 and 2004, and were based on individual linkage in national registers (most important, the Medical Birth Registry of Norway and the National Conscripts Service registry of draft board examinations).

The size of the effect of birth order on IQ is small, but highly significant on the group level. It is, however, hard to draw consequences on the individual level from this. In our comparison between brothers, we found that if two brothers had different scores, the probability would be 57% that the elder would have the higher score and 43% that the younger would have the higher score. This difference is not very predictive in my opinion.

As for the explanations: our research is based on register information, and it may be difficult to make inferences as to causes since have no data on details on parental personal resources and how the parents raise their children. However, we have sorted out the likelihood of some
alternative explanations, leaving interaction within the family as the most likely explanation. The alternative explanations that have been proposed are that this effect is an artefact or that the effect is biological (gestational). Both these possibilities have been weakened by our research, in my opinion. Family interaction as the remaining explanation means that the first born has the advantage of getting all the attention of the parents until the next child comes. Later, the parents’ resources have to be divided between two etc. Also the tutoring effect that is an element in one of the family interaction theories (Zajonc’s confluence model) may matter.

As the fourth child, I’m always interested in these things. The topic has been controversial, though. I’d always heard that firstborns had higher IQs; there’s a famous graph from 1973:


Then I read about this again in the book by Judith Rich Harris, where she mocked the birth-order studies and said the patterns had been debunked. Curious, I read a few articles on the topic, and it seemed like the pro-birth-order people and the anti-birth-order people were talking past each other, with the anti’s being very sure they were debunking the pro’s, but to me it wasn’t all so clear.

The issues are laid out in these discussions (from the journal American Psychologist) by Michalski and Shackleford, Armor, Zajonc, and Rogers et al.


  1. vasishth says:

    Cool. My elder brother is definitely more intelligent than me (he keeps telling me so all the time :-). Just kidding.

    I was wondering what the practical impact of a 3 point higher IQ level is, however. I recently read a very cool article in Psych Review, Deliberate Practice,

    title={{The role of deliberate practice in the acquisition of expert performance}},
    author={Ericsson, K.A. and Krampe, R.T. and Tesch-Romer, C.},
    journal={Psychological Review},

    which, if it correctly portrays performance, suggests that IQ per se might not get one very far. Are there any studies on what the effect size needs to be to have any practical significance?

    I would have rather concluded from these studies that birth order does not have any *practical* impact.

  2. Anonymous says:

    another paper taking an economic angle (and perhaps another identification strategy) here:

  3. John says:


    IIRC from my days as a psych major, IQ basically predicts school performance and that's about it. Wikipedia suggests otherwise (; however, I don't see much reported there to suggest that a 3-point difference will translate into anything meaningful.

  4. McCurry says:

    ….the concept & practice of "IQ Measurement" is still highly questionable 'science' — so drawing such broad conclusions from slight differences in "measured" IQ within a narrow sample is nonsense.

    But obviously, the popular news media loves this kind of social conjecture.

    At best, the inherent measurement-error in IQ testing is a couple of percentage points — negating the supposed scientific basis of this birth-order IQ study.

  5. p-ter says:

    I was wondering what the practical impact of a 3 point higher IQ level is, however.

    on the individual level, very little. however, a shift of 1/5th of an sd in the mean of a distribution is important when looking at a population. consider some IQ threshold above the mean (to get a college education, to become a doctor or scientists, etc): there should be an overrepresentation of first-borns, all other things being equal, and that over-representation should increase as the threshold increases. don't know if that's actually the case.

    IQ basically predicts school performance and that's about it.

    see here for references on other things:

  6. Denis says:

    IQ bashing is easy and fashionable. I suggest a very short but excellent introduction to intelligence testing:

    I. J. (2001). Intelligence: A very short introduction. Oxford: Oxford University Press.


  7. Andrew says:


    Easy and fashionable is good, right?

  8. The article says; " Altogether, these men counted 4627. In the comparison we included 236 683 men of birth order one to three who had not experienced the early loss of an elder sibling. "

    Shouldnt the comparison group be men who experience the early loss of a younger sibling and thus where still the eldest ? (form the size of the two groups I think they are comparing against men who didnt loose any siblings)

    Men who lose a older sibling compared to men who dont lose any siblings would in general come from less healthy families and so it seems unsuprising that they have lower IQs. Or am I missing something?

  9. Leon Edward says:

    first, re references to "IQ basically predicts school performance and that's about it."

    It does this yes , but it also transcribes into career performance and career development, communications especially with technical people.

    (In universities , the top of the class getting better starting salaries is no mistake! Employees know the value of higher IQ)

    But I'm sure they'll still higher your little brother or sister

    That Said..
    The sample size and 3 point conclusion gives no appreciable confidence level for the articles claims, sorry