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Post-hoc analyses related to birth order

Posted by JJMcGrath on 21 Apr 2009 at 23:56 GMT

The issue of birth order is certainly worth considering in research related to the impact of paternal age. In the complex matrix of highly inter-correlated factors like maternal and paternal age, sibship size, and socioeconomic status, sibling order could also be contributing to neurocognitive outcomes.

With respect to our results, on first principles, it is difficult to conceive of how factors related to sibling order could operate in one direction for the offspring of older mothers (who do better on IQ tests at age 7), versus the offspring of older fathers (who do worse on these same tests).

With respect to the literature linking birth order and IQ, the findings are inconsistent and plagued with methodological issues. Recent reviews [1, 2] have highlighted how cross-sectional studies (based on offspring of different families) can be misleading. These studies show that family size is associated with child IQ, and that parents with lower IQ tend to have larger families. Studies that have examined within-family correlations between IQ and birth order have generally not found a significant relationship [3-7].

For the sample analysed in the PLoS Medicine paper, information on birth order was missing in about one third of the cohort members. Birth order, which was highly skewed, ranging from zero (i.e. first born) to 22! In order to reduce the influence of infrequent but very large families, we restricted the pos-hoc analyses to the cohort members who were third born or less (n = 7,784). In Model 2, the association between maternal age and neurocognitive measures remained significant except for two measures (Bayley Mental at 8mo, and Graham Ernhart at age 4), and for paternal age, three measures remained significant while the findings with respect to Bayley Mental score at 8 months, and WISC and WRAT Reading scores at 7 years became non-significant (perhaps due to loss of power).

While we did not hypothesize that birth order was a major factor influencing neurocogntive outcomes, the results of this post-hoc analysis suggests that birth order may warrant closer scrutiny in future studies.

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2. Wichman AL, Rodgers JL, MacCallum RC: A multilevel approach to the relationship between birth order and intelligence. Pers Soc Psychol Bull 2006; 32(1): 117-27.
3. Bjerkedal T, Kristensen P, Skjeret GA, Brevik JI: Intelligence and birth order among young Norwegian men (conscripts) analyzed within and between families. Intelligence 2007; 35: 503-514.
4. Kristensen P, Bjerkedal T: Explaining the relation between birth order and intelligence. Science 2007; 316(5832): 1717.
5. Neligan GA, Prudham D: Family factors affecting child development. Arch Dis Child 1976; 51(11): 853-8.
6. Sulloway FJ: Psychology. Birth order and intelligence. Science 2007; 316(5832): 1711-2.
7. Zajonc RB, Sulloway FJ: The confluence model: birth order as a within-family or between-family dynamic? Pers Soc Psychol Bull 2007; 33(9): 1187-94.

No competing interests declared.