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Science 22 June 2007:
Vol. 316. no. 5832, p. 1717
DOI: 10.1126/science.1141493
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Brevia

Explaining the Relation Between Birth Order and Intelligence

Petter Kristensen1,2* and Tor Bjerkedal3

Negative associations between birth order and intelligence level have been found in numerous studies. The explanation for this relation is not clear, and several hypotheses have been suggested. One family of hypotheses suggests that the relation is due to more-favorable family interaction and stimulation of low-birth-order children, whereas others claim that the effect is caused by prenatal gestational factors. We show that intelligence quotient (IQ) score levels among nearly 250,000 military conscripts were dependent on social rank in the family and not on birth order as such, providing support for a family interaction explanation.

1 National Institute of Occupational Health, N-0033 Oslo, Norway.
2 Section for Preventive Medicine and Epidemiology, University of Oslo, N-0318 Oslo, Norway.
3 Institute of Epidemiology, Norwegian Armed Forces Medical Services, N-0015 Oslo, Norway.

* To whom correspondence should be addressed. E-mail: petter.kristensen{at}stami.no

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