New research on birth order

3 Jan

When I teach developmental psychology, I find that students are fascinated by the subject of birth order. Research over the years has yielded mixed results. I admit having been impressed by Sulloway’s fascinating book Born to Rebel. But I know that it is difficult to tease out the effects of birth order on behavior from the many factors affecting development. Steven Pinker says that he once believed in birth order effects, but now rejects them.

A study in the most recent issue of the journal Intelligence uses a novel research approach. The author looks at adoption cases where the children are not related either to each other or adopted parents. Here is the abstract:

“This study uses data on fully adopted sibling groups to test whether the explanation for the consistently observed negative effects of birth order are physiological or social in origin. Swedish administrative register data is used to construct full sibling data for cohorts born 1960–1982. Using a within-family comparison approach, I compare adopted siblings of different adopted birth order to one another to see whether birth order amongst adopted children (N = 6968) is associated with educational attainment by age 30, and the likelihood of having entered tertiary education by age 30. These same within-family comparison analyses are also performed on siblings in fully biologically related sibling groups (N = 1,588,401). I find that there is a negative relationship between adopted birth order and both educational attainment and the likelihood of entering tertiary education in fully adopted sibling sets. These findings strongly suggest that differences in educational attainment by birth order are driven by intrafamily social dynamics. I also conduct additional analyses in fully adopted sibling groups where age order and adoption order are reversed to test whether there is evidence for tutoring by siblings. These results do not indicate clear support for any tutoring effect.”


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