OK, here's another study I'm mulling over. Courtiol et al (2009) have just reported an experiment on cooperation in college students as a function of birth order. Their measure of cooperation is an objective one, taken from the results of a two- person game. The game provides numerical values for trust and reciprocity, determined by how much money each player sends or returns to his partner. Although birth order studies are infested with methodological problems, this design, as far as I can see, successfully avoids them.
The history of claims for birth order effects is not a happy one (e.g. see Judith Rich Harris' "Four Essays on Birth Order" (2004) at http://xchar.home.att.net/tna/birth-order/index.htm and also her more recent review in "No Two Alike" (2006)--the chapter headed 'Birth Order and Other Environmental Differences Within the Family"). So I paid attention when Courtiol et al reported positive effects of birth order on both trust and reciprocity. But here's the catch. They provided a complex statistical analysis (to me, anyway) but their analysis depends on a curious grouping of birth order: first-borns comprised one group, and later-borns the other. But the later-born group also included only children (without siblings). On logical grounds, one would think that only children belong in the first-born category instead. Their justification for doing this was inspection of the data. For trust: "Means of x [their monetary datum] for middleborn, lastborn and only children appeared much closer to each other than to the mean of x for firstborns (Table 2); these three categories were therefore pooled." For reciprocity: "Only children and laterborns were pooled because their average amounts sent (y) were closer to each other than to the average amount sent by firstborns (Table 2)." My own inspection of their data suggests that without this post- hoc categorization, they would not have been able to report significant results. Is their move kosher, or do we have a case of data-massaging here? _Science_ has a news item on the study at http://tinyurl.com/ylc4l34 It does not mention the peculiar definition of "later-borns". Stephen Courtiol, A. Raymond, M. and Faurie, C. (2009). Birth order affects behaviour in the investment game: Firstborns are less trustful and reciprocate less. Animal Behaviour, 78, 1405-1411. ----------------------------------------------------------------- Stephen L. Black, Ph.D. Professor of Psychology, Emeritus Bishop's University e-mail: [email protected] 2600 College St. Sherbrooke QC J1M 1Z7 Canada ----------------------------------------------------------------------- --- To make changes to your subscription contact: Bill Southerly ([email protected])
