Page 97 Adler’s fascinating theorizing on birth order has led to an almost
overwhelming amount of research. Yet, controlled studies of birth order effects are not only difficult to conduct, but often result in no effects at all.
Imagine the many variables that ought to be accounted for: the overall number, gender, and spacing of the siblings, and the events and timing of those events that occur in families (moves, divorce, death, disability, to name just a few). Few studies can include high enough numbers of participants and control for these many variables in a way that leads to meaningful results.
Critics have argued that for all these reasons, research can neither confirm nor deny Adler’s predictions about the impact of birth order position on individuals.
In 1996, Frank Sulloway published Born to Rebel: Birth Order, Family Dynamics and Creative Lives, in which he presented an evolutionary argument for birth order effects on personality. Siblings, he wrote, compete for an important and often scarce resource: parental affection and attention.
Children’s success in this competition reflects strategies that impact their personalities, and our birth order position predicts these strategic personality traits. Lending support to Adler’s theory, Sulloway proposed that first-borns are likely to be achievement-oriented, anxious, and conformist whereas later- borns tend to be more adventurous, open to experience, innovative, and
rejecting of the status quo. After all, they must find a way to earn their
parent’s love that’s different from their older sibling. So, “watch this, mom!”
is likely to be the later-born’s battle cry. Indeed, Sulloway’s historical analysis found that later-born scientists were much more likely to accept radical new theories when first proposed than first-born scientists. First- borns were more likely to stick to conventional and already established theories.
A fascinating study by Zweigenhaft and von Ammon (2000) tested Sulloway’s predictions about later-borns being more rebellious quite cleverly. The researchers interviewed a group of college students who had been arrested for engaging in civil disobedience. As predicted, there was a significantly higher percentage of later-borns among those who had been arrested than among a comparison group of their friends who had not participated in any civil disobedience or among a control group of college students simply drawn from classes at the same college.
These findings lend empirical support for the prediction of greater
conventionality and cooperativeness among eldest children, compared to their more radical, risk-taking younger siblings.
Although Sulloway has been criticized for his methodology (he collected biographical data on historical individuals), Born to Rebel breathed new life into birth order research, and since its publication, many and better studies have been conducted to test Adler’s predictions. Generally, “between- family” research designs (individuals from different families are compared) tend not to confirm Adler’s theory, perhaps because of the difficulty in these sorts of designs of controlling for the many variables that distinguish
families. In contrast, “within-family” designs ask respondents to compare themselves to their own siblings, and these studies are more likely to provide some confirmation for Adler’s theory. For example, Paulhus, Trapnell, and Chen (1999) conducted a within-family study of over 1,000 families and found that first-borns were nominated as most high-achieving and
conscientious, while later-borns were seen as most rebellious, liberal, and agreeable. In a very recent review of over 200 birth-order studies that did show significant differences between siblings, Eckstein and colleagues (2010) found support for Adler and Sulloway: first-borns and only children are seen as the most high-achieving, and later-borns as the most rebellious and socially interested.
Important to note in Adler’s theory of birth order effects is that he
hypothesized that it is the family constellation, not something biological or prenatal, that results in different personalities among siblings. That is, for Adler, the personalities of later- borns are shaped by older siblings’ and parents’ attitudes toward and treatment of them. But is this social hypothesis supported by the research? Research using within-family designs has been conducted to examine both hypotheses. One important biological theory concerns immunoreactivity. The theory is that histocompatibility-Y antigens, which are exclusive to males and located on the Y-chromosome, can induce an immune system response from mothers carrying male fetuses. Several studies have examined the immunoreactivity hypothesis for explaining sexual orientation in males, with evidence that the prevalence of homosexuality among males is higher among later-born males with a greater number of older brothers (e.g., Bogaert & Skorska, 2011). The reason the finding is explained biologically is that studies show that fraternal birth order does not predict homosexuality in males in adoptive or blended families, only in biologically related brothers (Bogaert, 2006).
Page 98 On the other hand, greater support has been found for other birth order effects, such as higher achievement among first- than later-borns, being explained by the more Adlerian social, family constellation perspective. For example, one very recent study used data on fully adopted sibling and fully biologically related sibling groups in a within-family design of Swedish sibling sets. Barclay (2015) found that older children indeed had higher educational attainment than their later-born siblings, regardless of whether they were from adoptive or biologically related families. This finding provides strong evidence for intrafamily dynamics such as resource competition, much as Sulloway predicted, being the driving force behind birth order effects.
In general, the specific predictions that Adler made about the traits of oldest, middle, youngest and only children have not found strong support in the research literature. However, within-family studies have indeed found support for some consistent (though small) differences between first- and later-born children in the realms of academic achievement,
conventionality and risk-taking. As well, within-family designs are
best suited to examine whether birth order effects are due to dynamics that occur in families, where later-born siblings may engage in “de-
identification” (finding one’s niche by observing what older siblings do and then finding a way to do the opposite), versus due to some prenatal or
biological cause. Here again there is some support for Adler’s theory that such effects are indeed social, and due to the constellation dynamics in families.