The influence of birth order (are you the oldest of your siblings, the youngest, the middle child or whatever?) is widely interesting to the public. For example, when I was a child in the 1960s, my being an only child was a subject of widespread envy among other children. Everybody had a theory about how I was going to grow up greedy or entitled or whatever due to my Only Child Privilege.
(It’s interesting how discussions of Privilege these days in, say, The Atlantic tend to be much cruder and more reductionist — White Privilege! — than among school children in 1969 — Only Child Privilege.)
But the effects of Birth Order has been a vexed subject for researchers. I reviewed a book on the subject back in the 1990s for National Review. Lately, psychiatrist Scott Alexander of his SlateStarCodex blog has gotten interest in the subject.
I used the 2019 Slate Star Codex survey, in which 8,171 readers of this blog answered a few hundred questions about their lives and opinions.
My impressions are that SlateStarCodex’s readers can be stereotyped as:
– extraordinarily high IQ,
– more logical than well-informed relative to their IQs (his readership originally growing out of a Rationalist cult),
– interested in mental health problems, often because of difficulties of their own. (Scott is, among much else, a psychiatrist and has done a lot of interesting Moneyball-style research on which psychiatric pharmaceuticals work for whom.)
– They tend more toward the tech industry and STEM academia than, say, the humanities academia or the arts.
Of those respondents, I took the subset who had exactly one sibling, who reported an age gap of one year or more, and who reported their age gap with an integer result (I rounded non-integers to integers if they were not .5, and threw out .5 answers). 2,835 respondents met these criteria.
Of these 2,835, 71% were the older sibling and 29% were the younger sibling. This replicates the results from last year’s survey, which also found that 71% of one-sibling readers were older.
So, that’s a sizable sample for a nice simple subset of this complicated issue.
In contrast, many birth order analyses get sidetracked into the weeds of complex family situations. For example, the book I reviewed worried how to categorize Hitler (everybody always asks about Hitler), who was something like his father’s 4th born child but the first born of his mother, who was his father’s third wife. Or something like that. (My general impression, by the way, is that family histories tend to be more complicated than you might assume: e.g., President Gerald Ford, the embodiment of American normalcy, only met his biological father once after infancy.)
But Scott has boiled it down nicely to a simple comparison: if you are one of two siblings, are you the older or the younger one?
And he gets a strong result: 71% for his readers who are one of two siblings are the older one.
Of course, that raises the question of what exactly is being selected for in this highly self-selected example: high IQ, powers of reason versus powers of memory, mental health issues, attention span (Scott writes some of the longest blog posts on the Internet), or what?
Okay, let’s assume it is IQ. Scott sums up:
Overall these results make me lean slightly more towards intra-family competition or parental investment as the major cause of birth order effects. I can’t immediately think of a way to distinguish between these two hypotheses, but I’m interested in hearing people’s ideas.
Scott summarizes the two leading theories in line with his results as:
1. Intra-family competition. The oldest child choose some interest or life path. Then younger children don’t want to live in their older sibling’s shadow all the time, so they do something else.
With a highly selected group like SSC’s readers, who are way out at the edge of the bell curve for logical ability and attention span, maybe younger siblings choose to develop other aspects of their talents?
2. Decreased parental investment. Parents can devote 100% of their child-rearing time to the oldest child, but only 50% or less to subsequent children.
Mom or Dad patiently explain logic to their logically inclined first child, but don’t have as much time for the second child. And the first child isn’t as good a teacher for the second child and maybe is actively aggressive toward the younger sibling: “You’re stupid for not understanding the Law of the Excluded Middle.”
The second surviving theory then would raise the Daycare Question. Should high IQ mothers work less to spend more time nurturing their high IQ children’s intellects? Or is a Mixtec-speaking nanny good enough?