I'm pretty sure that in the general population, there are at least as many people with older siblings as there are people with only younger siblings. But in this poll, it's 6 vs 19. That looks like a humongous effect (which we also found in SIAI-associated people, and which this poll was intended to further check). I could see some sort of self-selection bias and the like, and supposedly oldest children have slightly higher IQs on average, but on the whole I'm stumped for an explanation. Anyone?
ETA: Here's a claim that "it is consistently found that b...
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