I’m preparing to talk to a group of high school psychology teachers about “bad” science. (How is “Bad Science” not an eighties movie? Was “Weird Science” enough?) I’m reeling, as I have many times before in reporting on the science of only children, from the disconnect between what studies about only children show us, and what we tend to believe, regardless.

I visited birth order guru and scientific historian Frank Sulloway at Berkeley last year.  We talked about why bad scientific reasoning about onlies tends to occur outside the data sets. He believes a Darwinian impetus may well underlie the stereotype. Our relatives had an evolutionary imperative to spread their own genes.  “They need a good story to convince us …