Everything you think you know about the effects of birth order is wrong. Here’s what’s right:
until very recently there were no convincing findings that linked birth order to personality or behavior. Our common perception that birth order matters was written off as an example of our well-established tendency to remember and accept evidence that supports our pet theories while readily forgetting or overlooking that which does not. But two studies from the past three years finally found measurable effects: our position in the family does indeed affect both our IQ and our personality…
In 2007 Norwegian epidemiologists Petter Kristensen and Tor Bjerkedal published work showing a small but reliable negative correlation between IQ and birth order: the more older siblings one has, the lower one’s IQ.
In 2009 my colleagues and I published evidence that birth order influences whom we choose as friends and spouses. Firstborns are more likely to associate with firstborns, middle-borns with middle-borns, last-borns with last-borns, and only children with only children. Because we were able to show the effect independent of family size, the finding is unlikely to be an artifact of class or ethnicity. The result is exactly what we should expect if birth order affects personality. Despite the adage that opposites attract, people tend to resemble their spouses in terms of personality. If spouses correlate on personality, and personality correlates with birth order, spouses should correlate on birth order.
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