Are firstborns smarter?
This is an old hypothesis that Robert Zajonc devoted considerable work to in the '70s. He found an effect of up to 3 IQ points. A recent international study with large samples has however recently re-examined the theory. There is a popular account of the findings here. The journal abstract is as under:
Examining the effects of birth order on personality
Julia M. Rohrer et al.
This study examined the long-standing question of whether a person’s position among siblings has a lasting impact on that person’s life course. Empirical research on the relation between birth order and intelligence has convincingly documented that performances on psychometric intelligence tests decline slightly from firstborns to later-borns. By contrast, the search for birth-order effects on personality has not yet resulted in conclusive findings. We used data from three large national panels from the United States (n = 5,240), Great Britain (n = 4,489), and Germany (n = 10,457) to resolve this open research question. This database allowed us to identify even very small effects of birth order on personality with sufficiently high statistical power and to investigate whether effects emerge across different samples. We furthermore used two different analytical strategies by comparing siblings with different birth-order positions (i) within the same family (within-family design) and (ii) between different families (between-family design). In our analyses, we confirmed the expected birth-order effect on intelligence. We also observed a significant decline of a 10th of a SD in self-reported intellect with increasing birth-order position, and this effect persisted after controlling for objectively measured intelligence. Most important, however, we consistently found no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness, or imagination. On the basis of the high statistical power and the consistent results across samples and analytical designs, we must conclude that birth order does not have a lasting effect on broad personality traits outside of the intellectual domain.
So the effect was very small. In fact, if we look at the supplemental material, we see that the difference in the British sample was just one IQ point -- totally unimportant for all intents and purposes. So even the small effect found by Zajonc would seem to have been overstated.
Whether a difference of one IQ point requires explanation is unclear but several environmental explanations have been suggested in the links above.
What was NOT found is also interesting. That birth order had NO effect on any personality variable upsets a lot of theories -- but is consistent with genetics rather than the environment being the main influence on personality -- e.g. if you are a miserable whiner like most Leftists are, you were born that way.
One theory that would seem rather damaged by the findings even though it was not directly tested was the pet theory of Frank Sulloway. Firstborns are conservatives and later-borns are rebellious, says Sulloway. Rebelliousness would seem to be a personality variable. See here for a dissection of the strange Prof. Sulloway and his theory.