A massive (377,000 high school students can certainly be considered massive) study found that eldest children were more intelligent, BUT by only the equivalent of 1 IQ point! Eh, nothing. It is meaningless. This is in direct contradiction to standard thinking that first borns have an intellectual advantage because they get more attention from parents, more time before the other kids are produced. Ok, all you second and third borns – test your oldest sibs immediately.
Source: Damian and Roberts in the Journal for Research in Personality, 2015.