Belmont and Marolla conducted a study on the relationship
between birth order, family size, and intelligence. The subjects
consisted of all Dutch men who reached the age of 19 between 1963
and 1966. These men were required by law to take the Dutch army
induction tests, including Raven’s intelligence test. The results
showed that for each family size, measured intelligence decreased
with birth order: first-borns did better than second-borns,
second-borns did better than third-borns, and so on. And for any...