That reminds me of some studies on birth order and personality. There have been a lot of studies showing how birth order influences personality, but few of them are any good. Almost all of them depend on how other members of the family describe the siblings. When personalities are assessed by those who do not already know the birth order and have not observed the family interacting with each other, the observers don't find any correlation between birth order and an adult's personality whatsoever. Outside observers did however find some statistically significant correlations between birth order and the way individuals behave around those they have known since early childhood. It appears that birth order does not effect an adult's personality in general, but that family gatherings do make our personalities revert to a childhood state when birth order was relevant.
(Those studies linking IQ to birth order of course didn't depend on family members ranking each other, but they were still quite bad as they ignore an important confounding variable. There is a strong correlation between IQ and family size. Children from very large families tend to have much lower IQs, perhaps because they get less individual parental attention or perhaps because more intelligent parents tend to practice better family planning. Most of the first born children in studies that found them to be smarter don't actually have any younger siblings. When you only compare children with the same number of siblings, first born children are no smarter than middle children or youngest children.)