I've been thinking of better answers for this question ("Their favorite movie is Center Stage"; "They strongly dislike 69") but here it is: they're all firstborn. From the Guardian:
If you are the eldest child and female, you are statistically more likely to be the most ambitious and well-qualified of all your family.
The study followed 3,532 people in 1,503 sibling groups, and found specifically that "firstborn girls were 13% more ambitious than firstborn boys," and that the "probability of attending further education for firstborns is 16% higher than their younger siblings." (I wonder how much the latter number has to do with the allocation of parental resources?) Also: all 12 men who have walked on the moon are firstborns.
I like this study because I, like Beyoncé, am a firstborn girl, and I greatly value any occasion that enables me to use the phrase "I, like Beyoncé" in a sentence. I've also always been less laid-back than my younger brother. Anyone else have anecdotal sibling evidence to confirm?