I remember reading/hearing something about the effect of older brothers on the chances of people becoming certain types of people.
It went something like the oldest was most likely to be a pilot/doctor/CEO etc.
The second was most likely to be a teacher/middle management etc.
There's plenty of blogs that suggest the more older brothers a man has, the more likely they are to be homosexual. But this obviously doesn't look at whether or not Biological influences lead to this "birth order effect", or cultural influences.
I remember a suggestion that it was to do with antibodies (or some other substance) that builds up in the mother's womb after having son's. But again I can't find any evidence for that, so it might be pure speculation.
NB. I don't subscribe to this theory, but if you're really interested, it might be worth looking at the science behind it.