I had noticed it too... and, with the exception of the Female Dominates, and reading through the posts both here and in other threads, it almost seems to reason that first borns are more natural to this life. Although with males the natural inclination is to be Dominate, always in charge and with females, while we may be in charge in other areas (where we pretty much have to be) of our lives we enjoy (or is that need/desire/wish) to give up control when we can.
Have to admit though that I'm surprised that, so far the only male sub to take the poll, is an oldest child. For some reason I always assumed that male subs were the youngest in the family.
Still curious to see how the results finally come out, but honestly, at this point my previous personal hypothesis seems to be proving true, with a few exception (like noted above).