I don't think that it has a direct influence at all. In fact, it seems to imply an arrogant interpretation of proportion. The truth is that the universe atomizes our existence, which is at once beautiful and humbling. It is beautiful without needing to relate to us.
You'll find all different types of people born in every different month and even day. I have no doubt that if this were looked at neurologically we find different of brain activity and hormonal distribution.
Perhaps knowing your sign could influence how you act, but that's not a direct result of their actual sign. It is a result of their believing something about the sign and themselves.
Mine's probably the least fun answer though, for a lot of people.