The 12 star signs are based on the rising & falling of certain constellations and were laid down in ancient greece (I think). If you took the same line/horizon now, there would be 13 start signs as the shift in the heavens oner the millenia since they were laid down has been pretty significant. That makes me think the influence of the star signs on human behaviour is negligible.
That's not to say that people born in different time of year may not display different characteristics. love to see the blind test carried out with antipodeans, who will have contrary seasons for the start signs and see how that impacts on those who say they can pick star signs.
Caitlin's right, there is evidence that sportspeople are usually from certain months, but that is believed to be down to how they fall in the school year. it's not just American football either, true for athletics and other sports as well. A child born in August will always be competing against children nearly a year older and will tend to be beaten on those grounds. At school age, a year is a massive difference, such that the later births are less likely to be picked for the school teams simply on the bases that they lag behind in age, not necessarily ability. This is perpetuated up the school system, such that if you look at successful adult sportspeople they tend to be born between september & Christmas. it's not so much an effect of the time they were born, more when that time falls in an arbitrary date cut off system, in this instance.