To play devil's advocate for a second - do astrologist's claim that the stars 'cause' personality differences or just claim some correlation between the two? Maybe astrologist's have deciphered a pattern that has some validity as is is also a known scientific phenomena (by modern psychology), and the actual cause is still a mystery. This article seems to say that season of birth is correlated to certain personality characteristics due to seasonal temperature. I remember from developmental psychology (years ago, I admit) that children born in the spring are more likely to be schizophrenic in later life.
If I was not playing devil's advocate, I might say this article proves that astrology is bunk - that there are real differences between people that depend on when they were born, and we have a perfectly rational explanation why...