So I was thinking, if this astrology community really thinks they are correct (as I'm sure many of them think), why don't they simply try to prove it? Of course, anecdotes are quite fine to convince your friends, but the scientific community requires a little more than this, real proof. Something like: sort out a big number of random people in terms of their personality traits, and compare the resulting clusters with their date of birth, and check if there is any correlation (we're not even talking about causality here). I found out that these kind of tests are not very common, but not entirely non-existent, and as expected, they were mostly negative for astrology. Here are some examples (taken from here and here):
Psychologist Bernard Silverman of Michigan State University looked at the birth dates of 2,978 couples who were getting married and 478 who were getting divorced in the state of Michigan. Most astrologers claim they can at least predict which astrological signs will be compatible or incompatible when it comes to personal relationships. Silverman compared such predictions to the actual records and found no correlations. For example "incompatibly signed" men and women got married as frequently as "compatibly signed" ones.Any kind of scientific theory must be able to make predictions, otherwise it is considered false (that's what the scientific method is all about). Well, if astrology no longer wants to be considered pseudoscience, it should improve those statistics, which are currently at the same level as that old technique, guessing... Apparently your solar sign doesn't have any influence on your love life (except of course in self-imposed restrictions - Leo girls are off limits!), on the job you choose, or even on your own personality. So how come so many people will still tell you that it works for them and for the ones they know, while at the same time scientific statistical analysis tells exactly the opposite? Well, here we're entering psychology (or in this case, mass psychology), and this is the explanation I found most plausible: the human brain is hardwired to find relations where there are none, it is simply part of human evolution.
Many astrologers insist that a person's Sun sign is strongly correlated with his or her choice of profession. Indeed, job counseling is an important function of modern astrology. Physicist John McGervey at Case Western Reserve University looked at biographies and birth dates of some 6,000 politicians and 17,000 scientists to see if members of these professions would cluster among certain signs, as astrologers predict. He found the signs of both groups to be distributed completely at random.
Other tests show that it hardly matters what a horoscope says, as long as the subject feels the interpretations were done for him or her personally. A few years ago French statistician Michel Gauquelin sent the horoscope for one of the worst mass murderers in French history to 150 people and asked how well it fit them. Ninety-four percent of the subjects said they recognized themselves in the description.
Geoffrey Dean, an Australian researcher who has conducted extensive tests of astrology, reversed the astrological readings of 22 subjects, substituting phrases that were the opposite of what the horoscopes actually stated. Yet the subjects in this study said the readings applied to them just as often (95 percent of the time) as people to whom the correct phrases were given. Apparently, those who seek out astrologers just want guidance, any guidance.
Hartmann (a researcher in the Department of Psychology at Denmark's University of Aarhus) and his colleagues used computer analysis and statistical methods to study possible astrological connections between over 15,000 individuals. If connections existed over a rate of 5%, they were considered valid and not the result of random links. The scientists could find no relationship between the time and date of a person's birth and their personality traits.
So for instance, someone will easily relate with personality traits that are commonly associated with their star sign, tending to exaggerate positive 'hits' and overlook whatever does not fit. Also if most people were to somehow change to another sign, they would easily adapt themselves to their new traits, as most of them are generic enough for everybody to have them is some greater or smaller scale. And of course, the comfort of having some king of guidance... Telling how your star sign ought to behave could also change the way you act (similar to the pygmalion effect). This is quite common with other pseudosciences and superstitions (think about the placebo effect). There are many other arguments against astrology, but they seem a little overkill at this point: lack of a proper mechanism (if the stars influence us, how do they do that exactly? gravity?), the fact that star constellations are purely man made (they are simply drawing in the night sky), the conception time versus birth time issue (why does one count and not the other), the twins case (each one with very similar horoscopes, but in some cases entirely different personalities), and so on...
But the truth is clear: the date of birth does not affect an individual's personality. Of course, there may be some external forces that may influence our lives, but certainly not the sun, the planets and the stars. And for now, we can't/shouldn't talk about what we don't know!
::.. Astrology on wikipedia
::.. Astrology on about.com
::.. Astrology in skepdic.com