As the son of an Astronomer, it would be the height of heresy (not to mention treason) to say something along the lines of “but Astrology is scientifically defensible”.
I have, however, had a theory rattling around in my head for a while on this topic. My theory may be obvious and well known, perhaps even disproven, but I have yet to see any other references to the basic concept. A recent study on the factors determining the onset of menopause, however, shows a similar conclusion, so I felt like it was time to present the idea.
My assumption is that people’s personalities could be shaped by the time of year in which they are born, and that common traits may be found among people born in similar seasons. This is not to say that the boundaries are calendar months, nor linked to planetary or cosmic positions, with the exception of the relative positioning of the earth and sun. My thought is that the availability of vitamins and micronutrients in the diet have traditionally been seasonal, and that neonatal nutrition may have a subtle but real effect upon brain development. Obviously, there are a lot of caveats to this theory: primarily, it assumes that personality is (at least partially) a result of physical brain structure.
In any case, next time you hear someone rejecting Astrology out-of-hand with arguments about the relative gravitational fields of the planets and the doctor delivering a baby, this theory can be used to play devil’s advocate. And, after all, that’s where the fun is.