These are essentially the same question – astrological sign is determined by the day of one’s birth but our reaction to the question is different. Asked in the first way, this is a legit question about an Epidemiological signal known since antiquity and is officially known as «season of birth effect» (SBE). (Historical trivia: the first use of this term may be found in Ellsworth’s Huntington book: Season of Birth: Its Relation to Human Abilities published in 1938). The SBE appears to be relevant for many diseases, outcomes and human populations and countries. Since we westerners are more likely to bite the bullet from cardiovascular disease or cancer, rest assured that the association has been noted for the former and the latter (though not everyone agrees)
Asked in the second way, the same question can immediately bring ridicule and shame on the scientifically oriented person who dares to ask it. In fact there is a small cottage industry of papers that use astrological sign in order to caution/warn/ridicule/discredit subgroup analyses in clinical datasets. The most famous of this is a subgroup analysis of the ISIS-2 but there are certainly others.
So what makes us think that the Season of Birth Effect is real and worthy of study across disciplines, while the astrological sign association is bogus and should be discarded any time it is detected? The answer may be found in the framing effect, a form of cognitive bias in which our reaction and even answer to the same question critically depends on the way it is framed. Though the framing effect is particularly relevant in sociology, it is also operational in medicine particularly affecting the choices patients make depending on the format of presentation of the same information.
So if framing effects are also operational in scientific discourse (at least in the medical field) how do we deal with them? The first way is to acknowledge their potential presence so as to minimize their impact by appropriate use of neutral terminology and avoid sensationalism when describing associations and effects. As the season-of-birth effect example shows one can present the same information to evoke sharply different responses, so one should always be on the look for emotions when reading a story in either the scientific or the lay press.
So to come back to the initial question: does one’s day of birth or astrological sign affect later outcomes? The associations seem to be there and are relatively consistent and reproducible so some phenomenon must be at play. Whether this phenomenon is real or a statistical analysis artifact (e.g. could age-period-cohort effects and the troubled history of the 20th century be at play here) is something I don’t know. But I would certainly be willing to find more about it, irrespective of whether the information is presented by month of birth or astrological sign 🙂