How We Know Astrology Isn’t Real

This article first appeared in Issue 6 of our free digital magazine CURIOUS.

Astrology (not to be confused with astronomy) still has plenty of fans, some of whom even base major life decisions on planetary movements. Yet it's also so widely disreputable it's the most common point of comparison to dismiss something as baseless. However, not everyone who holds astrology in contempt knows why. Although physics and astronomy have undermined any plausible mechanism by which astrology might work, it is social sciences such as psychology that have bridged the gap between incredulity and demonstrated failure.

It wasn't always like this. Johannes Kepler, one of the greatest astronomers of all time, was an astrologer as well, and probably wouldn't have made his discoveries without his astrological investigations. Until Kepler published his work in the 17th century, planets were widely thought to travel on crystal spheres, possibly pushed by angels, so the idea they controlled events on Earth wasn't such a leap. Consulting a doctor also often came with a side serving of astrology.

The fault, dear Brutus, is not in our stars, but in ourselves

Even then, however, not everyone was convinced – in a play written when Kepler's work was just beginning, Shakespeare had Cassius say: "The fault, dear Brutus, is not in our stars, but in ourselves". The more we learned about the heavens, the less sense astrology made. The discovery that stars lie at vastly different distances, with the closest thousands of times further away than any planet, makes constellations a handy fiction. How then could a planet's movements "through" the Zodiac have any significance? Moreover, known forces all weaken with distance, yet astrologers treat Venus and Saturn as wielding equal, albeit different, influence over our lives.

These and other astronomical lessons justifiably encouraged skepticism among those aware of such things, but on their own do not amount to disproof. After all, the argument for Earth's "continental drift" suffered decades of ridicule primarily for lack of a plausible mechanism, only for one to be found at mid-Oceanic ridges. Carl Sagan refused to sign a statement by leading astronomers called "Objections to Astrology", in part because the absence of a mechanism alone was insufficient to prove it false.

Many scientists, with varying views on astrology's merits when starting, have set out to test its claims. Doing so, however, runs up against one of the key issues that made people suspicious of astrologers even before Galileo and Newton: the vagueness and contradictions in astrologers’ claims.

If a dozen astrologers all make different predictions based on planetary motions, it's a fair bet some will be right. Even if testing proves they on average did no better than chance, those who picked right will announce they are the true keepers of the sacred knowledge, and the rest are charlatans and fools.

Such claims are hard to test, since most people can see a bit of themselves in vague and largely complimentary descriptions, such as those typically provided by horoscopes.

That hasn't stopped scientists from trying, though. When predicting events for the year ahead, astrologers have consistently failed to outperform people merely extrapolating from the past. Beaten on this account, astrology's defenders moved to safer ground, claiming their practice tells us about personality, but not fate.

Such claims are even harder to test, since most people can see a bit of themselves, or their loved ones, in vague and largely complimentary descriptions, such as those typically provided by horoscopes.

Nevertheless, scientists have found innovative ways to add a little rigor. A few such studies do appear to provide support for astrology's claims. Even in these cases, however, a deeper look tends to encourage skepticism.

An attempt to see whether personality variations between twins could be explained by astrology claimed success, for example, but a second look found an abundance of flaws in the study. When these were removed, the effect disappeared.

Three Indian psychiatrists provided the birth dates, times, and places of 150 people, half of whom had been diagnosed with mental illness, to four astrologers. Based on birth details and gender alone, the astrologers were challenged to predict which birthdates belonged to those who had been diagnosed, their symptoms, and the state of their mental illness at the time. The astrologers did perform better than chance in predicting who had suffered mental illness at some point, and who was suffering at the time. They failed, however, at predicting specific symptoms. More significantly perhaps, the astrologers showed little agreement with each other in their predictions, normally a requirement for science.

More frequently, studies that report astrological influences fail the replication of results. A famous example was the so-called "Mars Effect", when astrologer and psychologist Michel Gauquelin claimed French athletes were more likely than the general population to be born when Mars is rising or peaking. Similar tests have found no such effect in the United States.

Some astrological claims, however, can be tested using enormous databases. One such study looked to see whether the star signs people marry are randomly distributed, or if they match astrologers’ compatibility advice. "If there is even the smallest tendency for Virgos to fancy Capricorns, or for Libras to like Leos, then we should see it in the marriage statistics," the University of Manchester's Dr David Voas said in a statement. By Leos, Voas presumably meant those born between July 23 and August 22, rather than those who share their name with a famous actor.

Despite a 20 million-person sample size he referred to as a "giant magnifying glass", Voas found no patterns at all.

Indeed, even the more plausible astrology-adjacent claims tend to fail such large sample tests. People who scoff at the power of planets may have no trouble with reports of the influence of the full Moon. Yet before becoming an Australian parliamentarian, Professor Andrew Leigh conducted a study to see if lunar cycles correlated with births, deaths, or even conceptions. Over a 29-year period, even modest effects were ruled out.

A meta-analysis of 37 studies testing the influence of the Moon on everything from crisis calls to mental hospital admissions showed lunacy is a misnomer. The few studies that reported a link had failed to control for other cycles, or failed other basic forms of analysis.

If even the Moon, close enough to rule the tides, has no measurable effect, it is hard to see how the planets could exert an influence.

Complicating matters is the fact that the timing of your birth can influence your fate in ways that have nothing to do with astrology. Rates of schizophrenia are higher among people born in early spring, at least far from the equator. Several studies have indicated this is due to fetal vitamin D deficiency during a key stage of development. The fact it occurs during different months in Tasmania and Scotland proves the issue is the amount of sunlight, not the stars behind the Sun at the time.

Athletes are more likely to be born just after the cut-off point for sporting age brackets, January or July. At elementary school, being the oldest by even a few months makes it easier to excel at sport, and it seems the effect lasts. Astrologers, however, might argue Capricorns and Cancers are just naturally athletic, although, strangely, that only applies to those born in the latter part of the horoscope period.

There is, however, one entirely plausible way astrology could influence our lives; if our beliefs make it so. Some online dating websites ask for people's star signs, presumably because at least a few of their users are paying attention. If people read enough columns telling them they’re incompatible with those born in particular months, it might indeed reduce the chances of them ending up with such a person, making it slightly surprising Vaos found no such effect.

One prominent study that claimed to show evidence for astrology reported that people whose Sun was in certain signs were more extroverted than those in the other six signs. Numerous papers by the senior author of the original study, Hans Eysenck, have since been classified as "unsafe" because of errors and possible manipulation of data, so when Jan van Rooij of the University of Leiden attempted to replicate the work it wouldn't have been surprising if he found nothing. However, van Rooij added a twist by asking the subjects how familiar they were with astrology. He found that those who knew the characteristics assigned to people of their sign did indeed match the predictions, while those who did not know what was expected of them showed no such pattern.

Nevertheless, most tests of astrology don't even find that sort of indirect effect.

Moreover, advances in astronomy have offered some opportunities to test astrology on top of psychological ones. Neptune, for example, was discovered because of its gravitational tug on Uranus. Astrologers quicky adopted it and assigned significance to its position in the sky when people were born. Yet not one of them made a prediction of a previously undetected planet, based on observing some shift in the collective psyche every 13.7 years as Neptune moves to a new Zodiac segment.

Astronomers are on the hunt to find a possible Planet X based on the movements of comets and other outer-Solar System objects whose orbits may have been shaped by its gravity. Don't hold your breath for astrologers finding its location first based on working backward from social observations.

CURIOUS is a digital magazine from IFLScience featuring interviews, experts, deep dives, fun facts, news, book excerpts, and much more. Issue 9 is OUT NOW.