Using probability to demystify astrology

Astrology assumes that astral influences are at work, including the date of birth—but why birth rather than conception or gestation? Such claims are very easily refuted through relatively simple experiments.

The first such observation dates back more than 2,000 years. If a person's birth chart determined their character or destiny, how could twins so often have different destinies? Carneades (a philosopher of the New Academy, born in 219 BCE) was probably the first to formalize this argument, which was then taken up and refined over the centuries.

The other side of the argument is that people whose birth charts span the entire range can all meet the same fate at the same time (in a plane crash, natural disaster, terrorist attack, and so on).

Put to the test by statistics --------------------------------------------

The controversy surrounding "astrological twins" resurfaced once precise statistical analyses became possible.

In France, the biologist Suzel Fuzeau-Braesch (author of a "Que sais-je?" book on the subject; see this brief article) published a study supporting astrology in 1992 in La preuve par deux (Robert Laffont). Based on 251 pairs of twins, the study would prove to be shamefully biased.

In 2006, the Dane Peter Hartmann published a report on the influence of birth date on personality and intelligence. The analysis used data from much larger samples—one of 4,000 people and another of 11,000—for whom birth dates and the results of various psychological and cognitive tests were available. These tests had originally been conducted to assess the impact of military service on young recruits. The conclusion was unequivocal: there was no correlation between birth date and assessments of personality and general intelligence!

Of the statistical studies conducted since 2000, Geoffrey Dean's is probably the most startling. This Australian psychologist systematically gathered every statistical study carried out around the world over several decades and stated unequivocally in a 2003 interview that the "principles of astrology are invalid" and that astrology books "are in fact nothing more than frauds". One small detail: Dean had been the founding president of the Federation of Australian Astrologers!

Other experiments have been conducted with astrologers taking part.

One such experiment, conducted by Shawn Carlson and published in the journal Nature in 1985, asked astrologers to choose, from three personality profiles, the one that matched an individual's birth chart. The result: no better and no worse than pure chance.

In another experiment, made public in the Netherlands in 1996, a number of astrologers were given the birth data of seven people, together with those same people's answers to a personality questionnaire devised by the astrologers themselves. As in the previous experiment, not only did the matching of people to profiles prove random, but the astrologers' answers showed no similarity whatsoever!

When belief takes hold... -------------------

Chinese astrology claims that an individual's destiny is shaped by their year of birth, with each year associated with one of five elements (fire, earth, metal, water, and wood). Certain elements are said to predispose people to particular illnesses: 1907, an earth year, would thus make tumors more likely, while 1908, a fire year, would favor heart disease.

A 1993 study of death records from 1969 to 1990 for 28,169 Chinese Americans and 412,632 White Americans found a correlation between year of birth and longevity among Asian patients with the corresponding illness (their life expectancy was about one year shorter).

Disturbing! But astrology's devotees should not take this as confirmation of their faith. The same study found no correlation in the rest of the sample. So what was happening? Did the stars influence only those who believed in them? Almost. The variations in longevity were caused by the belief itself. This highlights the importance of psychological factors in illness and, more generally, in human behavior.

2016, the year of the fire monkey

How can you put the odds in your favor? -------------------------------------------

"But, one may object, astrologers have happened to make many valid predictions. I would reply that they have made a far greater number of incorrect predictions. When they speak the truth, it is due either to chance, to the sheer number of predictions they have made, to their having foretold wrongful deeds—which are the deeds people commit most often—to their scrutiny and observation of those who consult them, or to their knowledge of certain secrets concerning the powerful or those who associate with them...".

It could hardly be put better today, yet Jean Gerson addressed this observation to the dauphin, the future Charles VII, in 1419. For that is precisely what charlatanism is:

• making a sufficiently large number of predictions that at least one of them is very likely to come true, while knowing full well that the audience will forget the others and remember only those that actually did;

• phrasing predictions in sufficiently general terms that events can more easily be made to fit them, or that the recipient can more easily make the prediction their own;

• predicting an action that the recipient will be more likely to take because it has been foretold to them;

• relying on sufficiently broad statistics that the predicted event has a strong chance of occurring—yes, probability is also a tool for astrologers;

• relying on information that the recipient does not yet know.

Then there is the Barnum effect (see box), which makes people recognize themselves in what they would like to be rather than in what they are.

Human credulity is likely to keep astrology thriving for many years yet, especially if its proponents use psychology and probability to bolster their influence.

The Barnum effect --------------

According to François Filiatrault

First demonstrated in 1949 by B. R. Forer and subsequently replicated dozens of times with similarly consistent results, the Barnum effect is the process by which people readily recognize themselves in what they believe to be an especially flattering or positive description of themselves.

In the basic experiment, each participant provides a personal detail: their date and time of birth, their result on a bogus personality test, an account of a dream, and so on. The participants are then led to believe that a personality analysis, which they are shown a few moments later, has been produced from this information, and they are asked to rate the accuracy of the resulting portrait. Of course, no analysis has taken place, and, without realizing it, all the participants read the same description. Yet they immediately feel that it fits them like a glove, never imagining that the text might describe their neighbors and friends equally well.

This conviction will lend credibility to the supposed expertise of whoever performed the analysis.

Why is it called the Barnum effect, a name coined by the psychologist Paul Meehl? Because the owner of the famous American circus explained his company's success in two sentences: "There's a sucker born every minute" and "There must be a little something for everyone."

Using probability to demystify astrology

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