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All About Astrology

Donate Astrology neither has plausibility nor any test data showing that its predictions are better than random chance.  

by Brian Dunning

Filed under Ancient Mysteries, Paranormal

Skeptoid Podcast #173
September 29, 2009
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Also available in Chinese


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Today I'd like to talk about a subject that's very silly at face value, so silly that anyone with any functional part of a brain laughs it off as childish and ridiculous: Astrology, the notion that the time of year you were born assigns you a zodiac sign, and that sign determines your personality, forecasts your future, and provides decision guidance. The problem is that laughing something off is not really following a very skeptical process, especially when you remember that a lot of important people (such as the late President Reagan) depend on it. It's appropriate to look at the basis of astrology to see if it has any scientific validity, but it's also appropriate to look at the real-world results to see if there might be some real effect due to a mechanism that's not yet known.

The hardest part about examining the foundations of astrology is trying to determine what they are. It would be nice to be able to at least state that there are 12 signs of the zodiac, but that's only one system. Other astrology systems have 14 or 24 signs. Obviously, they can't all be right.

Even Western astrology with its reliable twelve signs of the zodiac has a serious flaw. Each sign of the zodiac is 30° wide (1/12 of our 360° view of the sky). The precession of the Earth's axis causes our view of the heavens to change over time, and the zodiac used by most astrologers is now wrong by 24°. This means that about 80% of people who have been raised with the understanding that they are of a given birth sign are actually of the preceding sign. My birth date pegs me as a Capricorn, but according to the constellations, I'm actually a Sagittarius. Some astrologers correct for this; others don't. Again, they can't all be right.

Most astrology systems rely on "houses", basically chunks of sky corresponding to each constellation. When a planet moves through a particular house, it's supposed to have a meaning different from when it's in another house. Unfortunately, there are all sorts of varying systems for defining where these houses are (Campaneus, Regiomontanus, and Placidean are the most popular methods in Western astrology), and every astrology system around the world has a completely different interpretation of what the houses mean.

But these only scratch the surface. Most astrological systems are extraordinarily complex, requiring years of study to master, and take many details into account that are far beyond the scope of this show. While it's possible for astrologers to precisely codify exactly how their system is to be used, there are so many different systems, and so many different schools of thought within each, that there are probably as many different ways of doing astrology than there are astrologers. Every single school of thought contradicts another, and every overall system often profoundly contradicts the others. The question "How is astrology done?" has only one right answer: It depends on who your astrologer is.

But differing interpretations don't disprove that there might be some cosmic influence. Whether it's the day, time, or month of your birth, there may indeed be some cosmic force acting upon you that affects your personality. Astrology is pre-scientific. It was developed millennia before we knew about the actual fundamental forces in nature, thus it makes no claims to having a basis in any real science. That's good, because appealing to any of the real forces in nature would be implausible; each breaks down easily:

  • The strong and the weak atomic forces only have a range the size of an atomic nucleus.

  • Gravity has an incredibly long range: You are, right now, being affected by the gravity of Neptune. But you're being affected by the gravity from your computer mouse much more. And from your chair even more. And from that person standing in the room even more. Gravity vectors are cumulative. You're not being pulled this way and that way; those vectors add up to a single direction. You cannot decode a gravity vector to determine there was x amount of pull from this direction, and x amount of pull from that direction. Rearranging the furniture in your room has a gravitational effect on you orders of magnitude higher than the positions of the planets. The moon is the only celestial body that has a significant gravitational effect on us, but most astrological systems give it little or no significance.

  • Electromagnetism is the last of the four forces. None of the planets have magnetic fields strong enough to effect us here on Earth. Jupiter's field is the biggest and strongest, but even that is negligible. The sun is the overwhelming electromagnetic source in the solar system, blasting everything else away, but yet again, the sun is not much of a factor in most astrological systems.

  • In astrology only the planets, which do not have any effect on us, are assigned all the responsibility for all cosmic influence upon us.

And so, given that there is no detectable effect, you might feel inclined to ask astrologers how they were able to detect its existence themselves, to the point of making it their careers. Generally they'll say they know it's real because it works. Now, I don't want to get into the whole cognitive bias thing here about how people can fool themselves into thinking a metaphysical reading is real; so let's just stick with what we can test and see if astrology really does work.

I wanted to find out if people generally do have the traits that their zodiac signs say they should, and so I conducted an informal survey over Twitter. (To be responsible, I should stress that there was nothing scientific about the way this survey was conducted, and so its results can at best be considered interesting, and not scientific proof of anything.) I went out on the web and found widely available personality descriptions of the various zodiac signs. For example, the words describing a Sagittarius were generally positive, things that I felt most people would probably identify with. So I took the four phrases (optimistic and freedom-loving, jovial and good-humored, honest and straightforward, intellectual and philosophical) and asked people to assign a 0 to each if they felt it did not describe them at all, a 1 if they felt it somewhat described them, and a 2 if it described them very well. I added each person's points up to get a score from 0 to 8. Since these were generally positive traits, I bet that most people would come up with pretty high scores. The average score turned out to be 6.3, with a clear distribution shoved up to the high end of the graph. The average respondent considers himself a 79% match with the traits of Sagittarius.

People identifying with Sagittarius traits
0 = Does not describe me at all
8 = Describes me very well
Average: 6.3 (79% match)
Margin of error: 6.2%
People identifying with Leo traits
0 = Does not describe me at all
8 = Describes me very well
Average: 5.7 (71% match)
Margin of error: 5.2%
People identifying with Taurus traits
0 = Does not describe me at all
8 = Describes me very well
Average: 5.1 (64% match)
Margin of error: 5.3%
People identifying with Scorpio traits
0 = Does not describe me at all
8 = Describes me very well
Average: 3.7 (46% match)
Margin of error: 4.7%

I also asked the same question using the traits of Leo, Taurus, and Scorpio, adjectives which were (in my estimation) progressively less complimentary, and the average scores did indeed turn out to be 5.7, 5.1, and 3.7. Each of these graphs has a nice, clear bell curve. It's clear that when you ask people, without any context, whether they feel they are better described by words which happen to be positive (like Sagittarius' "optimistic and freedom-loving"), they tend to identify with those terms; but when you ask the same question with less positive words (like Scorpio's "determined and forceful") there is less identification. Armed with knowledge of this fairly obvious axiom, any astrologer should have no problem writing fortunes for just about anyone that will hit the mark 9 times out of 10.

In my survey, I also wanted to see how the results of these same questions might differ between people who are of that zodiac sign, from those who are not. I took the negative qualities of a Libra (indecisive and changeable, gullible and easily influenced, flirtatious and self-indulgent) and asked Libras if they thought it represented them, and asked the same question of non-Libras. If there's anything to astrology, the Libras would have recognized their own weaknesses in those descriptions. But guess what; they didn't. Both groups reported an average of 2.0 out of 6 points, or about a 33% match.

Non-Libras identifying with Libra traits
0 = Does not describe me at all
6 = Describes me very well
Average: 2.0 (33% match)
Margin of error: 4.7%
Libras identifying with Libra traits
0 = Does not describe me at all
6 = Describes me very well
Average: 2.0 (33% match)
Margin of error: 4.7%

This result was interesting, so I extended this line of investigation, and asked the same question again, but this time instead of using the zodiac sign's traits, I used randomly chosen readings from the Los Angeles Times horoscope. The first was for Capricorn, and it said:

The universe is sending out some muddled messages. Don't read too much into the signs. If you have to stretch to figure out what something means, it's just because you're not meant to know yet.

Neither Capricorns nor non-Capricorns felt that fortune applied to them much at all; the graphs look virtually identical with a big tall bar in the "Does not apply to me at all" column and only a smattering of results in the other two. Non-Capricorns reported a 12% match, and Capricorns reported a 17% match. While that five percentage point difference may seem significant, it's below the 7.8% margin of error that I calculated for this question.

Non-Capricorns identifying
with Capricorn horoscope
0 = Does not apply to me at all
2 = Applies to me very well
Average: 0.2 (12% match)
Margin of error: 7.8%
Capricorns identifying
with Capricorn horoscope
0 = Does not apply to me at all
2 = Applies to me very well
Average: 0.3 (17% match)
Margin of error: 7.8%
Non-Taurus identifying
with Taurus horoscope
0 = Does not apply to me at all
2 = Applies to me very well
Average: 0.9 (45% match)
Margin of error: 5.9%
Taurus identifying
with Taurus horoscope
0 = Does not apply to me at all
2 = Applies to me very well
Average: 0.9 (46% match)
Margin of error: 5.9%

I tried it again with a fortune for Taurus that sounded more positive:

You can do more than instruct people. You can inspire them. You focus on a beautiful potential and describe it with the passion that gets others moving in the same direction.

As you can probably guess, more people felt this applied to them, and this time it made almost no difference at all whether they were Taurus (46% match) or anything else (45% match). Grant a positive sounding fortune, and more people convince themselves it applies to them.

But my little Twitter survey is hardly the first time anyone has tested astrology. Many, many studies have been done; and better the study, the less of an effect has been found. Dutch researcher Rob Nanninga wrote:

One of the best examples was conducted by the Australian researcher Dr. Geoffrey Dean... Dean selected 60 people with a very high introversion score and 60 people with a very high extraversion score. Next, he supplied 45 astrologers with the birth charts of these 120 subjects. By analyzing the charts the astrologers tried to identify the extroverts from the introverts. The results were very disappointing. It was as if the astrologers had tossed coins to determine their choices. Their average success rate was only 50.2 percent.

He devised his own test in which seven people from diverse backgrounds filled out detailed questionnaires about themselves, and separately provided a list of the dates of important events in their lives. 50 experienced astrologers agreed to match the questionnaires to the date charts, and were offered 5,000 guilders if they could correctly match all seven. As a control, Nanninga also had a group of skeptics try to perform the same matches, to rule out successes based on subtle clues in the data.

The astrologers were asked to indicate how many correct matches they would have expected... Half of them predicted that they had matched all subjects with the correct charts. Only six astrologers expected less than four hits. In fact, the most successful astrologer achieved only three correct matches, whereas half of the participants (22) did not score a single hit. The average number of hits was 0.75...

Moreover, there was no evidence that the most experienced astrologers did any better than beginners. It is interesting to compare the entries of the participants with each other. Because they all had received the same information, one would expect many similar responses. Actually, the lack of agreement was striking. Each of the seven charts could be paired with seven questionnaires. Of these 49 possible combinations, none was selected more than twelve times. It was as if each astrologer had used a random generator to determine the correct matches.

Of the control skeptics, the most successful also scored three hits, the same as the best astrologer.

In addition to his 1987 study referenced by Nanninga, Dr. Geoffrey Dean also performed a meta analysis of nearly 300 empirical studies of astrology. He found no real effect, and attributed the perceived effect to perceptual and cognitive biases that he called hidden persuaders. In his conclusion he wrote:

To critics, astrology's failure to deliver is unremarkable because its alleged efficacy is explained by the same hidden persuaders that underlie proven invalid approaches such as phrenology and bloodletting... Each hidden persuader creates the illusion that astrology works, all are used routinely in consulting rooms, all lead to client satisfaction — and none require that astrology be true... If you are looking for something where nothing is true and everything is permitted, then astrology seems to be an excellent choice.

People who are big believers in their horoscopes are probably going to continue to remain so, no matter how much evidence they're shown that any perceived effect is imaginary. But for those who are on the fence, this information is crucial, given that some leaders in government and business employ astrology in their decision making. If someone with authority over your life is making important decisions based on magical beliefs, you should trust the science, not the authority.

By Brian Dunning

Please contact us with any corrections or feedback.


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Cite this article:
Dunning, B. "All About Astrology." Skeptoid Podcast. Skeptoid Media, 29 Sep 2009. Web. 15 Aug 2018. <>


References & Further Reading

Culver, R., Ianna, P. The Gemini Syndrome: A Scientific Evaluation of Astrology. Buffalo: Prometheus Books, 1984.

Dean, G. "Meta-analyses of nearly 300 empirical studies: Putting astrology and astrologers to the test." Astrology & Science, 28 Sep. 2008. Web. 28 Sep. 2009. <>

Grim, Patrick, editor. Philosophy of Science and the Occult, Second Edition. Albany: State University of New York Press, 1990. 15-84, esp. 51-81.

Jerome, Lawrence E. Astrology Disproved. Buffalo: Prometheus, 1977.

Nanninga, Rob. "The Astrotest: A Tough Match for Astrologers." Correlation, Journal of Research into Astrology. 10 Feb. 1997, Volume 15, Number 2: 14-20.

Watson, J. "Astrology is Bunk." Debate Unlimited by Martin Willett. Debate Unlimited, 14 Jan. 2008. Web. 28 Sep. 2009. <>


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