Astrologers claim they can tell your character, abilities, health, love life, and much more, just from your horoscope or birth chart. It seems amazing.

Could it actually work? Could astrology really be true? Astrologers have the perfect answer -- just try it. Put astrology to the test. Be convinced that it works. Just as they have been convinced every day for the past twenty centuries.

What could be more reasonable? So you try it.

Linda Goodman says Leos are warm, generous, independent, and dislike being told what to do. So you ask one hundred Leos if this is true. Ninety say yes, the rest say it depends but generally yes.

Cautiously, you press on. Astrologers say a Mars-Neptune conjunction signifies that the person is idealistic, concerned with values such as consideration for others. So you ask one hundred people with a Mars-Neptune conjunction if they are idealistic. Ninety-five say yes.

Still cautious, you have your birth chart read. The astrologer tells you things she could not possibly have known, like you have problems with money. Amazingly, everything fits. You are now absolutely, totally, completely convinced. Yes, astrology really works. You haven't the foggiest idea how it works, but it certainly works.

As it happens, none of this is make-believe. Millions of people have tested astrology this way. Millions have found that it works. Millions have been convinced that astrology is true. You are not alone.Now the important bit.

Suppose you repeat your tests with switched data, using non-Leos instead of Leos. people without a Mars-Neptune conjunction, and the wrong birth chart. And suppose the results are no different. Whereas 90% of Leos said they were like Leo, so did 90% of non-Leos, so Leos are no more like Leo than non-Leos. Are you still absolutely, totally, completely convinced that astrology works? Of course not.

Again, none of this is make-believe. Many tests with switched data have been made, all with results like these. Your sun sign and birth chart may fit you exactly but so do sun signs and birth charts that are not yours. Astrologers and their clients cannot tell the difference. No wonder astrology seems to work.

Take a set of charts or a set of authentic and switched charts. Get descriptions of their owners. Can astrologers match charts to owners? In astrology books they do it all the time. So we expect the proportion of successful matches to pile up close to 100% as shown below in grey. But in 47 studies totalling 700 astrologers and 1200 birth charts it was more like 50% as shown in red. The astrologers (many of them among the world's best) were no more successful than tossing a coin. They could not tell the difference between authentic and switched charts.

Before deciding whether astrology works we need to test switched data. We must stop asking if Leos are generous and ask instead if Leos are more generous than non-Leos. Otherwise our tests will be meaningless. So scientists have the perfect answer � just try it. Put switched data to the test. See for yourself whether astrology really works. What could be more reasonable?

But during twenty centuries astrologers have rarely tested switched data. So they cannot know whether astrology works. Even when they get a good fit to a chart later found to be wrong, they see it as proof of astrology's subtlety. They argue that astrology is right even when it is wrong. Vested interests 1, scientific integrity 0.

The case against astrology is that it is untrue. It does not deliver benefits beyond those produced by non-astrological factors, it has not contributed to human knowledge, it has no acceptable mechanism, its principles are invalid, and it has failed hundreds of tests. But no hint of these problems will be found in astrology books, which in effect are exercises in deception. But it doesn't end there.

Astrologers disagree on almost everything, even on basics such as which zodiac to use. They rarely test switched data, which is why scentists see astrologers as crazy or even crooks. In fact astrologers are mostly nice people who genuinely wish to help others. But the claim they repeatedly make (astrology is true because based on experience) is simply mistaken � what they see as its strength (experience) is actually its weakness (no switched data).

The case for astrology is that a warm and sympathetic astrologer provides low-cost non-threatening therapy that is otherwise hard to come by. You get emotional comfort, spiritual support, and interesting ideas to stimulate self-examination. In a dehumanised society astrology provides ego support at a very low price. Where else can you get this sort of thing these days?

In short, there is more to astrology than being true or false. But note the dilemma to get the benefits you have to believe in something that is untrue. The same dilemma can apply elsewhere as in psychotherapy and even religion, so it is not unique to astrology. Nevertheless it presents an ethical problem that astrologers have generally failed to recognise let alone resolve.