Dowsing is the ability to detect hidden materials or objects using non-scientific means. A common form is water divining, where the person can detect underground water using devices such as divining rods. Other common forms attempt to detect valuable minerals, oil, lost objects, etc. The general concept is the same. (see note 1)
The supposed mechanism behind dowsing works vary depending on the source (see note 2). Sometimes mystic forces are invoked, sometimes there is a pseudo-scientific reliance on magnetic or electric fields, other times the explanation is simply that we don't know why, but it works.
Strangely enough, there is some evidence that it actually might work sometimes (see note 3). If it does work, the most likely reason is the ideomotor effect (see note 4) combined with the dowser's subconscious knowledge of what he is trying to detect. Basically how this works is that the diviner is subconsciously interpreting the environment looking for clues about where the water (or other target) might be. Without realising it, he causes the divining rods (or other prop) to move when the target is likely to be near. So the target is found. There is nothing mystical about this explanation. Its just based on well known psychological phenomena and human nature, as well as knowledge the person may or may not consciously realise they have.
Other possible explanations include confirmation bias (see note 5). This basically means that the people who believe in this phenomenon take a lot of notice of the successes, but ignore the failures. This is a common effect in many paranormal effects, and is one reason why proper scientific investigation is necessary to determine whether something really works or not. There will always be a certain success rate for an event, even if its quite unlikely, so its important to allow for this success which occurs purely through chance.
In fact, the chances of success are probably not as low as many people assume. For example, dowsing is popular in Australia. This might be because Australia is very dry on the surface, but sits on top of a huge water table so that the chances of finding subterranean water are actually relatively high (see note 6).
There has been some research, both formal and informal, into how dowsers operate, and the ideomotor effect has been demonstrated quite conclusively (see note 7). When people are told to use dowsing skills to detect an object and are then given reason to believe the object is in a particular place the dowsing technique will indicate the object is there, whether it really is or not. So its not whether the object exists or not that matters, its whether the dowser thinks it exists. This is a clear indication of a subconscious psychological effect and is a common finding in psychological research.
Dowsing has been a popular phenomenon used in attempts to claim the $1 million JREF prize (see note 8). In every case the dowser has failed. Many claim this is because the testing procedure is unfair, but the testing is always done on a basis which is agreed by the person being tested and the JREF so that really isn't valid. The same observation applies to other phenomena JREF test, such as psychic powers, etc.
So, in summary we know the following: First, dowsing often doesn't work, but adherents tend to take more notice of the successes and ignore the failures which makes it seem more successful than it really is. Second, when it does work its usually due to the knowledge and experience of the dowser operating through subconscious effects such as the ideomotor effect. Third, the dowser could detect the target just as easily, or better, using more scientific instruments and analysis.
Some people might be able to use subtle, sub-conscious cues to achieve limited success in dowsing, but there's nothing really extraordinary happening there. Therefore I give this a moderately high score on the crap-ometer!
1. There is a good, short description of dowsing, along with a summary of its history, and some notes on criticism of the phenomenon, at Wikipedia.
2. Here are a few examples of explanations I found on various web sites: pendulums are a powerful antenna that receives information from the vibrations and energy waves emitted by people, places, thoughts and things; the pineal gland is the fulcrum, the pivotal point and nodal point of our consciousness [it goes on]; all living things (and many non living things such as water...) have an energetic field [plus more pseudo-science].
3. Some researchers have produced positive results - for example the "Scheunen" experiment carried out in 1987 and 1988, which involved 10,000 double-blind tests set up by physicists near Munich. But the methodology of many tests has been suspicious, and no one has been able to design a test which produces replicable results.
4. The ideomotor effect was first described over 100 years ago by William Benjamin Carpenter while researching a ouija board effects. Research by Hyman in 1999 indicates that intelligent, honest people can believe there is an external influence in action when the real effect is entirely caused by the ideomotor effect.
6. The water tables were formed millions of years ago when parts of Australia were covered by oceans and the water became trapped underground as the land dried out.
7. For example, Ray Hyman (1999) The Mischief-Making of Ideomotor Action. The Scientific Review of Alternative Medicine (Fall-Winter).
8. JREF is the James Randi Education Foundation, which is an organisation set up by famous magician and skeptic James Randi to investigate, debunk and educate people on paranormal subjects. The million dollar prize is fairly administered and the rules are clearly stated, yet despite many attempts there has never been a success in claiming it. In fact many high profile psychics have found convenient excuses to not even try!
Sources of Further Information
There are many web sites with information on this subject. Below I have shown some which present the information for both sides of the argument.