John Hale, of Dunedin, asks :-
Is there a scientific basis for the practice of water-divining? Why does the related folklore stress that the dowser does it with a pronged hazel branch?
Vicki Hyde, a representative of the New Zealand Skeptics organisation (www.skeptics.org.nz), responded.
For hundreds of years people have claimed to be able to find underground water, lost people, or other hidden materials using a rod, forked stick or pendulum in the process called dowsing.
Some have claimed it works via paranormal powers, others have put forward various electromagnetic or geological explanations. A more likely explanation involves well-known factors such as the ideomotor effect, where involuntary muscle action makes things swing or waver, and post hoc reasoning and confirmation bias, where positive results are celebrated and failures ignored or explained away.
The Australian Skeptics have offered diviners numerous chances to demonstrate their powers over the past 30 years in hundreds of agreed-upon tests; they even produced a Great Water Divining DVD collating material from 1980 to 2003.
Like other carefully controlled trials conducted around the world, the results have been dismal. In one test, the diviners predicted that their success rate would be better than 92%; the actual result was 13.5%, only very slightly above basic chance. The only time dowsers have done significantly better than chance is when they knew where the sought-after material was before they started.
Despite this consistent failure, extraordinary claims, and successful businesses, have been built around this practice. Desperate drought-affected farmers have forked out thousands for dowsing services; oil companies have invested in searches using a map and a pendulum.
A more updated version has seen modernised metal dowsing rods backed up with fancy-looking electronic circuitry (often non-functioning!) which claim to be able to discover everything from lost golf balls to hidden bombs. They have proven as equally useless as the traditional hazelwood stick.
skepdic.com/dowsing gives an historical view.