In this article (extracted from a longer one) from the early journals of the BSD, a member, Dr J. A. Simpson Emslie, applies his medical expertise to assert that dowsing reactions are purely a result of reflex actions. 

http://www.dowsing-research.net/blog_extracts/BSD_No10_1935_p106.pdf

He says that normally the brain controls these, but during dowsing, this control is relaxed, permitting the dowser’s experience. He goes on to make the claim, but it is not clear how this is substantiated, that he knew two dowsers who had suffered damage to those nerves that control reflex actions. The resulting loss of control meant that their dowsing reactions were very much more exaggerated.

He points out that a dowsing reaction can be obtained with one’s tongue, “…if it held midway between the roof and the floor of the mouth. You will find that the reaction will make it rise towards the palate.” What me might refer to as “Oral dowsing”, was the subject of  a very interesting study into the dowsing effect, undertaken at Guy’s Hospital, London, by a Dr Lintott. It was entitled “Some observations on so-called water divining” and published in Guy’s hospital gazette, June 24; 1933. A summary of the work was published in the first BSD journal:  No1 p9-10

The study involved recording the changing tension in the jaw muscle of the subjects as they traversed a water pipe. This was achieved by having the subject hold a rubber bulb in their mouth! Interestingly, “all the experiments were carried out in a strong spirit of scepticism and under critical observation, and, where possible, control experiments were made.”

It was found that the subjects fell into three groups: 1) those who were completely insensitive; 2) those for who had some sensitivity, but this could vary over time; 3) those with constant and marked sensitivity. In addition, they noticed how the subjects’ attention was important, suggesting “some action of the higher cerebral centres” which prevented them from “tuning in”.

http://www.dowsing-research.net/blog_extracts/BSD_No1_1933_p109.pdf

In conclusion, many dowsers would most likely agree with the Doctor and a lot of evidence seems to support this. However there are some documented instances of what appears to be psychokinetic influences at work. This will be the subject of a future post.