Andrewmcleod wrote: "The other complication to note is that in many cases people may have visual cues, from years of experience of poking around caves and digs, that they aren't consciously aware of. It's not the dowsing.".
I'm glad you said that (and I agree). A certain person we tested the effectiveness of as a diviner arrived with good prior knowledge of the layout of the known caves in the area in question. (Uncannily good; it was as if he'd read up on it just beforehand.) When he started he chose to begin from a known cave entrance, from where he "divined" in the general direction of another known cave which even a basic understanding of cave developement would suggest is the likely route of the newly discovered passages of ours. Unsurprisingly, he strode confidently off in the expected direction.
But our survey showed the passage headed at 180 degrees away from his dowsed "survey". His demonstration of the "effectiveness" of dowsing was hardly a blind test - and he rarely refers to this one when discussing his "successes".
On another occasion, I witnessed the late Monty Grainger dowsing, up on the Ingleborough Allotment. (Monty passed away many years ago so I'm comfortable in naming him; those who knew him would agree he was something of a character.) It was when Lizard Pot was being dug into. Monty strode off and the rods twirled as he went over the line of the cave as shown by the survey, which he genuinely hadn't seen. I was (as a young and gullible caver) rather impressed. It was only later that he let slip his knowledge of where the Rift Pot fault lay - and of course it was a fair bet that Lizard Pot was closely associated with it. This fact had been well discussed before the day of the dowsing in conversations which Monty would almost certainly have heard. But he was convinced of his strong "reaction".
I have a genuinely open mind on dowsing and if it can be shown to work (through proper experimentation) I'll take more notice of it. But as any "results" are clearly so unreliable, my current opinion is that the method is worse than useless as it merely provides a distraction from the truth and clouds logical thinking.
The above is said with absolutely no intended offence to anyone who uses dowsing.