If you want to purchase a lie detector you must notice that it is not really easy to find something cheap, but still very accurate.
There are many lie detectors based on voice stress analysis, but no scientific paper has even been written that would support such a lie detection method.
Serious lie detectors (that are used for example in police stations) are based on following parameters:
- Skin resistance: The most important parameter - when you lie you sweat (not like while you exercise, but very small quantity of sweat). Think on a situation where you are telling some important lie. You feel high amount of stress while you are doing that. This stress induces the sweat. The whole phenomena is known in medicine for years and is called psychogalvanic reflex. You can read a lot on the Internet about this phenomena.
- Blood pressure
- Pulse
- Breathing rhythm
These parameters are quite redundant, but all together they provide very fast lie detection - with just one parameter you would probably have to repeat lie detection session for several times.
So, it is not necessary to use all these parameters. Thus, if you want to buy a lie detector that is not too expensive, one good hint would be to find a device that measures one or two of these parameters what would be much cheaper than a fully equipped device. Typically measuring of skin resistance would be the parameter that simple device would measure. Instead of purchasing a polygraph you can just purchase some low noise computer based skin resistance meter.
How to Buy a Lie Detector