Final answer:
The presumption behind the polygraph is that lying is associated with physiological reactions that can be measured, but its reliability is questionable due to variable factors affecting physiological responses.
Step-by-step explanation:
The presumption behind the polygraph is that there are distinctive physiological responses associated with lying. Polygraphs measure various physiological indicators such as heart rate, respiration, and sweating to detect signs of stress and arousal that are often presumed to accompany deception. However, the accuracy of polygraphs is controversial due to the lack of evidence supporting the idea that lying produces a unique physiological response pattern. Factors such as emotional state, beliefs, and individual physical characteristics can influence these physiological responses, leading to potential false positives or negatives making the polygraph an unreliable tool for determining truth.