Most of us have heard of or seen a dramatization of a lie detector test being given to a suspect. Often an innocent suspect is depicted confidently refuting accusations while hooked up to a machine with moving needles that remain steady, supposedly indicating he or she is speaking the truth. Or, a guilty suspect is attempting to hide his or her guilt by fooling the machine only to learn that the body doesn't lie.

​

Lie Detector

Lie detectors, or polygraph tests, measure a person's involuntary physiological response to psychological arousal. A person's sympathetic nervous system is activated in stressful situations, such as being caught committing a crime or lying. Lie detectors measure signs of an activated stress response by measuring the following types of information:

​

Galvanic Skin Response/Electrodermal Activity

Sweaty palms? Must be lying! The electrodermal activity (EDA), or the electrical conductivity of your skin can be measured. Salt water, such as sweat, increases conductivity. When a person is psychologically aroused the autonomic system is activated. Sweat is a normal response to psychological arousal. Be careful here--arousal is a deliberately vague term. Measuring increased EDA does not help us understand the cause of the arousal.

Respiration Rate, Heart Rate & Pupil Dilation

When the limbic system perceives a threat, your body's stress response is activated. There are several observable signs of a person with an activated stress response: increased and shallower respiration, increased heart rate and dilated pupils. Be careful here too--observing signs of an activated stress response does not help us understand the cause of the stress response.

​

​

Why Law Enforcement Do Not Routinely Use Lie Detectors/Polygraph Tests

A person's autonomic nervous system can become aroused for many reasons, and his or her individual response to those conditions varies, making it challenging to standardize measurements of those responses as well as to determine the cause of the arousal. Arousal can be anger, fear, anxiety, being startled, excited, or mental stress. Environmental conditions, such as ambient temperature, can also affect the autonomic response.