Racial biases in polygraphs and their legal implications

Widely used in research since the 1800s, SCR (also known as the galvanic skin response or electrodermal activity) measures changes in the electrical conductivity of the skin due to sweat gland activity. SCR has historically been considered a ‘gold standard’ for the objective biological measurement of fear and anxiety. However, myriad emotional, cognitive and physical factors can influence sweating and thereby SCR magnitude, which demonstrates it indexes general arousal. Individuals may exhibit increased SCR when stressed (for example, being deceptive or simply attempting to understand a difficult question) or for reasons unrelated to stress (for example, positive emotions such as happiness or non-emotional processes such as focusing attention⁴). As such, SCR has inherently poor specificity and discriminant validity as a measure of anxiety⁵.

Since the 1920s, SCR has been a major component of polygraph testing, which has entrenched it in the criminal legal system⁴. Currently, polygraph examinations remain controversial owing to concerns about reliability and validity. Although judges have substantial discretion over whether polygraph results can be presented to the jury⁶, about half of US states still allow polygraph evidence with stipulated agreement by both the defence and prosecution before administering the test. Furthermore, polygraph examinations are also widely accepted as evidence in criminal cases across Europe, in civil cases in China and for all cases in Colombia⁶. Polygraphs are also used at earlier stages of law enforcement to verify witness statements and to justify further interrogation of suspects⁶, and at later stages to track progress under court supervision (for example, monitoring of individuals convicted of a sexual offence).