Why are polygraph results not always accepted as evidence in court?

Polygraph results are often excluded from judicial proceedings because their scientific validity is widely questioned by psychologists and legal experts. While the polygraph is designed to measure physiological indicators—such as heart rate, blood pressure, and respiration—it primarily detects nervous excitement rather than deception itself.123

The primary reasons for their limited admissibility include:

• Inability to distinguish stress: Critics argue that the device cannot reliably differentiate between the physiological responses caused by lying and those caused by general anxiety, nervousness, or fear, which are common during interrogations.132
• Susceptibility to manipulation: Subjects can potentially influence their own physiological responses, making it difficult for examiners to definitively determine truthfulness.3
• Lack of universal reliability: Because the test relies on the interpretation of physiological data under specific conditions, it is not considered a surefire method for establishing guilt or innocence.32

Due to these significant concerns regarding accuracy and the potential for error, polygraph tests are generally not admissible as evidence in the United States, England, or many other jurisdictions.24