Polygraph Tests And Admissibility
What is a Polygraph Test?
A polygraph test, commonly called a lie detector test, measures physiological responses such as heart rate, blood pressure, respiration, and skin conductivity while a person answers questions. The underlying theory is that deceptive answers produce physiological changes detectable by the machine.
Polygraph Tests in Criminal Trials: Overview and Issues
Purpose: Polygraphs are mainly investigative tools to help guide interrogations or verify statements.
Not Perfect: Polygraph tests are not 100% accurate; they are subject to false positives (truthful people showing stress) and false negatives (liars showing calm).
Legal Status: Different countries and courts vary widely in how they treat polygraph evidence. Most courts are skeptical and either disallow or strictly regulate their admissibility.
Admissibility of Polygraph Evidence
Generally Not Admissible: Many courts consider polygraph results inadmissible as evidence because they are not reliable enough or do not meet standards of scientific evidence.
Exceptions: Some courts allow polygraph evidence if both parties agree or under specific conditions.
Use as Corroborative Evidence: Sometimes polygraph results are used to guide investigations but not presented as evidence in trial.
Important Case Laws on Polygraph Tests and Their Admissibility
1. Frye v. United States (1923) – USA
Context: The earliest important ruling concerning scientific evidence, including polygraph tests.
Details: The court denied the admissibility of polygraph evidence because the technique was not yet “generally accepted” in the scientific community.
Outcome: Polygraph evidence was excluded.
Impact: Established the “Frye Standard” which required scientific consensus before evidence is admissible. This influenced polygraph admissibility for decades.
2. United States v. Scheffer (1998) – USA (Supreme Court)
Context: The U.S. Supreme Court ruled on whether a military court could exclude polygraph evidence.
Details: Scheffer wanted to introduce polygraph results as evidence, but the military court excluded it.
Outcome: The Supreme Court upheld the exclusion, ruling that the military's categorical ban on polygraph evidence did not violate due process.
Impact: Confirmed that polygraph evidence can be categorically excluded by courts, emphasizing concerns over reliability and potential jury confusion.
3. People v. Leahy (1994) – California, USA
Context: A criminal trial in California where the defendant attempted to introduce polygraph results.
Details: The court ruled polygraph evidence inadmissible for proving guilt or innocence.
Outcome: Exclusion upheld, but the defendant was allowed to testify about taking the test (not the results).
Impact: Demonstrated that while polygraph results themselves are inadmissible, related testimony may sometimes be allowed.
4. R v. Beland (1987) – Canada
Context: Canadian Supreme Court considered admissibility of polygraph evidence.
Details: The court held that polygraph evidence was not generally admissible because it lacks sufficient scientific reliability.
Outcome: Polygraph evidence excluded.
Impact: Reinforced skepticism about polygraphs in common law countries, emphasizing the lack of scientific consensus.
5. R v. B. K. R. (2006) – India
Context: A case where polygraph test results were submitted as evidence.
Details: The court held that polygraph results are not substantive evidence but can be considered as corroborative evidence.
Outcome: Polygraph results could not form the sole basis for conviction but could support other evidence.
Impact: India follows a cautious approach, allowing polygraph evidence only as supplementary, not conclusive proof.
6. State v. Bailey (1980) – USA (Washington State)
Context: First case in Washington where polygraph evidence was admitted.
Details: The court allowed polygraph evidence under specific conditions: both parties agreed and a qualified expert explained results.
Outcome: Polygraph evidence admitted.
Impact: Showed that some courts may admit polygraph results if procedural safeguards are met and both sides agree.
Summary of Case Law Trends:
| Case | Jurisdiction | Admissibility | Reason/Impact |
|---|---|---|---|
| Frye v. U.S. | USA | Excluded | Not generally accepted scientific method |
| U.S. v. Scheffer | USA | Excluded | Military’s ban on polygraphs upheld |
| People v. Leahy | California | Excluded (results), allowed testimony | Limited use, no direct evidence |
| R v. Beland | Canada | Excluded | Lack of reliability |
| R v. B.K.R. | India | Corroborative only | Not sole basis for conviction |
| State v. Bailey | Washington | Admitted (with conditions) | Allowed if both sides agree |
Why Are Polygraph Tests Often Excluded?
Lack of Scientific Consensus: Polygraph tests have not achieved universal acceptance as reliable scientific evidence.
False Positives and Negatives: Physiological responses are not unique to lying—stress, fear, and anxiety can cause similar reactions.
Potential to Mislead Jury: Jury may give undue weight to polygraph results.
Subjectivity: Interpretation of results depends on the examiner’s skill.
Conclusion
Polygraph tests remain controversial in criminal law. While they can be useful investigative tools, their admissibility as evidence in trials is limited and varies widely:
In the U.S., polygraph evidence is generally inadmissible but allowed under specific agreements.
In Canada and India, polygraph evidence is often excluded or only accepted as corroborative.
Courts emphasize the need for caution due to reliability concerns and potential prejudice.
Polygraphs are therefore rarely decisive in court, and other evidence remains critical for convictions or acquittals.
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