Lie Detectors Don’t Catch Lies — They Catch Nerves

A polygraph measures changes in heart rate, blood pressure, breathing patterns and skin conductance (sweating). The core assumption is that deception causes stress, which then produces measurable bodily responses. The flaw lies in equating stress with dishonesty.

“Polygraph machines measure physiological responses such as heart rate, blood pressure, breathing patterns and skin conductivity,” explains Dr M Gokul Reddy, senior consultant cardiologist at Apollo Hospitals. “These are indirect indicators of stress — not deception itself.”

On the podcast Good Hang With Amy Poehler, Dakota Johnson recalled seeing others’ lies quickly flagged on polygraphs. When Poehler joked her test might have been rigged, Johnson attributed it to her naturally low blood pressure — joking it could make her a “good spy.”

According to Dr Reddy, this isn’t far-fetched. “Individuals with low blood pressure, slower heart rates, or a calmer autonomic nervous system may not show the stress responses polygraphs rely on,” he says. “Some are physiologically less reactive, so even deliberate falsehoods may not trigger measurable changes.”

Why arousal is not the same as deception

Mental health experts say the issue runs deeper. “Physiological arousal is not specific to deception,” says Dr Jyoti Kapoor, psychiatrist and founder-director of Manasthali — Mental Health & Wellness Services. “Anxiety, fear, embarrassment, trauma, or performance pressure can trigger the same responses.” Anxious but truthful individuals may seem deceptive, while calm, emotionally detached liars can pass. “Those trained in emotional control can sometimes beat the test,” Dr Kapoor adds. “Polygraphs measure emotional responsiveness, not truth, penalizing the anxious and favoring the composed”

Why polygraphs fail the science test

Clinical studies show that individuals with psychopathic or dissociative traits often exhibit limited emotional arousal when lying, making deception harder to detect. Others deliberately use countermeasures — like controlled breathing, mental arithmetic, or self-induced discomfort — to manipulate results.

Medical factors further complicate results: cardiovascular conditions, medications, anxiety disorders, and prior stress can all alter physiological signals, making interpretation unreliable. This is why courts remain cautious. Professional bodies rate polygraph accuracy as only moderate, and many countries, including India, do not admit results as evidence. “The biggest myth is that polygraphs reveal truth,” says Dr M Gokul Reddy. “They reveal physiological arousal — and arousal does not always equal lying.”

Experts caution against overestimating lie detectors; until science can clearly separate emotion from intent, they remain far from foolproof — and a calm heart can sometimes outsmart them.

What the law says

The Supreme Court has made it clear in Selvi vs State of Karnataka (2010) that lie detector and narco-analysis tests can be conducted only with the accused’s voluntary consent, recorded before a Judicial Magistrate and with legal representation. Any involuntary test violates Article 20(3) of the Constitution and its results cannot be treated as material evidence.

This position was reiterated in Amlesh Kumar vs State of Bihar (2025), where the court held that an accused has no indefeasible right to demand a narco or lie detector test.”

— Fizani Husain, Supreme Court Advocate & Former Public Prosecutor, Delhi High Court

Highlight : detectors got it wrong: In the US, serial killer Gary Ridgway passed a polygraph in the 1980s, delaying his arrest for years. In India, polygraph and narco tests conducted during the Aarushi Talwar and Nithari killings investigations were later disregarded by courts due to questions over reliability.

Quote: People with lower autonomic reactivity may not show measurable changes even when lying. Polygraph machines measure stress responses.”

— Dr M Gokul Reddy, Senior Consultant

Cardiologist, Apollo Hospitals