Lie Detector Test: How Does a Polygraph Examination Work?

A lie detector test sounds like a TV drama: wires, needles, one dramatic question. In reality, a polygraph examination is a calm, structured procedure with clear rules. This guide explains each step, what the sensors measure, how scoring works – and where the method's limits are.

At DV Company in Hamburg we offer the polygraph for companies as a supplementary tool for applicant screening, regular checks of key staff and internal investigations. Our founder Vadim Dischewski has worked in close protection since 2008; discretion and careful procedure go hand in hand for us.

How does a lie detector work?

A polygraph does not "detect lies". It records several physical reactions at once, controlled by the autonomic nervous system and hard to influence deliberately. The examiner compares how strong these reactions are across different types of questions and draws a professional assessment from the pattern – not a certainty.

Typically recorded:

• Breathing – two elastic belts around chest and abdomen.
• Skin conductance – electrodes on the fingers, reflecting sweat-gland activity.
• Blood pressure and pulse – a cuff on the upper arm, as at the doctor's.
• Movement – depending on the device, a seat sensor that flags movements which could distort the recording.

None of this is painful. The cuff may feel tight during longer series, which is why there are breaks.

The procedure step by step

1. Clarifying the assignment. With the client we define the issue and the exact question. Sometimes the honest answer is that a polygraph is not the right tool.
2. Consent. The test is voluntary and requires written consent. The person is told what is measured, why and who gets the results, and may stop at any time.
3. Pre-test interview. The longest and most important part. The examiner explains the technique and all questions are worded and reviewed together in advance – no surprise questions.
4. Questions. A mix of neutral questions, relevant questions on the actual issue and comparison questions that provide a reference for reaction strength.
5. Recording. The person sits still and answers only "yes" or "no". Questions are asked in several rounds, often in varying order, with pauses.
6. Scoring. The examiner evaluates the charts using a defined scheme. The outcome is one of three: no significant reactions, significant reactions, or inconclusive.
7. Debrief and report. The client receives a written report describing the procedure, the assessment and its limits.

How long does it take?

The recording itself is fairly short, but with the interview, several rounds and the debrief you should plan for several hours. A serious test cannot be rushed.

What the result means – and what it doesn't

• Not evidence. In 1998 Germany's Federal Court of Justice ruled that polygraph examinations are unsuitable evidence in criminal proceedings (BGH, 1 StR 156/98).
• A supplementary tool. In a company it can help weigh information, narrow down suspicion or clear a person – but decisions should never rest on it alone.
• Errors are possible. Illness, medication or misunderstood questions can affect results; that is why "inconclusive" exists.
• Data protection. Results are sensitive data, and employment relationships are subject to specific rules (e.g. Section 26 BDSG within Art. 88 GDPR). Check with your legal adviser and, where relevant, the works council. This is not legal advice.

When do companies use it?

Applicant screening for positions of trust, regular checks of key staff, internal investigations into data leaks or theft, and suspected industrial espionage. If eavesdropping is suspected, bug sweeping (TSCM) often makes sense too. Detective work is handled by our partner AC Detective, a detective agency in Hamburg. Our core business remains close protection – see bodyguard cost for pricing.

Confidential first consultation

Considering a polygraph test in your company? In a free, confidential first consultation we discuss whether it suits your case – and what to do instead if it doesn't. Send an enquiry.