Don’t always trust the results of lie detector (polygraph) tests! The Neill Group (TNG) do not use lie detectors for any investigation.
When the facts are disputed and trust has broken down, people often ask whether Lie Detectors (Polygraph) tests can provide a clear answer. The honest answer is more measured: a properly conducted polygraph examination can be a useful investigative tool, but it is not a machine that simply identifies truth or deception.
For employers, legal representatives, insurers and private clients, the value lies in using the examination at the right point in an investigation, with informed consent, appropriate safeguards and realistic expectations. It can help test information, identify issues requiring further enquiries and support decisions about where an investigation should focus next. It should never replace evidence, sound judgement or a fair process.
What lie detectors actually measure
The term “lie detector” usually refers to a polygraph. A polygraph does not read thoughts or directly detect lies. It records physiological responses while a person answers carefully structured questions. Depending on the equipment and process, this may include breathing patterns, pulse, blood pressure, skin conductivity and movement.
The underlying principle is that a person may show a measurable stress response when answering a relevant question deceptively. However, stress is not the same as dishonesty. An innocent person can be anxious, frightened, exhausted, unwell or intimidated by the situation. Equally, some people may appear outwardly calm while being deceptive.
That is why professional polygraph work relies on far more than a chart or a single reaction. The examiner’s preparation, the wording of questions, the person’s medical and personal circumstances, and the quality of the pre-test interview all matter. Questions must be narrow, clear and capable of a direct answer. Broad questions such as “Are you a trustworthy person?” are not useful. A question tied to a defined event, date, action or item is far more meaningful.
When a polygraph examination can assist
A polygraph examination is most useful where there is a specific factual issue that cannot be resolved through records, witness accounts, CCTV, digital evidence or ordinary enquiries alone. It may be considered during investigations into suspected theft, fraud, unauthorised disclosure of information, workplace misconduct, relationship disputes or allegations involving missing property.
For a commercial client, the examination may assist where a small group of people had access to stock, funds, confidential information or a restricted area, but the available evidence does not identify a clear offender. It can also help investigators assess conflicting accounts and decide which lines of enquiry warrant further attention.
For a private client, it may be considered where there are serious, specific allegations between parties and both the emotional and financial cost of uncertainty is high. These matters require particular care. A polygraph is not an appropriate shortcut for controlling a partner, pressuring a family member or resolving ordinary relationship insecurity. The person being tested must participate voluntarily and understand what the process involves.
There are also situations where a polygraph is unlikely to add value. If objective evidence can be obtained through financial records, access logs, mobile data, surveillance or witness enquiries, those sources should usually be prioritised. If the allegation is vague, the questions cannot be fairly framed, or a person is not capable of informed consent, an examination may be unsuitable.
Consent is not a formality
A valid examination begins with voluntary, informed consent. The person should understand why the test is proposed, what subjects will be covered, who will receive the results and how those results may be used. They must be free to decline or stop the process.
This is especially significant in workplace matters. An employee may feel pressure to agree even where no express threat has been made. Employers should seek appropriate employment and legal advice before proposing any test, particularly where disciplinary action may follow. A polygraph result should not be treated as a substitute for a proper investigation, an opportunity to respond to allegations or a fair employment process.
Consent also has a privacy dimension. The questions asked, the information disclosed during the pre-test interview and the final report can contain highly sensitive personal information. Access should be limited to people with a legitimate need to know. Information should be collected, stored and disclosed consistently with applicable privacy obligations and the agreed scope of the assignment.
A reputable provider will explain these boundaries clearly rather than promising certainty or encouraging a rushed decision.
What a properly managed process looks like
The process should start well before the examination itself. An investigator needs to understand the allegation, review the known facts and identify the precise questions the examination can reasonably address. This stage prevents a common failure: testing the wrong issue because the client has not separated suspicion from evidence.
A pre-test interview then gives the examinee an opportunity to discuss the matter, understand the questions and raise relevant health, medication or wellbeing issues. The examiner should not proceed where a person appears unable to participate freely or safely. There may be reasons to defer or decline an examination, including acute distress, medical concerns, intoxication or an inability to comprehend the process.
During the examination, questions should be asked in a controlled, consistent manner. They should avoid assumptions, double meanings and emotionally loaded language. For example, a useful question may concern whether a person removed a specified item from a particular location during a defined timeframe. It should not combine several allegations into one question or require the person to interpret unclear terms such as “steal”, “betray” or “misuse”.
The final output should be a professional report that records the scope, relevant limitations, consent process and examiner’s findings. It should distinguish between factual material, the examinee’s account and the examiner’s assessment. A report that overstates certainty is not serving the client well.
How much weight should results carry?
Polygraph results should be treated as one source of information, not the final word. Their greatest practical value is often investigative direction. A result may support the need for further document review, a targeted interview, asset tracing, digital enquiries or another evidence-gathering step. It may also reveal that an allegation needs to be reconsidered rather than pursued on assumption alone.
The reliability of an examination depends heavily on circumstances. Question quality, examiner competence, the examinee’s condition and the presence of independent corroborating evidence all affect the usefulness of the result. No examination can remove every uncertainty from a contested matter.
For this reason, decision-makers should avoid language such as “proven guilty” or “proven innocent” based solely on a polygraph outcome. In a business setting, decisions involving employment, contracts, insurance or recovery action should rest on the full body of available evidence. In personal matters, the result should be handled carefully and without escalation or public disclosure.
Legal and privacy considerations in New Zealand
Whether a polygraph result is relevant or admissible in a legal proceeding depends on the forum, the purpose for which it is offered and the circumstances in which it was obtained. It should not be assumed that a result will be accepted as evidence in court, or that it will determine the outcome of a dispute.
Where criminal allegations, family violence, harassment, fraud, employment consequences or civil proceedings may be involved, independent legal advice is sensible before arranging an examination or relying on its outcome. A polygraph process must never interfere with a police investigation, discourage a person from reporting a crime or place a vulnerable person at further risk.
Privacy, confidentiality and data handling must also be considered from the outset. Sensitive reports should not be circulated casually among managers, colleagues, family members or social media contacts. The more serious the allegation, the more disciplined the handling of information needs to be.
Choosing a private investigator for sensitive personal and business matters
If you are considering a polygraph examination, ask practical questions. Who will conduct it? What experience do they have? How will consent be obtained and recorded? What questions are proposed? How will personal information be protected? What will the report say, and what are its stated limitations?
You should also expect a private investigator to challenge an unsuitable request. A professional service is not measured by how quickly it agrees to test someone. It is measured by whether it can assess the risk, define the issue properly and recommend the most defensible path to reliable information.
For complex or urgent matters, TNG’s investigative approach is built around lawful enquiries, discreet handling and evidence that can be acted on. A polygraph may have a place within that work, but the strongest outcomes come from combining careful process with verified facts.