What is a polygraph test?
A polygraph test, also known as a lie detector test, is a device used for lie detection.
While one is asking and answering a series of questions posed by the relevant authorities, it measures and records several psychological indicators such as: –
- Pulse;
- Breathing;
- Blood pressure; and
- Skin conductivity.
The rationale in the use of the polygraph is that untruthful answers will produce physiological responses that are different from those associated with answers answered truthfully.
Who will take a polygraph test?
Anyone who is assisting in police investigations may be asked to take a polygraph test.
When can a polygraph test be taken?
There is no determinative time during the course of police investigations where one will be directed to take a polygraph test.
One who is assisting in police investigations may be directed to take a polygraph test by the police authorities anytime during the police investigation process.
In Took Leng How v Public Prosecutor [2006] 2 SLR(R) 70; [2006] SGCA 3, arrangements were made for the accused to undergo a polygraph test some three (3) weeks after several rounds of questioning by the police. No formal arrest was made at that time.
Where can a polygraph test be taken?
The polygraph test will be taken at a place directed by the Investigation Officer.
Why is a polygraph test taken?
This is one of the methods employed by the police to determine if one is telling the truth during police investigations. This is part of the police investigations.
How long does a polygraph test last?
According to the American Polygraph Association, a polygraph test may take between 2 to 3 hours. There are three phases. Please refer to the table below for more elaboration.
| S/N | Phase | Elaboration |
| 1 | Pre-test interview |
|
| 2 | Collection of Charts |
|
| 3 | Analysis of Charts |
|
A series of questions may be asked by the polygrapher. These may include questions relevant to the crime and intentional questions to set a baseline for a true and false result. For the latter question, the subject may be asked to lie to an irrelevant question.
How does a polygraph test work?
A polygraph test traces the changes in a person’s physiological conditioning while being questioned. These changes will be recorded directly on the polygraph charts for further review. Upon the final chart’s completion, the examiner analyses the charts to determine whether the answers to the questions indicate truth or deception.
During the Collection of Charts (Phase 2) in Question 6 above, these are the components that may be attached to the subject to generate the polygraph charts:
| S/N | Components | Elaboration |
| 1 | Pneumograph |
|
| 2 | Electro Dermal Activity (E.D.A) |
|
| 3 | Cardiosphygmograph |
|
Is a polygraph test admissible evidence in Court?
No. As highlighted by the Deputy Public Prosecutor in Public Prosecutor v Ling Chengfeng @ Ling Koh Hoo [2018] SGDC 77 at paragraph [103], a polygraph test result is inadmissible in Singapore.
It is worth pointing out, in the case of Siew Yit Beng v Public Prosecutor [2000] 2 SLR(R) 785; [2000] SGHC 157 at paragraph [37], it was held that the act of conveying the results of the polygraph test to the examinee would not render any subsequent confession inadmissible. This is provided that nothing that could be construed as inducement, threat or promise was done.
Is a polygraph test determinative of one’s guilt?
No. This is one of the methods employed by the police to determine if one is telling the truth during police investigations.
The police will consider all evidence in totality before recommending a charge to the Prosecution.
Can I say ‘No’ to a polygraph test?
Yes, you can refuse a polygraph test. A polygraph test can only be taken upon one’s consent.
Can I have someone to accompany me while I am taking a polygraph test?
Usually no. However, this is up to the police’s discretion on whether they allow your lawyer or any family member to be present while you take the polygraph test.
The police are under no obligation to do so.
Do I have to answer all the questions during a polygraph test?
The polygraph test is part of the police investigations.
In the case of Public Prosecutor v Lye Yoke Ping Jenn [2016] SGDC 337 at paragraph [27], the Investigation Officer informed the Court that use of a polygraph test is optional and not standard procedure. It is for the benefit of the suspect(s) and no adverse inference can be drawn from the refusal to take the polygraph test.
This would likely mean that the refusal to answer questions during the polygraph test would not result in any adverse inference being drawn against you.
Do I get a copy of my results of the polygraph test?
Typically, the police will inform you the results of your polygraph test. Whether or not you get a copy of your results of the polygraph test is up to the police’s discretion.
What are some factors that can affect the results of a polygraph test?
As mentioned in Question 1, the polygraph test measures physiological responses such as pulse, breathing, blood pressure and skin conductivity.
However, these responses may be influenced by psychological factors, such as embarrassment, outrage or distress. Such emotional responses are particularly common when the questions involve highly sensitive and potentially distressing matters.
Consumption of alcohol or drugs prior to the polygraph test may also impact the results.