Be very cautious if you are asked to take a lie detector test. 

A lie detector test is not scientifically reliable. The test may say that you are lying when you are not. Defence lawyers stopped using this test several years ago because of its unreliability. 

If you are asked to take a lie detector test, it may be because people want to play mind games with you, to manipulate you. You may be totally thrown off balance by being asked repeatedly, “Why don’t you just tell us the truth?” If you do take the test, you may be told, “You failed the test. Now tell us the truth.” Some police interviewers may use “little white lies.” Some police officers have been known to “colour the truth.”

However, there are times when taking the lie detector test will prove your innocence and allow the police to look elsewhere. Weigh your options carefully before deciding because there are risks involved.

Mennonite Central Committee Canada. (2011). Getting through the maze: A guidebook for survivors of homicide.