In fiction, we often see a suave criminal tricking the lie detector or some low-level thug cracking under the weight of his own deception. Just how accurate are these portrayals? Can people really fool a polygraph?
To answer both those questions, it isn’t as simple as it seems. Whether your characters are innocent or guilty, they have plenty to worry about with a lie detector test. Let’s discuss what actually happens during this examination.
- 45-90 minute pre-test interview
- 15-20 minutes of questioning with the machine
- Analysis and Review of results
This is the longest part of the test. During the interview, the examiner will explain the process and the machine, though the bulk of the time will be spent discussing the issue at hand. If the subject is a personal matter such as infidelity, both parties may be present. Otherwise, the whole process is done with only the examiner and examinee. Test questions are also reviewed during this time – there should be no surprises during the exam.
Here’s a diagram of the polygraph, how it’s hooked up, and what it measures:
Examinees typically do not spend much time attached to the machine, though that may not be the case in a criminal examination. Regardless, questioning is limited to one subject at a time to avoid muddling the results. For example, someone accused of killing two people will be questioned separately about each murder as they may be innocent of one but guilty of the other. Questions will be repeated three to four times to get a more complete picture of the response.
Being attached to the machine should not hurt, though the blood pressure cuff can be uncomfortable. The examinee is generally free to stop the test at any time, though that may increase the appearance of guilt.
The results, called charts or polygrams, show the various readouts from each sensor:
These charts are typically reviewed by a computer using algorithms, then scored by the examiner.
Once the results have been analyzed, the examinee may be given a copy which they will be asked to sign and date. Before signing, they can ask to be retested on specific questions and should have an opportunity to explain any answers that are determined to be deceptive.
Rules of examination
- In most cases, the test is voluntary
- The subject should not be tested while under the influence (alcohol or drugs)
- Only Yes/No questions are allowed
- No compound questions
- No conjunctions
The Truth about Lie Detector Tests
Simply put, polygraph tests are unreliable and potentially misleading. While they do a great job of measuring the subject’s response, they’re incapable of discerning the reason for that response. Polygraph supporters claim otherwise, but an innocent person’s nervousness is indistinguishable from a guilty person’s fear. Drugs such as beta blockers can affect the results without being detectable, and psychopaths do not experience typical reactions to lying. Involuntary bodily responses can also be a factor.
Beyond that, it can be easy to manipulate examinees into false confessions. The test is a stressful situation, so by increasing that stress (narrating the readouts of the machine or pretending to detect a lie in real time, for example), examiners can lead people into saying things they don’t mean.
All these factors can completely change the results, which is why polygraphs aren’t used in court more often. Polygraphers claim an accuracy rate of 85-95%, but in reality, the test has an error rate between 25-75%.
For more insight into polygraph tests, check out Mythbusters (Season 5, Episode 24 – “Confederate Cannon”).
Dee is a moderator and blogger for Story Scribes. In her downtime she tries out various crafts, plays video games, and makes music. Currently, she’s working on a fantasy story that’s been trying to escape for a few years.