What is a polygraph?

The so-called lie detector, truth machine or polygraph is a particular type of measuring instrument used to record physiological responses, generally registering variations in blood pressure, heart rate, respiratory rate, nervous stimuli, and galvanic skin response generated in response to certain questions posed to the subject undergoing the test.

How does the polygraph work?

To explain the functioning of this measuring instrument, it is necessary to expose that the brain of a healthy person reacts autonomously and involuntarily to numerous stimuli, and that these provoke various physiological reactions that can be measured through sensors placed in different parts of the body, usually on the torso, arms, and hands.

The act of lying or concealing something serious produces sudden emotional physiological reactions -activation phase- in the organism of the questioned subject, which in no way can be controlled in a brief period of time. The polygraph measures up to 25 seconds per question, enough time to detect the different phases of emotional activation. During that short period of time, blood pressure, respiration, and electrical conductivity of the skin undergo appreciable modifications, which can be measured by an algorithm that determines the probability of truthfulness or deception in the response.

Being stimulated by compromising questions produces stimuli that are practically impossible to hide. In fact, it can provoke fear or the fear of being discovered, so our body will react involuntarily, activating to face the challenge of fighting (denying/lying) or choosing to flee (confessing/admitting) in order to protect ourselves, as ultimately our most basic impulse, that of survival, will take control of our actions.

What reliability do polygraph results have?

As with other techniques, the reliability of the polygraph depends on several factors, the first of which is the type of instrument used, which is important to be properly calibrated according to the manufacturer’s standards. At Utilizamos, we use the latest generation Stoelting CPSpro polygraphs, equipped with up to 13 channels of physiological measurement. In addition, as official representatives of the brand, our equipment is constantly updated (software) and renewed.

Another fundamental factor in obtaining reliable results is the use of techniques validated by the scientific community linked to the American Psychological Association (APA) and the European Polygraph Association (EPA), who, following the guidelines of ASTM (*), promote standards and professional use standards for obtaining results with a reliability not less than 90%, having achieved in real and laboratory tests ratios ranging between 94 and 98% accuracy in the results.

To obtain results with a reliability greater than 90%, the polygraph test must consist of at least three graphs and cannot include more than 4 relevant questions, and ideal technical and environmental conditions must be observed.

The training and empirical experience of the examiner is an important factor to consider when obtaining the best results.

(*) ASTM, the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary technical standards agreements for a wide range of materials, products, systems, and services.

Can you fool a polygraph?

Certainly nothing is infallible, as both machines and people are susceptible to error, whether intentional or not.

Some people who fear that their less than honorable actions will be discovered study ways to deceive the system. In the case of the polygraph, up to three methods of deception have been described:

1. By ingesting large amounts of drugs and medications that can affect the nervous system to the point of blocking emotional response. This method is not very effective when facing an experienced examiner, as it is easy to detect during the pre-test interview. Additionally, some examiners may require the examinee to undergo a urine or MULTIDRUG TEST capable of detecting ingestion of these types of substances up to a month before the test.

2. Through voluntary body movements during the test, the examinee tries to identify the questions by making muscle tensions, anal sphincter contractions, or including a stone or nail in the shoe, so that when pressed, an emotion is transferred to the graph that alters the result.

This type of trick was resolved more than 15 years ago by the polygraph industry, with the inclusion of sensors for motor activity, under the seat, under the arms, and under the feet of the person undergoing the test.

3. Through mental countermeasures, which are the most difficult to exercise, one tries to evade the test by thinking of mental calculations with abstraction of the environment. To avoid this, the examiner will rotate the order of the questions so that the person undergoing the test cannot anticipate what type of stimulus they want to avoid through mental abstraction. In the best case scenario, this type of strategy leads to inconclusive results, and even incriminating or induced false positives.

What is the minimum age for taking a polygraph test?

The polygraph test is applicable to adults (18 years or older), provided they express their written consent. For minors over 14 years of age, the test can be administered if they are willing and their parents or legal guardians provide written consent.

In addition, the polygraph test can be applied to any person whose mental faculties are not diminished and who is aware of the scope of the test.

Are there any limitations for a person to undergo a polygraph test?

Yes, in the following cases:

• When the examiner detects that the person being evaluated is a victim of any type of violence, pressure, or coercion.

• Minors who do not have the proper authorization or the presence of parents or guardians will be excluded from evaluation. As a general rule, tests will not be given to minors under 14 years of age in any circumstances.

• People with serious heart pathologies.

• Pregnant women in an advanced state of pregnancy.

• People diagnosed with any type of psychotic mental disorder (schizophrenia, paranoid delusional disorder, schizophreniform disorder, schizoaffective disorder) even if they are under pharmacological treatment.