Effectiveness of the forced-choice coin test for detecting malingering during forensic psychiatric examinations

In general clinical psychiatric practice, open questions are favored over closed ones because they are considered more therapeutically effective and less likely to make the patients pander to us. However, in forensic psychiatric examinations, suspects may attempt malingering.

Using a simple examination based on a forced-choice technique, the author proved that the level of intelligence of a theft suspect pretending to have an intellectual developmental disorder was not so low. The author prepared two sets consisting of a few coins each and asked the suspect to choose which set had a higher total value. The suspect was questioned repeatedly over multiple trials. He always selected the wrong set over the course of more than 10 trials.

If the suspect really did not know the correct answer, the probability of getting the answer right or wrong in a binary choice question is 50% for both. The probability of answering the question wrong by chance 10 times in a row is (1/2),¹⁰ in other words, about 0.1%. It was evident that the suspect intentionally kept answering incorrectly. When suspects who pretend to have an intellectual developmental disorder answer only "I do not know" to all questions without actively playing out the symptoms, it is difficult to demonstrate that the symptoms are psychiatrically conflicting and prove that they are malingering. Even in such cases, this type of test based on a forced-choice technique can be used to prove that suspects are behaving falsely.