First, I want to apologize for this rather long post. But, there is an issue that has always bothered me when it comes to detecting deception. An issue which I don’t think gets near enough attention.
There is a fallacy underlying most of the research on detecting deception. And the fallacy goes something like this:
- People’s nonverbal behavior changes when telling the truth versus lying.
- If we can identify some of the nonverbal differences involved, then we train people to detect deception.
This argument seems pretty logical and straightforward, but it’s not.
There is already much debate surrounding the idea that the nonverbal cues of deception can be reliability identified. But, just for the sake of argument, let’s assume that it can be done.
Here’s where I think the real problem starts. There is a big difference between identifying nonverbal cues associated with deception, and being able to use those cues to detect deception.
Specifically, the problem is that the nonverbal cues which have been identified are based on statistically significant differences. These significant differences are not the same as diagnostic differences; that is, differences which can be used to distinguish group members from each other.
I am going try to explain this distinction using a concrete example, but first some basics about statistical significance and research on detecting deception.
To begin with, a significant difference refers to the idea that an observed outcome is probably not due to chance. And a great description of
statistically significant differences can be found on the Cancer Guide website.
But, for our purposes let's use an example. Let’s assume that we watched two groups of 60 people each. People in the fist group were instructed to tell the truth about their favorite vacation and include as many details as possible. Now, the individuals in the second group were given the same instructions, but told to lie. We could videotape everyone’s stories and count how often certain types of nonverbal behaviors occurred. Watching the tapes, we might just notice that people in the lying group touched their face more often than people in the truth telling group.
And there might even be a significant difference between the two groups with respect to this nonverbal behavior. Let’s say that, on average, liars touched their face 5 times, while truth tellers only touched their face 3 times. Even though we’ve found a statistically significant difference – a difference that is unusual – this does not necessarily mean that we can use this information to detect deception. Significant differences cannot always be used in a diagnostic way. That is, in way to reliability distinguish group members (liars from truth tellers) from each other.
Ok, let me show you a concrete example of why this can’t typically be done.
I often teach the same course during the semester - a day course and a night course. And every time this happens, the students in the day class earn better grades than the students in the night class. I think this difference occurs because students taking night course are more likely to work full-time during the day and have little extra free time for studying.
Here are 2 sets of grades from the last time this happened (see, Table 1).
Table 1 - Scores from two separate classes (full data is provided in Table 3).
Day Students' Scores
| Night Students' Scores
|
92.37
| 81.84
|
94.47
| 75.90
|
...
| ...
|
83.69
| 38.09
|
87.54
| 97.15
|
Now, the average scores for the students in the day class is 85.02 or a "B" while the average score for students in the night class is 80.31 - right around a "C+" or "B-."
And there is a significant difference in grades between these two classes – the difference observed is probably not due to chance (
t[118]=1.679,
p < .05). It’s a small difference, but it’s still a statistically significant difference. In other words, I can say with some confidence that students in my day class really did earn better grades than students in my night class. So, far so good. I’ve identified a significant difference that exists between two groups.
Or to think about it in terms of detecting deception, I’ve found two groups which statistically differ from each other – just like noticing a difference between liars and truth tellers with respect to some nonverbal behavior.
But, here is the twist and where the problem emerges. I’ve now combined the two classes and reordered their scores from highest to lowest (see, Table 2). You know the two groups of students are significantly different with respect to their grades, but can you tell them apart based on their scores?
In other words, can you reverse engineer the problem?
This is the same problem as trying to catch a liar by looking at his or her nonverbal behavior. Give it a try. Here are all of my students' scores. Which class does each student come from – the day class or the night class?
Table 2
Combined Students' Scores | Guess What Class? |
| 100.00 | D or N |
| 100.00 | D or N |
| 98.44 | D or N |
| 97.88 | D or N |
| 97.61 | D or N |
| 97.15 | D or N |
| 96.82 | D or N |
| 96.59 | D or N |
| 96.57 | D or N |
| 96.30 | D or N |
| 96.29 | D or N |
| 96.05 | D or N |
| 95.85 | D or N |
| 95.85 | D or N |
| 95.84 | D or N |
| 95.78 | D or N |
| 95.60 | D or N |
| 94.47 | D or N |
| 94.02 | D or N |
| 93.90 | D or N |
| 93.84 | D or N |
| 93.78 | D or N |
| 93.71 | D or N |
| 93.51 | D or N |
| 93.25 | D or N |
| 93.15 | D or N |
| 92.73 | D or N |
| 92.64 | D or N |
| 92.49 | D or N |
| 92.47 | D or N |
| 92.37 | D or N |
| 92.09 | D or N |
| 91.96 | D or N |
| 91.85 | D or N |
| 91.85 | D or N |
| 91.85 | D or N |
| 91.70 | D or N |
| 91.57 | D or N |
| 91.44 | D or N |
| 91.44 | D or N |
| 90.92 | D or N |
| 90.66 | D or N |
| 90.15 | D or N |
| 89.77 | D or N |
| 89.74 | D or N |
| 89.50 | D or N |
| 89.37 | D or N |
| 89.10 | D or N |
| 88.84 | D or N |
| 88.73 | D or N |
| 88.33 | D or N |
| 88.17 | D or N |
| 88.17 | D or N |
| 88.08 | D or N |
| 88.05 | D or N |
| 87.89 | D or N |
| 87.81 | D or N |
| 87.54 | D or N |
| 87.54 | D or N |
| 87.29 | D or N |
| 87.13 | D or N |
| 87.08 | D or N |
| 86.77 | D or N |
| 86.60 | D or N |
| 86.56 | D or N |
| 86.34 | D or N |
| 86.06 | D or N |
| 85.81 | D or N |
| 85.75 | D or N |
| 85.73 | D or N |
| 85.72 | D or N |
| 85.03 | D or N |
| 84.95 | D or N |
| 84.44 | D or N |
| 83.69 | D or N |
| 83.68 | D or N |
| 83.16 | D or N |
| 82.63 | D or N |
| 82.40 | D or N |
| 82.13 | D or N |
| 82.10 | D or N |
| 81.89 | D or N |
| 81.84 | D or N |
| 81.84 | D or N |
| 81.06 | D or N |
| 80.79 | D or N |
| 80.55 | D or N |
| 80.36 | D or N |
| 80.28 | D or N |
| 79.51 | D or N |
| 79.25 | D or N |
| 79.00 | D or N |
| 78.70 | D or N |
| 78.48 | D or N |
| 78.22 | D or N |
| 78.21 | D or N |
| 77.96 | D or N |
| 77.95 | D or N |
| 77.43 | D or N |
| 75.90 | D or N |
| 75.13 | D or N |
| 75.01 | D or N |
| 74.33 | D or N |
| 72.41 | D or N |
| 67.33 | D or N |
| 66.83 | D or N |
| 66.55 | D or N |
| 65.75 | D or N |
| 57.99 | D or N |
| 47.20 | D or N |
| 45.77 | D or N |
| 44.30 | D or N |
| 44.19 | D or N |
| 44.19 | D or N |
| 40.42 | D or N |
| 38.09 | D or N |
| 38.09 | D or N |
| 38.09 | D or N |
| 36.30 | D or N |
| 32.64 | D or N |
Now, even if you were to play it safe and assume that anyone with a grade above the middlemost score was most likely from my day class, and anyone below that score was in my night class… you’d still not get it right. Take a look at the data again, this time with the right answers provided.
Table 3 – Best Guess Plus Real Answer
Combined Students' Scores
| Best Guess
| Actual Class
| Correct Guess
|
| 100.00 | D | D | Correct |
| 100.00 | D | N | Incorrect |
| 98.44 | D | N | Incorrect |
| 97.88 | D | D | Correct |
| 97.61 | D | D | Correct |
| 97.15 | D | N | Incorrect |
| 96.82 | D | D | Correct |
| 96.59 | D | D | Correct |
| 96.57 | D | D | Correct |
| 96.30 | D | D | Correct |
| 96.29 | D | D | Correct |
| 96.05 | D | D | Correct |
| 95.85 | D | N | Incorrect |
| 95.85 | D | N | Incorrect |
| 95.84 | D | N | Incorrect |
| 95.78 | D | D | Correct |
| 95.60 | D | D | Correct |
| 94.47 | D | D | Correct |
| 94.02 | D | N | Incorrect |
| 93.90 | D | D | Correct |
| 93.84 | D | D | Correct |
| 93.78 | D | N | Incorrect |
| 93.71 | D | D | Correct |
| 93.51 | D | N | Incorrect |
| 93.25 | D | N | Incorrect |
| 93.15 | D | D | Correct |
| 92.73 | D | N | Incorrect |
| 92.64 | D | D | Correct |
| 92.49 | D | N | Incorrect |
| 92.47 | D | N | Incorrect |
| 92.37 | D | D | Correct |
| 92.09 | D | D | Correct |
| 91.96 | D | N | Incorrect |
| 91.85 | D | D | Correct |
| 91.85 | D | D | Correct |
| 91.85 | D | D | Correct |
| 91.70 | D | N | Incorrect |
| 91.57 | D | D | Correct |
| 91.44 | D | N | Incorrect |
| 91.44 | D | N | Incorrect |
| 90.92 | D | N | Incorrect |
| 90.66 | D | N | Incorrect |
| 90.15 | D | N | Incorrect |
| 89.77 | D | D | Correct |
| 89.74 | D | D | Correct |
| 89.50 | D | D | Correct |
| 89.37 | D | N | Incorrect |
| 89.10 | D | N | Incorrect |
| 88.84 | D | N | Incorrect |
| 88.73 | D | D | Correct |
| 88.33 | D | N | Incorrect |
| 88.17 | D | D | Correct |
| 88.17 | D | D | Correct |
| 88.08 | D | D | Correct |
| 88.05 | D | N | Incorrect |
| 87.89 | D | D | Correct |
| 87.81 | D | N | Incorrect |
| 87.54 | D | N | Incorrect |
| 87.54 | D | D | Correct |
| 87.29 | D | N | Incorrect |
| 87.13 | N | D | Incorrect |
| 87.08 | N | D | Incorrect |
| 86.77 | N | N | Correct |
| 86.60 | N | D | Incorrect |
| 86.56 | N | D | Incorrect |
| 86.34 | N | D | Incorrect |
| 86.06 | N | D | Incorrect |
| 85.81 | N | D | Incorrect |
| 85.75 | N | N | Correct |
| 85.73 | N | N | Correct |
| 85.72 | N | N | Correct |
| 85.03 | N | D | Incorrect |
| 84.95 | N | N | Correct |
| 84.44 | N | N | Correct |
| 83.69 | N | D | Incorrect |
| 83.68 | N | D | Incorrect |
| 83.16 | N | N | Correct |
| 82.63 | N | D | Incorrect |
| 82.40 | N | D | Incorrect |
| 82.13 | N | D | Incorrect |
| 82.10 | N | N | Correct |
| 81.89 | N | D | Incorrect |
| 81.84 | N | N | Correct |
| 81.84 | N | N | Correct |
| 81.06 | N | D | Incorrect |
| 80.79 | N | N | Correct |
| 80.55 | N | N | Correct |
| 80.36 | N | D | Incorrect |
| 80.28 | N | N | Correct |
| 79.51 | N | D | Incorrect |
| 79.25 | N | N | Correct |
| 79.00 | N | N | Correct |
| 78.70 | N | D | Incorrect |
| 78.48 | N | N | Correct |
| 78.22 | N | N | Correct |
| 78.21 | N | N | Correct |
| 77.96 | N | D | Incorrect |
| 77.95 | N | D | Incorrect |
| 77.43 | N | D | Incorrect |
| 75.90 | N | N | Correct |
| 75.13 | N | N | Correct |
| 75.01 | N | D | Incorrect |
| 74.33 | N | N | Correct |
| 72.41 | N | D | Incorrect |
| 67.33 | N | N | Correct |
| 66.83 | N | D | Incorrect |
| 66.55 | N | N | Correct |
| 65.75 | N | N | Correct |
| 57.99 | N | N | Correct |
| 47.20 | N | N | Correct |
| 45.77 | N | D | Incorrect |
| 44.30 | N | N | Correct |
| 44.19 | N | D | Incorrect |
| 44.19 | N | D | Incorrect |
| 40.42 | N | N | Correct |
| 38.09 | N | N | Correct |
| 38.09 | N | N | Correct |
| 38.09 | N | N | Correct |
| 36.30 | N | D | Incorrect |
| 32.64 | N | N | Correct |
Even in the best case scenario, you’d only be right 53.3% of the time. “Just guessing” or flipping a coin would get you that type of answer.
This example illustrates just one of the problems that can occur when crying to catch liars based on significant differences in nonverbal behavior. And decades of research on detecting deception reveals a very similar pattern of results. Significant differences between truth tellers and liars are identified. Training programs are created. Testing shows only modest gains in people’s ability to detect deception. In fact, most studies on detecting deception show that people are not very good at it – the accuracy rate is usually around 50 to 60%.
Personally, I believe the main problem underlying research on detecting deception is due to the fact that significant differences do not necessarily identify diagnostic differences.
For the most part, nonverbal cues associated with deception, can only be seen when looking at group averages, not specific individuals.