Polygraphs, also referred to as lie detectors, seem to be
part of a social experiment that never ends. Though many of us have heard of
polygraphs (especially given their widespread presence in old detective shows),
it was not until recently that I looked into the experimental design behind the
polygraphs, and the highly debated believability of their results.
First, a little background on polygraphs… The polygraph was
invented in 1921 by John Augustus Larson, a 21 year-old physiology student at
University of California, Berkeley who moonlighted as a police officer. The
device, which Larson referred to as the cardio-pneumo psychogram, could measure
blood pressure, pulse, respiration, and skin conductivity, all with the premise
that these are measurable indicators that change when the subject of the tests
is being deceptive.
Like any good experiment, polygraphs use controls – control
questions, that is. A series of control questions, usually broad statements
meant to inquire into the subject’s past character and truthfulness, are asked
by the administer of the test, and then a series of relevant questions
concerning the case at hand are asked (a repeated-measures design, if you
will). If a subject is being deceptive, or perhaps rather, is being anxious or
nervous, the polygraph should record a change in the physiological measures.
Someone who is being deceptive concerning the case should theoretically be more
anxious when asked to answer relevant questions, while someone who is not being
deceptive concerning the case should be more anxious when answering control
questions.
Though the polygraph has been regarded as one of the
greatest inventions (in fact, the original polygraph device constructed by
Larson is housed in the Smithsonian), its validity has been debated since its
invention. Larson conducted many tests on his own, compiling a list of cases
where his device helped solve murders, thefts, etc. (interestingly, Larson
first tested his device on his wife…). However, many scientists today regard
the basis of polygraphs as pseudoscience. Former US Attorney General John
Ashcroft estimated the false-positive (or type I error, if you will) rate of
polygraphs to be 15 percent. Yet, the Journal
of General Pyschology published an analysis of 41 criminal cases where
polygraph tests were used, concluding an accuracy over 90 percent.
If the test seems to be so accurate, then why the ongoing
debate (as with all science, right)? Well, studies are conflicting in their
results of accuracy, which could be a result of “p-hacking”, where researchers
throw out inconclusive polygraph results to improve the accuracy rate of
polygraphs. One also has to question the experimental design of the polygraph,
where neither a subject or administer is blinded. One could potentially skew
the polygraph results from either perspective by causing undue nervousness or
anxiety during questioning, or by being aware of how the measured indicators
change and resisting (common spy trick). The best summary of the flaws of the
polygraph test come from William Iacono, a psychophysiologist at the University
of Minnesota: "A big problem is that it's not
really a test of anything,” highlighting the fact that little is really known about
how the body behaves when lying, and therefore these physical measurements may
not actually be measuring marks of deception.
Needless to say, given all of the
debate, I would not want to be on the wrong side of a polygraph in a courtroom.
Sources:
1.
“Telling the Truth About Lie Detectors.”
USA Today. http://usatoday30.usatoday.com/news/nation/2002-09-09-lie_x.htm
2.
“The Truth About Lie Detectors.” American Physiological Association.
http://www.apa.org/research/action/polygraph.aspx
