to do. However, the terrorist’s desire to avoid detection and the “cat and mouse” game that is played by terrorists and their pursuers make such a verbal mode of information-gathering highly unreliable.

Because verbal reports can be manipulated and controlled so easily, we might turn to biological systems that are less susceptible to voluntary control or that provide detectable signs when they are being manipulated. Once we move to the biological level, however, we have abandoned direct observation of terrorist behavior and moved into the realm of inference of likely behavior from more primitive and less specific sources. Biobehavioral methods can be powerful and useful, but they are intrinsically subject to three limitations:

  • Many-to-one. Any given pattern of physiological activity can result from or correlate with a number of quite different psychological or physical states.

  • Probabilistic. Any detected sign or pattern conveys some likelihood of the behavior, intent, or attitude of interest but not an absolute certainty.

  • Errors. In addition to the highly desirable true positives and true negatives that are produced, there will be the troublesome false positives (an innocent person is thought to be guilty) and false negatives (a guilty person is thought to be innocent). Depending on the robustness of the biobehavioral techniques involved, it may be possible in the face of countermeasures for a subject to induce false negatives by manipulating his or her behavior.

In addition, even if deception or the presence of an emotion can be accurately and reliably detected, information about the reason for deception, a given emotion, or a given behavior is not available from the measurements taken. A person exhibiting nervousness may be excited about meeting someone at the airport or about being late. A person lying about his or her travel plans may be concealing an extramarital affair. A person fidgeting may be experiencing back pain. None of those persons would be the targets of counterterrorist efforts, nor should they be—and the possibility that their true motivations and intents may be revealed has definite privacy implications.


Most behavioral methods are based on monitoring the activity of neural systems that are thought to be difficult to control voluntarily or that reveal measurable signs when they are being controlled.

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