I’ve noted before that people get nervous when stopped and questioned by the police. Being nervous is not by itself a sign that the person is lying or a criminal or doing something wrong. Although of course NCIS/OSI/CID will often say it is so.
As the writer notes here,
The very presence of a police officer is, in most cases, enough to unnerve the general public. Think about how you respond when a police car appears in your rearview mirror.
See, Christian Griessel, Voices: Authority of Fear, Tucson Weekly, 9 May 2009. The writers of the Reid Technique in part address the issue of nervousness and some reasons why nervousness may not indicate criminal activity is afoot or the suspect (or witness) is lying.
a. Nervousness because of the possibility of being erroneously considered guilty.
b. Nervousness about the treatment the person will receive from the police.
c. Nervousness about some other wrong or issue.
d. And while not a Reid factor, the way in which the military suspect is set-up should be considered. Being ordered to accompany a supervisor to law enforcement, being told that the supervisor doesn’t know why they are going to law enforcement.
On their website the Reid folks have this to say about nervousness when confronted by the police.
Especially during the initial contact with a police officer, most subjects will experience nervousness (hand tremor, eye blinking, dry mouth). As a recent incident illustrated, a subject may appear agitated and anxious because of a medical emergency. However, extreme nervousness, or fearfulness that increases during the course of questioning may be an indication of guilt to something beyond the initial traffic violation. Symptoms of extreme anxiety include excessive physical movement (pacing, crossing, uncrossing arms, constant hand movements) and mental blocks (inability to recall simple information like an address or an inappropriate response to a simple question).
Here is a suppression case that looks at nervousness as a factor of reasonable suspicion. While continued and severe nervousness can be a factor for police intrusions, the physiological response has to be supported by and consistent with other objective factors.
Third, Duenas appeared nervous throughout the encounter and, according to Taylor, "[t]he more I spoke with [Duenas], the more nervous he got." The district court found that "Duenas acted extremely nervous, out of the ordinary nervousness that [Taylor] sometimes encounters in making his traffic stops." This finding is supported by the record. We acknowledge that "nervousness is a sufficiently common–indeed natural–reaction to confrontation with the police that unless it is unusually severe or persistent, or accompanied by other, more probative, grounds for reasonable suspicion, it is ‘of limited significance in determining whether reasonable suspicion exists.’" Santos, 403 F.3d at 1127 (quoting United States v. Williams, 271 F.3d 1262, 1268 (10th Cir. 2001)). However, Duenas’ nervousness was accompanied by the foregoing circumstances. We agree with the district court that, combined with these two additional factors, the nervousness supported Taylor’s decision to detain Duenas until arrival of the canine unit.
United States v. Duenas, No. 08-3108, 2009 U.S. App. LEXIS 10857, at *10–11 (10th Cir. May 21, 2009)(emphasis added).
When a motorist detained for a routine traffic violation, such as speeding, shows unusual signs of nervousness, this may be considered as part of the totality of circumstances a reasonable law enforcement officer would analyze in investigating possible crimes.
United States v. Santos, 403 F.3d 1120, 1127 (10th Cir. 2005).
Reid has an interesting discussion of this topic, Behavior Symptom Analysis During Roadside Interviews, May-June 2009.