Typically, people can see whether a movement is intended to communicate (a.k.a. a gesture) or whether the movement’s producer has some other intention, be it practical or just fidgeting. There are however plenty of examples where the movement is ambiguous: it could be a gesture but it could also be a meaningless incidental movement. Barack Obama produced such a movement during a speech. Watch and judge for yourself.
Did Obama just flip off Clinton or was he merely scratching his cheek?
Again, like in many other cases where the nature of a movement was debated, there is a potential insult to be considered. It is almost as if people are more sensitive to potentially insulting gestures then to other gestures. Some people, like Lehmann or Mr Wood even use this sensitivity to their advantage. They camouflage their insulting gesture and thus create ambiguity on purpose. Those who have a reason to feel offended are insulted by the ‘gesture’. Other people only see a cactus or someone scratching his head.
I would predict that if people must judge if a movement is intended to communicate they will do so more often when that would mean it is an insult than when that would mean it is some other gesture. (Question: Can you think of an experiment to test this prediction?)
BTW, there is a very interesting related paper on this topic from a psychiatric perspective:
Bucci, Sandra, Mike Startup, Paula Wynn, Amanda Baker, & Terry J. Lewin. (2008). Referential delusions of communication and interpretations of gestures. Psychiatry Research, 158(1), 27-34. (Scopus)
Gestures are an important aspect of non-verbal communication, but people with schizophrenia have poor comprehension of them. However, the tests of gesture comprehension that have been used present only scenes in which interpersonal meaning is communicated, though there is evidence that people with psychotic disorders tend to perceive communications where none were intended. Such mistakes about non-verbal behaviour are the hallmark of a subtype of delusions of reference identified as delusions of communication. Thus we hypothesised that patients with delusions of communication would tend to misinterpret incidental movements as gestures and, since delusions are often derogatory to the self, they would also tend to misinterpret gestures as insulting. Patients with acute psychotic symptoms (n = 64) were recruited according to a 2 × 2 design (presence vs. absence of delusions of communication by presence vs. absence of auditory hallucinations). They, and 57 healthy controls, were presented with 20 brief video clips in which an actor either made a well-known gesture or an incidental movement. After each clip, they selected one of four interpretations: a correct interpretation if a gesture had been presented; the interpretation of a different gesture; an insulting interpretation; no gesture intended (correct for incidental movements). The patients made significantly more errors of all kinds than the controls, perceived significantly more of the incidental movements as gestures, and selected significantly more insulting interpretations of the clips. These differences between patients and controls were almost wholly due to patients with delusions of communication. These results suggest that the difficulties that people with delusions of communication experience in understanding gestures can be explained, at least in part, by the misattribution of self-generated internal events to external sources.
Perhaps we all suffer from delusions of communication to some degree when we are in a situation where we expect to be insulted (rightly or wrongly). I know I always check for fingers when I feel I did something impolite in traffic. Don’t you?