Speakers tend to produce very similar-looking gestures in very similar discourse contexts. This paper probes the sources of such uniformity with reference to a particular gesture form. When producing this gesture form, a speaker’s hand moves towards her interlocutor with the palm facing up, as though an object were being presented. Some researchers have claimed that this gesture form is pervasive because people metaphorically conceptualize ideas as though they were objects – the gesture reflects this conceptualization. An alternative hypothesis, however, is that the gesture form is constrained by a cultural convention, in which case the gesture form may no longer be motivated by metaphoric imagery. The present paper presents an experiment which explores these two hypotheses. Participants were asked to provide a rating of naturalness for non-canonical versions of the gesture form described above and non-canonical versions of a gesture form which is assumed to be wholly conventional. If a gesture form is conventional, participants should be resistant to seeing its normal production form violated. Instead, however, there was a high degree of variability among participants in what was considered acceptable for both gesture forms. These results produce a more complex picture of how convention shapes gesture, which will have implications for theories of language production.