The facial expression of pain has recently attracted considerable interest in experimental and clinical research based on an increasing awareness that it supports the communication of pain as a second signal system besides the verbal one [5,11]. In line with this, facial activity provides the possibility to develop pain assessment tools in individuals with limited abilities to communicate pain verbally (e.g. newborns, individuals with pronounced cognitive impairments and dementia [18,20,21,25]). An early and very important observation as regards facial responses associated with pain, was that there is a subset of key facial muscle movements that are displayed consistently across different pain modalities [27]. This subset of pain-relevant facial responses includes brow lowering, orbit tightening, levator contraction and eye closure [27,29]. Although there is convergent evidence that these facial responses constitute the core of the ‘‘pain face”, there are also other facial muscle movements that have frequently been observed in the context of pain. The most unexpected accompaniment of pain is the oblique raising of the lip, a facial muscle movement that is caused by contraction of the zygomatic major muscle. Ordinarily, this facial movement is seen when a person is smiling and it is assumed to be associated with happiness or positive affect [8,9]. Therefore, it is surprising that in the majority of the studies on facial responses during pain an increased frequency of oblique lip raise has been found compared to that of baseline conditions. This observation has been made regardless of whether pain was induced experimentally in pain-free individuals (cold pressor task [4,19,27], thermal heat pain [14,15], pressure stimulation [16,17,27], electrical current [17,24,26,27]) or whether clinical pain conditions were investigated (e.g. patients with shoulder or knee pathologies undergoing physiotherapy [3,11,28,29]). Moreover, the percentage of individuals displaying ‘‘smiles” during painful stimulation is rather high. To illustrate this, we computed the percentage of subjects that showed oblique lip raise at least once during phasic or tonic noxious stimulation in previous studies of ours. As can be seen in Table 1, this percentage varied between 22% and 57% depending on pain induction method and