## 5 Limitations and Future Direction  
An important consideration with regards to the present study is the limited number of participants and the touched-touchee pairs that they formed. On the one hand, although the number of participants was limited, we collected a moderately large sample (i.e., 5664 facial area pre-touch data points) from them. On the other hand, since these data were all collected from the same individuals, our data acquisition procedure could have been affected by two important factors: habituation and sensitization effects. For instance, in the context of present study the habituation could have taken place if the touchee felt more relaxed after a few trials (due to repeated facial pre-touch attempts by the same toucher) and subsequently allowed the toucher's hand to approach closer to the face area. Conversely, the sensitization effect could have occurred if the repeated attempts by the toucher resulted in higher sensitivity of the touchee, thereby asking the toucher to stop in ever quicker and therefore farther distances from the touchee's face. The occurrence of such effects could also become more possible, considering the limited number of touchee pairs in the present study whose members were fixed. Specifically, although the participants in our study were all strangers to one another, it would be foreseeable that the toucher-touchee pairs could feel more comfortable (or conversely more sensitive to the presence of a stranger) after the first few trail attempts. Therefore, it is crucial for the future research to further investigate the potential effect of such factors by including a greater number of participants. In this context, it is also interesting to study how swapping the individuals between the pairs may affect the responses of the touchees on the one hand and the confidence of the touchers for approaching new individuals' faces on the other hand. With regards to the distribution of touchers' data around the face area of the touches, we observed that the mean and median of their data were close to zero. This indicated that the touchers' data was quite fairly and smoothly distributed along the azimuth axis. On the other hand, we observed that their elevation angles were more elongated towards the above-lips area. Perhaps such a stretch of data towards the area above the lips is to an extent expected when we consider the fact that the touches were seated and the touchers were standing in front of the touchees' seat. However, this observation also highlights a shortcoming in our setting in which the fixed height of the seat did not allow for the lower portion of the faces to be more conveniently exposed to touches' reach. This indicated that our consideration for the position of the touches' seat according to the touches' arm length needed to be complemented with subsequent adjustment of the seat's height as well. Therefore, future research is necessary to analyze the potential effect of this shortcoming on the data collection and the performance of different regression models that were studied in the present study. Another issue that requires further consideration is the physical characteristics of the touches. In the present study, we did not collect information about the participants' height. We also did not acquire touches' subjective impression of the touches' appearances on their decision for allowingor stopping the reaching hand. In this respect, it is worthy of note that we did ask the touches to constantly look straight ahead (i.e., to avoid eye-contact and/or to look at/follow the approaching hand) to alleviate such potential effects as touchers' appearance. However, these measures would probably be insufficient to eliminate these effects. Therefore, the future research should consider collecting data about participants' subjective feelings (e.g., in the form of questionnaires), thereby allowing for more robust analysis of such non-verbal human behaviour.