This is a very interesting and well-conducted and reported study. However there are still certain issues that require clarification. Please find my detailed comments below:

1. Materials and methods: “To elicit natural spontaneous smiles, participants were not told that smiling represented the primary outcome of the study.” This is understandable, but the authors need also to report more precisely on what the participants were told about the study aim. This affects interpretation of the outcomes.
2. Sample calculation: The authors refer to differences detected a statistical test, but which is this test? This needs to be reported.
3. Group differences for specific tests between subgroups of the 22-participant groups are underpowered. For example 5 participants in the NZ group did not smile. On the same line there are several pairwise comparisons, increasing the possibility for false positive results. The limited sample size affects negatively these parameters. For sure some comparisons are underpowered, especially those that test variables with high variation, e.g. those for personality traits.
   The same is true for the linear models, where several factors are tested simultaneously. This results in very small groups that present a certain combination of factors limiting the power of the tests. Although statistical differences were detected by the study, these are limitations that need to be reported.
4. Please define precisely the statistical models in the statistics section. Which factors were included etc.
5. The authors need to define precisely the AU6, AU12, AU25 variables. How were these measured exactly? Differences between which landmarks etc. They might also want to refer the readers to the following website tom facilitate understanding: https://imotions.com/blog/learning/research-fundamentals/facial-action-coding-system/#the-facial-action-coding-system.

Congratulations for this nice work!