The face is the main, non-verbal emotional interface between humans (at least, that is my not-so-bold claim as a non-expert). How emotions are translated into expressions and why certain expressions look the same / are the same when they are responding to different, seemingly unrelated stimuli are relevant topics on a psychology and neuroscience Q&A-site, no? I am tempted to mention the the recognition of facial expressions is also a relevant topic, but that may already be included in the
recognition tag, not sure because it doesn't have a description.
If facial-expression is too specific to be a tag, then face could be a tag. The hypothetical face tag could include everything about facial expressions, as well as the recognition of familiar faces (again, this may be included in the
recognition tag), how certain facial features are often perceived (which varies over cultures and ethnicities obviously) and correlates between facial features and psychological / neurological traits. There's probably a host of other relevant topics that could go under a face tag.