In sociology this is defined as a bias towards people who are physically attractive, have a nice smile, or whatever. It basically means that most folks (whether falsely or not) see outwardly attractive people as intrinsically good. Simplified, if a person seems nice then they must be nice.

This theory has come under criticism from some psychologists who view it as a natural extension of maternal/paternal bonds and not as a social phenomenon. While this is worthy of evaluation you have to wonder why the reverse often holds true. When people see someone who is in obvious distress whether they're dirty, having a public conversation with Satan, or whatnot they tend to avoid that person. This is presumably under the assumption that the person will only cause harm through association. This reaction is also addressed by psychology but under the umbrella of a defense mechanism of instinctively fearing strangers. There is some contradiction between those two theories but I'll leave that tangle for the early childhood development specialists to deal with.