Everyone loves dimples. For how much dimples are talked about, it's strange how little we know about a feature up to 40 percent of the population sports. One problem, as SciShow points out, is it's not exactly an urgent subject for research. A 2001 study highlights that facial expressions have been studied frequently, but not always from an evolutionary perspective.
As for what causes them, it's believed a variation of the zygomaticus major muscle is the cause of dimples. In most people, it's a single muscle connecting the corner of your mouth with the cheekbone, allowing you to fake a smile when your boss calls you into their office. However, for others that muscle splits into two and connects in two places, causing a dimple to form. At least that's what scientists have surmised from studying cadavers, which aren't particularly smiley.