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.
Further muddying the dimpled waters, dimples are often cited as a dominant genetic feature. However, it's not that simple. Researcher John H. MacDonald writes, "The myth is that dimples are controlled by one gene with two alleles, and the allele for dimples is dominant." SciShow highlights a study from genetics company 23andMe suggesting there are up to nine genes associated with dimples.