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.
Making the truth about dimples more obfuscated, dimples aren't permanent. Some people have them as kids and then they go away. For others, they only appear in adulthood. That's added evidence for a variety of genes being involved as well as the possible impact of environmental factors like skin elasticity and the amount of fat in an individual's face.
This sets aside that you can have surgery (warning: that link is gross) to create dimples. Others get piercings to simulate dimples. However, short of these measures, you either have them or you don't and we aren't 100 percent sure why.
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