Most of the muscles that shape the human face are strikingly consistent from person to person. The path each one takes, from bone to skin, is essentially the same blueprint whether the face belongs to an infant or an octogenarian, on any continent. One muscle, however, breaks that pattern often enough to be noticeable, and it happens to break it in exactly the spot most likely to draw attention: the dimple on the corner of the mouth, every time a person smiles.
For decades, biology classrooms have offered a tidy explanation for why some faces show dimples and others don’t: a single dominant gene, inherited the same clean way eye color is sometimes (incorrectly) taught, plotted out on a straightforward Punnett square. It’s a memorable lesson, and it’s part of why the trait became a genetics-class staple in the first place. It also isn’t how the trait actually works.
The Muscle Is The Real Story Behind The Smile Dimple
The muscle in question is the zygomaticus major, a band of tissue that normally runs unbroken from the cheekbone down to the corner of the lips. In some people, that band splits into two slips as it develops, and one of those slips attaches directly into the skin of the cheek instead of just the mouth. When the muscle contracts, that extra anchor point tugs the skin inward. The dimple is the visible pucker made by skin getting pulled against a tether it isn’t supposed to have.
That’s an anatomical quirk, not a hidden genetic switch. Anatomists have documented this branching pattern in dissections for well over a century, long before anyone was talking about a “dimple gene” at all — a point reinforced by a 2019 study published in the Journal of Craniofacial Surgery, which pooled data from cadaver and imaging studies across several populations and found the split muscle in roughly one in five people.
Where We Go Wrong About Smile Dimples
So why does biology class keep pairing dimples with dominant inheritance? Because dimples, along with things like attached earlobes or the ability to roll your tongue, were adopted decades ago as friendly classroom shorthand for Mendelian genetics.
The trouble is that most human traits, dimples included, don’t actually behave that cleanly once real families are studied — as a literature review published in Myths of Human Genetics, a University of Delaware genetics reference, concludes, no genetic study has ever actually backed a simple dominant model for cheek dimples.
Geneticists now treat the trait as shaped by several genes acting together, plus a fair amount of chance during development, whether a muscle fiber happens to split, whether it fuses to the skin, whether it does so on one side of the face or both. This is the kind of variability — skipping a generation, showing up on one side only, appearing in a child whose parents have none — you’d expect from a trait shaped by several genes, not a single dominant one.
Where Else Do Humans Have Dimples?
Cheek dimples aren’t the only place the body relies on this kind of tethering, and seeing the pattern repeat elsewhere makes the mechanism easier to trust. A cleft chin works on a related principle, though the anatomy differs.
In some people, the two halves of the jawbone don’t fully fuse at the midline during fetal development, or the muscle beneath the chin (the mentalis) splits the way the zygomaticus major does on the cheek. Pop culture has long lumped cheek dimples and cleft chins together as though one gene controlled both, but a companion review, also published in Myths of Human Genetics, found cleft-chin rates swinging from under 5% to over 70% within a single studied population, depending on age and sex, a spread no simple dominant gene could produce.
Biologically they’re separate quirks that happen to produce a similar look using the same basic trick: a fixed anchor point pulling skin where it wouldn’t otherwise pucker.
The same idea shows up even farther from the face. Two shallow indentations sometimes visible on the lower back, just above the hips and informally nicknamed the “dimples of Venus,” involve no muscle contraction at all.
Some anatomists attribute them to a short ligament tethering the skin directly to a bony landmark on the pelvis, which identifies that landmark as the posterior superior iliac spine. There’s no smiling required, no muscle firing, just a fixed pull whenever the skin stretches. It’s the same underlying principle as the cheek dimple — soft tissue anchored a little too tightly to a fixed structure — carried out by a completely different piece of anatomy that has nothing to do with the face.
What Dimples Tell Us About How Bodies Are Built
Seeing that mechanism recur — ligament instead of muscle, pelvis instead of cheekbone — reinforces that dimples were never a single trait with a single genetic story. They’re a pattern the body seems to fall into wherever skin, muscle and connective tissue meet at a fixed point, showing up in slightly different forms across the body, each with its own developmental explanation. The classic dominant/recessive story is real and useful for a small number of traits, like certain blood types or specific inherited disorders. It was never meant to explain most of what makes faces, or bodies, different from one another.
So the dimple isn’t a gene expressing itself. It’s a muscle that took a small detour on its way to becoming a mouth, leaving a permanent, visible signature every time its owner smiles.
Curious how much you really know about the muscles, ligaments, and dimples that shape the human body? Put your knowledge to the test: Human Anatomy IQ Test