A dimple is a small, natural indentation in the skin resulting from an underlying structural variation. While many associate the word “dimple” only with the cheeks, this term describes several distinct anatomical variations across the body. These indentations are not uniform; they can be caused by muscle changes, bone structure, or specialized connective tissues. Understanding the specific cause for each type reveals that dimples are a collection of different physical characteristics.
Facial Dimples and Muscle Anatomy
The most familiar type, the cheek dimple, results from an anatomical variation in the Zygomaticus major, the primary muscle responsible for smiling. This muscle usually runs from the cheekbone directly to the corner of the mouth. In individuals with dimples, the Zygomaticus major muscle may be divided into two bundles, known as a bifid structure, during fetal development.
The inferior bundle of this split muscle attaches to the dermis, the inner layer of the skin, along its midpoint. This dermal attachment acts as a tether, pulling the skin inward when the muscle contracts during a smile. This tethering effect explains why many cheek dimples only appear when the individual is smiling or laughing. For adults, a permanent dimple visible even at rest may indicate a more significant dermal attachment or a shorter muscle fiber.
Anatomical Variations in Chin Dimples
The indentation found on the chin, often called a cleft chin or fovea mentalis, is structurally different from a cheek dimple. It typically presents as a Y-shaped fissure in the center of the chin. The primary cause of the cleft chin is related to the underlying bone structure of the lower jaw, or mandible.
The chin bone is formed from two halves that fuse together in the midline during fetal development. If this fusion is incomplete, a notch remains in the mandibular symphysis, the joining line of the two halves. This bony indentation causes the overlying soft tissue and skin to dip inward, creating the characteristic dimple. Unlike cheek dimples, the chin dimple is a more permanent feature resulting from a variation in the skeletal structure.
Non-Facial Dimples of the Lower Back
Dimples can also occur on the lower back, known as “Dimples of Venus” or fossae lumbales laterales. These are bilateral, appearing as a pair of symmetrical depressions just above the gluteal cleft. They are situated directly over the sacroiliac joints, where the sacrum connects to the ilium of the pelvis.
The cause of these indentations is neither muscle variation nor bone fusion. They are created by a short, specialized fibrous chord, or ligament, that stretches from the posterior superior iliac spine—a bony landmark of the pelvis—to the overlying skin. This short ligament pulls the skin inward, creating the indentation. Their visibility cannot be altered by muscle exercise, but they may become more noticeable with lower body fat.
The Role of Genetics in Dimple Inheritance
The presence of any type of dimple is strongly influenced by genetics, though the mechanism is more complex than once believed. Dimples were historically classified as a simple autosomal dominant trait, requiring only one copy of the gene for expression. However, the inheritance pattern is now understood to be more nuanced.
Current research suggests that dimples are a polygenic trait, meaning their expression is controlled by multiple genes working in combination. Many different genetic variants contribute to the likelihood of having cheek dimples. This polygenic nature explains the unpredictability of inheritance, as a child may develop dimples even if both parents do not visibly possess the trait. The specific anatomical variations are the physical manifestations of this complex genetic blueprint.