Flat feet develop through a combination of genetics, childhood development patterns, and physical changes that happen over a lifetime. Some people never develop arches as children, while others lose their arches later due to tendon wear, weight gain, or underlying health conditions. Roughly 3 to 25% of adults worldwide have flat feet, with the wide range reflecting genuine differences across populations and ethnicities.
Every Baby Starts With Flat Feet
All infants are born with flat feet. This is completely normal for two reasons: a baby’s bones and joints are extremely flexible, so the foot flattens under any weight, and a thick fat pad along the inner border of the foot physically hides whatever arch structure exists underneath.
For most children, arches gradually form as the bones harden and the foot’s supporting structures tighten. This process typically finishes by age 6. If the arch hasn’t appeared by then, a child likely has what’s called flexible flat feet, the most common type. With flexible flat feet, an arch is visible when the foot is off the ground or on tiptoe but disappears under body weight. This form rarely causes pain and often runs in families.
The Role of Genetics and Connective Tissue
Flat feet clearly cluster in families, but the specific genes responsible remain surprisingly unknown. A 2024 systematic review in Clinical Genetics searched for genetic variants directly linked to arch shape and found none. The researchers noted significant regional and ethnic variation in how common flat feet are, yet no study has mapped the inherited blueprint behind those differences.
What is well established is the connection between flat feet and connective tissue disorders. Conditions like Ehlers-Danlos syndrome and Marfan syndrome cause unusually loose ligaments throughout the body, including in the foot. When the ligaments that hold the arch together are too stretchy, the arch sags under load. If you have flat feet along with joint hypermobility in your fingers, elbows, or knees, a connective tissue condition may be the underlying cause.
What Holds the Arch Up (and What Lets It Fall)
Your foot’s main arch runs along the inner edge from heel to big toe. It’s built from a chain of bones: the calcaneus (heel bone), talus, navicular, three small cuneiform bones, and the first three metatarsals. But bones alone don’t hold this shape. A network of ligaments, tendons, and a thick band of tissue along the sole called the plantar fascia work together like cables supporting a bridge.
The most important active supporter is the posterior tibial tendon, which runs from a muscle in the calf, behind the inner ankle bone, and attaches to several bones in the midfoot. When this tendon contracts, it locks the arch into position and prevents it from collapsing inward. A ligament called the spring ligament cradles the talus bone from below, while the plantar fascia connects the heel to the toes and tightens with every step. If any of these structures weaken or tear, the arch loses support.
How Adults Develop Flat Feet Over Time
Adult-acquired flat foot most often results from gradual degeneration of the posterior tibial tendon. This isn’t usually a single injury. Instead, the tendon slowly breaks down over months or years, losing its ability to hold the arch up. As the tendon stretches and weakens, the ligaments it normally keeps taut also begin to fail, especially the spring ligament. The arch progressively collapses, the foot rolls inward, and eventually the forefoot shifts outward.
This process unfolds in stages. Early on, the tendon is inflamed but still intact, and the foot can be repositioned manually. In later stages, the tendon becomes permanently elongated or thinned out, and the bones shift into new positions. In the most severe cases, the deltoid ligament on the inner ankle also fails, causing deformity in both the foot and the ankle joint. The progression from early tendon inflammation to full structural collapse can take years, which is why many people don’t notice the change until their foot shape has already shifted noticeably.
Risk Factors That Speed Up Arch Loss
Several factors make adult-acquired flat feet more likely:
- Obesity places sustained extra load on the arch’s supporting structures, accelerating tendon and ligament wear.
- Aging reduces blood supply to tendons and decreases tissue elasticity, making the posterior tibial tendon more vulnerable to degeneration.
- Diabetes damages small blood vessels and nerves, weakening the soft tissues that maintain foot shape.
- Rheumatoid arthritis causes chronic inflammation in joints and tendons, gradually eroding arch support.
- Prior foot or ankle injury can damage ligaments or alter how weight is distributed, setting the stage for later collapse.
Women are affected more often than men, and symptoms most commonly appear after age 40.
Shoes in Childhood May Matter More Than You Think
Growing evidence suggests that what children wear on their feet influences whether their arches develop fully. Children who habitually walk barefoot tend to develop higher arches, longer feet, and wider feet compared to children who regularly wear shoes. A 2023 systematic review found that children in rural areas who mostly wore sandals had better-developed arches than urban children who wore closed shoes more than 90% of the time.
The timing matters too. Children who start wearing shoes earlier in life tend to have lower arches and higher rates of flat feet. Poorly fitting or rigid shoes appear to interfere with the natural strengthening process that builds arch structure during early childhood. Barefoot movement allows the small muscles and ligaments of the foot to engage and develop naturally. This doesn’t mean shoes cause flat feet in every child, but the research consistently points toward more barefoot time being better for arch formation.
Flexible vs. Rigid Flat Feet
Not all flat feet are the same, and the distinction between flexible and rigid types matters for understanding where yours came from. With flexible flat feet, the arch appears when you sit down or stand on your toes but flattens when you bear weight normally. This is by far the more common type and is usually inherited. Most people with flexible flat feet have had them since childhood and experience no pain.
Rigid flat feet show no arch in any position, whether standing, sitting, or on tiptoe. This type is rare and more likely to signal a structural problem: bones that fused abnormally during development, significant tendon damage, or advanced arthritis. Rigid flat feet are also more likely to cause pain, stiffness, and difficulty moving the foot side to side or up and down. If your foot was once arched and has become flat and stiff over time, that pattern points toward acquired causes like tendon degeneration or joint disease rather than a developmental trait you’ve carried since childhood.