Crooked teeth are extremely common, and they rarely have a single cause. A 2022 systematic review of children and adolescents found that crowding alone affects roughly one in three people, while broader alignment issues (overbites, underbites, crossbites) push the total even higher. Your teeth ended up where they are because of a mix of inherited jaw size, how your jaw developed during childhood, habits that shaped your mouth’s growth, and forces that continue to act on your teeth well into adulthood.
Your Jaw May Be Too Small for Your Teeth
The most fundamental reason teeth crowd is a mismatch between jaw size and tooth size, and genetics plays a big role in both. You inherit the width and length of your jaw from one parent and the size of your teeth from another, and those two don’t always match up. If your teeth are wider than the arch of bone they’re supposed to fit into, they overlap, rotate, or get pushed forward or backward to compete for space.
But genetics isn’t the whole story. Human jaws have been getting smaller over thousands of years, largely because our food has gotten softer. Chewing tough, unprocessed food generates strain on the facial bones, and that strain stimulates bone growth, especially in the lower face. Research comparing animals raised on raw versus cooked diets found that chewing raw food produced up to twice the strain on facial bones, and animals on soft diets had roughly 10% less growth in the lower and back portions of the face. Modern diets full of processed, pre-cut, and cooked food give the jaw far less mechanical stimulation during the critical years of childhood growth. The result: jaws that don’t grow to their full genetic potential, leaving less room for teeth.
Childhood Habits That Reshape the Mouth
Thumb sucking and pacifier use are normal in infancy, but when they continue past a certain age, they physically reshape the developing jaw. The American Academy of Pediatric Dentistry notes that pacifier use beyond 18 months can influence the developing facial structures, leading to an open bite (where the top and bottom front teeth don’t meet), a crossbite, or a protruding upper jaw. Children who used a pacifier for 36 months or longer had significantly higher rates of open bite compared to non-users. The good news is that if the habit stops before age 3, an open bite often corrects itself. Past that point, the changes become harder to reverse on their own.
Thumb sucking follows a similar pattern. The thumb applies sustained pressure on the upper front teeth and the roof of the mouth, pushing teeth forward and narrowing the upper arch. The longer the habit persists and the more forcefully the child sucks, the more pronounced the effect.
Mouth Breathing Changes Jaw Shape
If you spent much of your childhood breathing through your mouth, whether from allergies, enlarged tonsils, or a deviated septum, it likely affected how your jaw developed. Mouth breathing drops the tongue from its normal resting position on the roof of the mouth. That matters because the tongue acts as a natural expander for the upper jaw. When it sits on the palate, it counterbalances the inward pressure of the cheeks and helps the upper arch grow wide enough to accommodate all the teeth.
Without that tongue pressure, the cheeks compress the upper jaw inward. Research using digital dental casts found that mouth-breathing children had significantly narrower upper arches and higher, sharper palatal vaults compared to nose breathers. A narrow, V-shaped upper jaw doesn’t provide enough room for permanent teeth, leading to crowding, crossbites, and misalignment that can persist into adulthood.
Tongue Thrust and Swallowing Patterns
Every time you swallow, your tongue pushes against something. In a typical swallowing pattern, it pushes up against the roof of the mouth. But some people push their tongue forward against the back of their front teeth, a pattern called tongue thrust. This happens during swallowing, speaking, and sometimes even at rest.
You swallow hundreds of times a day, so even a small amount of misplaced force adds up. Over time, the constant forward pressure can push front teeth outward, creating an open bite, an overbite, or an underbite. Tongue thrust is especially impactful in children whose bones are still growing, but it can shift teeth in adults too.
Losing Baby Teeth Too Early
Baby teeth aren’t just placeholders. They hold space in the jaw for the permanent teeth developing underneath. When a baby tooth is lost early, whether from decay, injury, or infection, the neighboring teeth drift into the gap. This process, called mesial drift, narrows the available space before the permanent tooth is ready to come in.
The numbers are surprisingly specific. After the early loss of a baby molar, the space shrinks by about 1.5 millimeters in the lower jaw and 1 millimeter in the upper jaw. That might sound small, but permanent teeth need every fraction of a millimeter to erupt properly. Losing a second baby molar causes even more space loss than losing a first. The permanent tooth then erupts into a space that’s too tight, forcing it to come in crooked, rotated, or impacted (stuck beneath the gum).
Why Teeth Shift in Adulthood
If your teeth were straight in your twenties and have started crowding in your thirties or forties, you’re not imagining it. Teeth move throughout your entire life. A slow, forward-directed movement called mesial drift gradually reduces the length of your dental arch over the years. At the same time, the tiny contact points between neighboring teeth wear down, reducing the friction that held them in stable positions.
Lower front teeth are especially vulnerable. Their small crowns and narrow contact areas give them less resistance to rotation and shifting than larger back teeth. As contact surfaces flatten with wear, even minor space discrepancies show up as visible crowding rather than being distributed across the arch.
Several factors speed this process up. Gum disease reduces the bone support around tooth roots, making teeth more mobile and easier to push out of position. Grinding or clenching your teeth applies heavy lateral forces that nudge teeth over time. Habits like lip biting or resting your chin on your hand create asymmetric pressures. And if you had braces as a teenager but stopped wearing your retainer, the underlying forces that caused your original crowding never went away. They simply resumed their work.
Injuries and Trauma
A blow to the face, a sports injury, or a car accident can knock teeth out of alignment immediately or cause a jaw fracture that heals in a slightly different position. Even a fracture that seems to heal well can leave the upper and lower jaws meeting at a slightly different angle than before, changing how your bite fits together. In children, trauma to the jaw during growth years can alter how the bone develops on the injured side, leading to asymmetric alignment that becomes more noticeable as the face matures.
Health Effects of Crooked Teeth
Crooked teeth aren’t purely cosmetic. Crowded or overlapping teeth create tight spaces that are harder to clean with a toothbrush and floss, allowing plaque to accumulate in areas you can’t reach effectively. A meta-analysis of studies on children and adolescents found that those with malocclusion had 66% higher odds of developing gingivitis (gum inflammation) compared to those with well-aligned teeth. Crowding, excessive overbite, and crossbite all showed associations with poorer oral hygiene scores.
Beyond gum health, misaligned teeth can cause uneven wear on enamel, jaw pain from an unbalanced bite, and difficulty chewing food thoroughly. Some alignment issues contribute to tension headaches or clicking and discomfort in the jaw joint. The severity of these effects varies widely. Mild crowding might cause no problems beyond appearance, while significant misalignment can affect daily comfort and long-term dental health.
What Determines How Crooked Your Teeth Get
For most people, crooked teeth result from several overlapping factors rather than one clear cause. You might have inherited a narrow jaw, breathed through your mouth as a child due to allergies, lost a baby tooth early to a cavity, and then stopped wearing your retainer after braces. Each of these contributes incrementally, and the final alignment you see is the sum of all those forces acting over years.
The factors you can still influence include breathing patterns (treating nasal obstruction), tongue posture, oral habits, and maintaining gum health to preserve the bone that keeps teeth stable. For teeth that have already shifted, orthodontic treatment can reposition them at any age, though keeping them in place afterward requires ongoing retention since the biological forces that moved them in the first place never fully stop.