If mouths were votes, then crooked teeth would win every election.

Occlusion is the way the top and bottom teeth line up and fit together when the jaws are closed. The points of the molars should be in the grooves of the opposing molars. Upper teeth fit slightly over lower teeth. The dentist’s term for crooked teeth is “malocclusion”. Malocclusion (“bad occlusion”) is any deviation from this ideal alignment, so it covers a lot of ground with a lot of variety.

A system of three broad categories of malocclusion was set up up 1899 by a Dr. Edward Angle, and is still in wide use today, mainly by researchers studying the prevalence and causes of these conditions.

The fact of 21st-century life in America is that more of us develop some type of malocclusion than don’t. There’s a malocclusion majority! The exact numbers are elusive, since they depend on the definitions and criteria – how crooked is crooked? – but most experts would agree that between 50% and 70% of American kids become candidates for braces. About 15% of Americans have malocclusion severe enough to affect functionality or social acceptance. About 4 million of us are wearing some kind of orthodontic appliance, mainly braces, right now.

Strange, isn’t it? A condition we consider to be physically abnormal is more common than what we think of as normal.

Some experts believe that it wasn’t always this way. Skulls dating from more than about 10,000 years ago rarely show evidence of misaligned teeth. An early 20th-century researcher who sought out human communities isolated from modern societies reported that malocclusion was virtually non-existent among those tribes.

One theory is that changes in the human diet brought about a mismatch between our food and the “equipment” we use to eat it, and that this incompatibility is the fundamental cause of what has become an epidemic of crooked teeth. The theory suggests that our way of life causes kids’ jaws to develop differently, not that we have evolved. The time frame is much too short for evolutionary changes.

10,000 years ago is roughly the time when farming was becoming fashionable, and so our ancestors began using jaws and teeth meant for raw meat to munch softer foods from the crops and the dairy. It does appear that the shape of our lower jaw (mandible) began to change around that time, as seen in human remains examined by researchers, but our teeth themselves did not change.

Teeth and jaws became increasingly mismatched. In this view, the last hundred years or so saw a major shift to consumption of industrially produced, softer foods, and with this, the notable increase in the prevalence of malocclusion.

It does appear to be the case that what’s called “masticatory stress” – chewing hard stuff – during early childhood affects the development of kids’ jaw bones. This, in turn, is related to the likelihood of dental malocclusion. Animal experiments have shown that a diet of softer foods results in shorter and thinner mandibles. Our modern diet may be doing the same to us.

In any case, we can’t go back and change history, so let’s instead focus on what we know about the causes of malocclusion in our time.

First, there’s a clear relationship between genes and malocclusion. The shape, size, and eruption pattern of teeth are inherited. So is jaw size, though behavior can affect the expression of these genes. When the jaw is too small in relation to the size of the teeth, overcrowding necessarily results. Teeth overlap and are crooked. This is probably the most common type of malocclusion. It’s certainly the typical reason adults get braces.

Genes and behavior interact in other ways, too. Sometimes the cause-and-effect relationship can is unclear, and possibly even two-way.

Mouth breathing is an example of a behavior associated with malocclusion. About half of kids do at least some mouth breathing, but most have stopped by the time they are eight years old.

Chronic mouth breathers are often seen to have the “long faces” resulting from abnormal development of the jaw. As in the case of jaw size, when the shape of the jaw doesn’t jibe with the size, number, or eruption pattern of the teeth, malocclusion becomes more likely.

In monkey experiments in which the nasal passages were blocked, mouth breathing was found to produce the same kinds of skeletal-dental patterns seen in human mouth breathers. Researchers observed that the individual experimental monkeys would adopt one of three distinct mouth-breathing patterns, and each of the patterns was associated with a specific type of malocclusion.

Tongue thrusting when swallowing is a related behavior that parents need to be alert for. About 80% of tongue thrusters have an airway problem.

In a normal swallow, the tongue is pressed to the roof of the mouth, with the tip behind the front teeth and not in contact with them. Tongue thrusters tend to push the tongue forward when swallowing, past the teeth. A variation is seen in some people who spread the tongue when swallowing, pressing the sides and/or the tip against the teeth. This chronic, repetitive pressure on the teeth, over time, moves them out of alignment. Tongue thrusting can sometimes be spotted when a child speaks, visually or by hearing a lisp.

Another common childhood behavior associated with crooked teeth is self-soothing by sucking fingers or pacifiers. The longer these habits continue, the more likely they are to lead to malocclusion. Kids should be weaned away from these behaviors by the age of three at the latest to reduce the chances of needing braces.

Bottle soothing is involved in another common cause of crooked teeth. When kids keep bottles in their mouths beyond what’s needed for nutrition – for example, when they’re allowed to fall asleep with a bottle in their mouths – the sugars in the milk or other liquid clings to the child’s’ teeth and promotes tooth decay.

Premature loss of baby teeth due to decay or infection greatly increases the likelihood that permanent teeth will end up crooked. Baby teeth are like homing beacons for permanent teeth, guiding them to their proper places.

Baby teeth and permanent teeth can also go missing as the result of injury, resulting in the other teeth shifting and becoming crooked. Many of the most severe malocclusions are the result of trauma that damages or knocks out teeth or injures the jaw.

The interactions between genetic endowment, environment, and behavior are complex in relation to dental malocclusion. There are characteristic types, such as overcrowding, overjet, overbite, open bite, crossbite, and others, and the chain of cause and effect in a particular case is not always clear. We have action options for responding to some of the known causes. We can deal with thumb sucking, for example. Other factors, such as genes, are not under our control.

Prevention of malocclusion, therefore, is not always possible. What we can do is become familiar with the known causes, identify them early, intervene with preventative measures where possible, and consider orthodontic intervention where required.