There are several reasons why some people’s teeth grow in crooked, overlapping, or twisted. Some people’s mouths are too small for their teeth, which crowds the teeth and causes them to shift. In other cases, a person’s upper and lower jaws aren’t the same size or are malformed, resulting in either an overbite, when there is excessive protrusion of the upper jaw, or an under bite, when the lower jaw protrudes forward causing the lower jaw and teeth to extend out beyond the upper teeth.