A crossbite is when the upper teeth are sitting inside your lower teeth, rather than all of them sitting outside, as they should. We also typically diagnose this as a narrow smile. Some are localised crossbites involving single pairs of teeth, while other crossbites are more complex and involve one or both entire sides of the mouth.While they can be hereditary, crossbites can also be caused by overcrowded teeth, and by behaviours like thumb-sucking.