Do you really get buck teeth by sucking your thumb?

Depending on the frequency, duration, angle of the thumb and sucking force, thumb sucking may or may not have an affect on one's occlusion. Usually, it doesn't, especially if one stops prior to obtaining one's adult teeth. When it does have an affect it's usually an open bite with, perhaps, a cross bite. An open bite is defined by the upper and lower teeth not coming together when the mouth is in a closed position and occurs when the thumb prevents the normal growth of the teeth as they project outside the gums as they occlude. A cross bite is defined by the upper teeth resting inside the arc formed by the lower teeth and occurs, in the thumb sucker, due to the inner force the cheeks exert during sucking motions. If "buck teeth" (A Class 2, division 1 maloclussion) does form, however, it may occur due to genetic factors and/or the action of the lips and tongue while swallowing, where the upper teeth may have been placed in a way that results in the lips and tongue exerting a forward and upward force thus exaggerating the maloclusion.