Wisdom teeth are the last teeth at the back of the mouth. They often come in when people are in their late teens or twenties.

But some people never get them. A large study found that about 23 percent of people in the world are missing at least one wisdom tooth from birth. So it is quite common. The number is very different from region to region.

Wisdom teeth, or third molars, usually appear in the late teens or twenties, and they often cause pain when there is not enough space. However, many people never develop some of them at all. In 2015, Carter and Worthington combined data from 92 studies involving more than 63,000 people. They estimated that about 23 percent of people worldwide lack at least one wisdom tooth from birth, although the figure varied from about 5 to 56 percent between studies.

Another study in 2016 found a simple rule. Each tooth affects the growth of the next one, so the last molar tends to be the smallest. This may help explain why it is sometimes missing.

Wisdom teeth, the third molars, are notorious for erupting late and becoming impacted, yet many people never form some of them. In a 2015 meta-regression of 92 studies covering 63,314 people, Carter and Worthington estimated that about 23 percent of people worldwide lack at least one third molar. The estimates ranged from about 5 to 56 percent between studies, women were somewhat more likely to be affected than men, and absence was more frequent in the upper jaw.

Why should the last molar be especially vulnerable? A 2016 study of human and fossil hominin teeth found that molar sizes follow an "inhibitory cascade", in which each developing tooth releases signals that restrain the next. In our genus, tooth proportions were tightly tied to size, so one tooth could predict the rest of the row, and the rule tends to make the third molar the smallest.

Jaw shape also matters. Comparing populations worldwide, von Cramon-Taubadel found in 2011 that lower jaw shape was linked more to diet than to ancestry, with hunter-gatherers having longer, narrower jaws than farmers. But regional variation and differing diagnostic methods make it risky to claim that humans are simply "evolving" to lose wisdom teeth.