Biocultural markers of adulthood
Due to the inexact nature of the adolescence-adult transition, many societies have rites of passage to culturally define the adult status of young women and men. The second Monday in January marks Japan’s official Coming of Age Day (???, Seijin-no-hi). It is celebrated annually by men and women who celebrated their 20th birthday the year prior. As may be seen in the photographs above, the rite often involves the wearing of traditional clothing. Participating women typically wear long-sleeved kimonos with fur. Men wear kimonos in the style of their region. After visiting shrines, the new adults celebrate at parties with family and friends. The ceremonies are hosted by local government offices recognizing the celebrants as official adults able to drink, smoke, gamble, and drive. Many societies acknowledge adulthood by granting young women and men the right to purchase alcohol and other controlled substances, to drive motorized vehicles, to be inducted into the military, or to vote in political elections. These sanctions may occur at a range of ages and stages of biological maturity, and therefore are primarily social markers of adulthood.
One biological event that has been used to determine the end of adolescence is «wisdom tooth» eruption, the appearance through the surface of the gums of the third and final molar teeth. This takes place at about age 18-21 years in both young women and men, which is a narrower age range than most other biological or social markers of adulthood. The lower image above shows profile x-rays of a gorilla, chimpanzee, and human skull. The red arrows point to the third molar. These apes (yes, people are one of the apes!), all other apes, and many African monkeys have the same types and numbers of teeth. This is true for both the «milk teeth» (deciduous dentition) and the permanent teeth. The differences between species are the rate of formation of the teeth and the age at which each tooth erupts through the gum. Human beings, generally, form teeth more slowly and have later ages of eruption than any other primate species. In contrast to people, the gorilla and chimpanzee erupt their third molars between the ages of 11-12 years. The x-rays above show from the left a gorilla with third molar fully erupted, a chimpanzee with the upper third molar present but no third molars erupted, and a human with the third molars fully erupted. Soon after third molar eruption the females most non-human ape species become pregnant for the first time and this makes third molar eruption a reliable indicator of adulthood for the non-human apes. For the human species the situation is complicated by biological and social factors. One biological problem is that about 15% of humans worldwide never form the third molar, called «third molar agenesis», but these people do become adults. Agenesis of the third molar is likely a sign of on-going human evolution due to natural selection. When present the third molar may impact against the second molar, causing pain, infection, and without medical treatment serious pathology that can debilitate or kill the victim. Prior to the advent of modern dental interventions, including antibiotics and surgery, adolescents and young adults with third molar impaction would have reduced fertility and increased mortality; hence, the selection for agenesis. [Barry Bogin]