Newsweek has an interesting article on cognitive dissonance - basically a term used by psychologists to describe the intense discomfort of having two conflicting thoughts, beliefs, or perceptions at the same time. Humans, they say, will justify almost anything.
Here's an example how the Nuer and Dinka tribes of southern Sudan "justify" toothlessness as beauty:
The Nuer and the Dinka tribes of southern Sudan share an unusual custom. Both of these cattle-herding societies remove several of their kids' permanent front teeth as soon as they sprout: two on the top and four to six on the bottom. It's a very painful procedure, done with a fish hook, and it leaves all tribe members with a distinctive slack-jawed look and speech impediments.
This practice probably started long ago, when tetanus was rampant in central Africa. Tetanus causes "lockjaw," but the tooth removal would have allowed children afflicted by this infectious disease to drink liquids even when their jaw muscles clamped shut. Although there has been no tetanus or lockjaw in the southern Sudan for ages, both the Nuer and the Dinka continue the custom of extracting the front teeth. Indeed, they believe the sunken jaw and lower lip are beautiful. People with front teeth, they say, look like jackals.
Social psychologist Carol Tavris and Elliot Aronson described this in terms of cognitive dissonance:
In the case of the Nuer and Dinka, they "choose" to believe that the toothless look is aesthetically pleasing in order to justify the infliction of such trauma on their children. Any connection to health and survival is long gone.