The bones of Japanese mandibles are relatively small, but they have many teeth. As a result, the same teeth grow in a small environment and are squeezed for a long time, causing the teeth to grow unevenly. Because the cost of orthodontic treatment in Japan is expensive, it is not covered by social security. Therefore, when Japanese people correct their teeth, it often costs 1 million yen or more, which is about more than 60,000 RMB, so most people will not correct their teeth.