The cause of dental crowding is the imbalance of tooth and bone volume, with relatively large tooth volume and small bone volume, and insufficient arch length to accommodate the full number of teeth in the arch. Dental volume and bone volume imbalance are influenced by racial evolution, genetic and environmental factors, as follows: 1. Racial evolution refers to the tendency of degeneration of the masticatory organs during human evolution due to the consumption of cooked food and more and more refined food. The degeneration of the masticatory organs is fastest in muscle, followed by bone and slowest in teeth. The unbalanced degeneration constitutes the racial evolutionary background of dental crowding in humans. 2. Genetic factors refer to the fact that the number, size and shape of teeth are controlled by genetics. The size, position, and morphology of the jaws are influenced to some extent by genetic factors, and oversized teeth, multiple teeth, and crowded teeth due to underdeveloped jaws are significantly related to genetic factors; 3. Environmental factors refer to local and systemic factors that can lead to dental crowding during growth and development, such as early loss of milk teeth, especially early loss of the second milk molar, which will cause the first permanent molar The proximal movement of the first permanent molar during eruption will cause a reduction in the length of the dental arch, resulting in insufficient space for the eruption of the premolar teeth and crowding.