This ADA Critical Summary looked at a 2012 systematic review by Hafez et al. The aim of the review was to assess the relationship between dental crowding and the development of caries.
The summariser notes that the search was comprehensive but the inclusion criteria were not clear. Studies were assessed for quality and all the included studies were cross-sectional in nature. It was also noted that the quality of reporting and the heterogeneity of indexes and criteria did not allow a meta-analysis to be performed. Overall the currently available evidence is not sufficient to confirm or refute a causal relationship between crowding and caries
Hafez HS, Shaarawy SM, Al-Sakiti AA, Mostafa YA. Dental crowding as a caries risk factor: a systematic review. Am J Orthod Dentofacial Orthop. 2012 Oct;142(4):443-50. doi: 10.1016/j.ajodo.2012.04.018. Review. PubMed PMID: 22999666.