Constipation was present in 8.7% of 676 children (338 twin pairs). Monozygotic twins were concordant for constipation almost four times as frequently as dizygotic twins, indicating a genetic predisposition for constipation. Further evidence of the hereditary nature of constipation is shown by the family studies: the incidence of constipation in the children was increased whan a parent, a sibling, or a twin was constipated.
Bakwin H, Davidson M. Constipation in Twins. Am J Dis Child. 1971;121(2):179-181. doi:10.1001/archpedi.1971.02100130133018