A group of people who experience a significant event, such as birth or leaving school, during the same period of time. Cohort analysis traces the subsequent vital history of cohorts; see Frejka and Callow (2001) Pop. & Dev. Rev. 27, 1 on cohort reproductive patterns in low-fertility countries. A cohort effect is any effect associated with being a member of a group born at roughly the same time and bonded by common life experiences; Cohen and Iams (2007) Int. Mig. Rev. 41, 3, for example, see a US cohort effect in the fact that foreign-born people have lower retirement incomes than do US-born people. See Franklin and Plane (2004) Geograf. Anal. B 60, 1 on the contribution of the cohort effect to fertility decline in Italy. The major problem of cohort analysis is to distinguish between the effects on the cohort of getting older (age effects), of common experiences like National Health orange juice (cohort effects), and particular historical events, like a war (period effects).

Cohort fertility is the total of live births born to a particular birth or marriage group; see Lutz and Skirbekk (2005) Pop. & Dev. Rev. 31, 4.