Cohort Effect

What it is

Bobby Duffy's Generations separates three things that are easy to confuse. Period effects touch everyone at once, as when an event changes a whole society. Lifecycle effects come with age, so that people tend to behave in similar ways at similar stages of life. Cohort effects stay with a group that shared an early experience, and they remain visible as that group ages.

The distinction matters for forecasting. The consumer behaviour textbook by Cathy Neal, Pascale Quester and Del Hawkins warns against assuming that today's 40 year olds will adopt the lifestyles and consumption patterns of today's 50 year olds, because the younger group grew up under different conditions.

In effect

In the article on why people cannot get enough of Harry Potter, the cohort frame is applied to a book series. Readers who met the novels at a similar stage of life share a common reference point, in the way any cohort carries a shared early experience. The seven novels were published between 1997 and 2007, so a child who began the first could finish the last as an adult, and the story grew up alongside its first readers. That application is an interpretation made on this site.

What it does not say

It does not say that everyone born in the same years is the same. A cohort effect is a tendency of a group, and it sits beside period and lifecycle effects, which are hard to separate in practice. It also does not claim that a series creates a cohort by itself; the idea describes shared early experience in general.

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