A cohort effect occur when a commonly aged group of people in research indirectly affect results due to their common age-related influences. A cohort is a group of people who share a common identity in some way. A college freshman class could be said to be a cohort. In research a cohort effect can be seen when people who are around the same age affect experimental results indirectly. People who are in the same age group were exposed to the same historical cultural events, traditions, social situations, and trends as each other. These make age groups different from each other in their uniqueness.
For example, think about how different a group of American children who grew up in the Great Depression would be from children who grew up in the prosperous 1990s. An example of a cohort effect could be seen in an experiment in which participants use a computer to perform a cognitive task. The results might show that participants in their 20s did vastly better on the cognitive test that participants in their 60s. But a cohort effect is responsible for the significance- the participants in their 20s have been using computers their entire lives and were more comfortable and proficient with the computerized testing format than the participants in their 60s.