What are cohort effects?
From PsychWiki - A Collaborative Psychology Wiki
The effects of being born at about the same time, exposed to the same events in society, and influenced by the same demographic trends and thus, having similar experiences that make the group unique from other groups. Cohort effects are most likely to be a problem during a cross-sectional study as it is difficult to separate effects of developmental changes from cohort effects when examining age effects across a wide range of ages (Cozby, 2009).
Example: In this hypothetical study of learning to use computers, the result indicates that people in the 50-year-old group scored lower on the learning measure when compared with people in the 40-year-old group (Cozby, 2009).
Application: Should we conclude that the difference in the ability to learn to use a computer decreases with age due to developmental age changes or was due to a cohort effect - people in the 50-year-old group scored lower on the learning measure because while growing up, they had less experience with computers when compared with people in the 40-year-old group (Cozby, 2009)?
Cozby, P. C. (2009). Methods of Behavioral Research: Tenth Edition. New York, NY: McGraw-Hill.