A research study finds that 40-year-olds are better able to solve math problems than 80-year-olds. It is suspected that the differences in problem-solving ability result from the fact that 80-year-olds had fewer opportunities to study math and did not have access to calculators or computers. If true, the study has revealed:

a) experimenter bias.
b) a placebo effect.
c) age-related differences.
d) cohort effects.

ANS: D, Differences that occur between people who were born and raised during different time periods are called cohort effects.