Compare Groups These diverse groups seem ideal for cross-sectional research. The younger ones (left) have their hands all over each other and express open-mouth joy—but with age, even smiling classmates, like these high school friends (right), are more restrained. However, with cross-sectional research, it is not certain whether such contrasts are the direct result of chronological age or the result of other variables—perhaps income, cohort, or culture.
CORBIS FLIRT/ALAMY
CATHY WALSH
CATHY WALSH