Making sense of development: What within-person and between-person patterns reveal

Developmental science seeks to understand how humans change. Annual longitudinal approaches provide insights into development but miss the micro-level in-between moments in young people's daily experiences that connect annual changes. Intensive longitudinal methods such as daily diaries capture these micro-level fluctuations; when analyzed with multilevel models, these methods simultaneously examine daily within-person and between-person processes. Drawing on our studies of ethnically and racially minoritized young people in the United States, we synthesize three recurring patterns: (1) nonsignificant within-person associations but significant between-person associations, (2) significant within-person associations but not between-person associations, and (3) significant effects at both levels but in opposite directions. Discrepancies across levels reveal mechanisms. These patterns can refine developmental theory and inform interventions.