Parental and child problematic smartphone use over time: Six-year within-dyad analysis in two Korean cohorts.

Emerging evidence suggests a positive link between parental and child problematic smartphone use (PSU), yet the direction and reciprocity of this relationship over time remain unclear. Here, we address this gap by examining six years (2018-2023) of PSU data from 4,086 parent-child dyads in South Korea, comprising two cohorts: a child cohort (1,984 dyads; mean child age = 11.00, 51.1 % female; mean parent age = 42.17, 95.0 % mothers) and an adolescent cohort (2,102 dyads; mean child age = 14.00, 46.6 % female; mean parent age = 44.32, 94.6 % mothers). Random-intercept cross-lagged panel models (RI-CLPMs) revealed consistent patterns across both cohorts. At the between-dyad level, parental and child PSU covaried, such that children of parents with higher PSU also reported higher PSU. At the within-dyad level, their PSU levels co-fluctuated over time; that is, in years when parents' PSU rose above their typical level, their children's PSU rose as well. However, no significant cross-lagged effects emerged, indicating that increases in parental PSU did not predict subsequent changes in child PSU and vice versa. Together, these findings suggest that longitudinal co-fluctuation patterns in PSU, rather than causal transmission, exist within a shared family digital ecology, thereby highlighting family-level dynamics in understanding PSU.