What ergodicity means for you

This paper explores the relation between within-person and between-person research designs using the concept of ergodicity from statistical mechanics in physics. We demonstrate the consequences of ergodicity using several real data examples from previously published studies. We then create several simulated examples that illustrate the independence of within-person processes from between-person differences, and pair these examples with analytic results that reinforce our conclusions. Finally, we discuss the plausibility of ergodicity being the general rule rather than the exception for social and behavioral processes, address common arguments against heeding the implications of ergodicity for behavioral research, and offer several possible solutions. •Ergodic processes give the same results for within-person and between-person designs.•Non-ergodic processes give arbitrarily distinct results regardless of sample size.•Most processes in developmental cognitive neuroscience are unlikely to be ergodic.•Longitudinal, spatial, and temporal aggregation mask individual processes.•To understand within-person processes and causes, within-person designs are needed.