Over the past decades, researchers who study adolescent development slowly reached an unnerving conclusion: much of the scientific knowledge on development that we have generated by studying groups, does not directly describe processes that unfold within individuals over time (this so-called non-ergodicity is explained in the next section). This is a shocking realization indeed, as one would expect developmental psychology to describe individual-level processes, and individual differences in change therein. Instead, most of what we have generated is knowledge about groups of individuals, and truths about averages that may or may not be true for any one individual (Blaauw, 2018; Molenaar, 2004). Subsequently we have a huge gap in our knowledge (see also Lichtwarck et al., 2008), which spans all domains of adolescent development. Uncertainty about whether and when group-to-individual generalizations are valid is particularly detrimental for our ability to effectively inform interventions (e.g., Fisher et al., 2018), as these typically deal with changes within individuals over time.