2 A systematic review 69 between parenting and adolescent adaptation. This review showed an exponential growth in publications within the last years, as more than half of the included studies were published between 2015 and 2018. These within-family studies examine parenting processes at a level that fits better with contemporary parenting theories and demonstrate how different parenting processes sometimes may occur within different families. That is, even though a handful of studies highlighted that adolescents are better adapted in periods when their own parents are more supportive or controlling, other empirical estimates suggested that some adolescents are less well adapted in such periods. Notwithstanding the strengths, there were also limitations. First, the review did not contain meta-analytic estimates of the included associations, because standardization methods of within-family (or within-person) effect sizes are still developing (Wang et al., 2019). Hence, it was not yet possible to compare within-family effect sizes across studies. Additionally, many associations were studied by a limited number of unique studies and samples, and mostly either on a daily or (semi- or bi-)annual timescale. Therefore, future methodological advances regarding standardization methods of multilevel effect sizes, uniformity among researchers in the use of such standardization practices, and additional studies on similar within-family parenting processes and with different timescales, appear vital for future meta-analytic assessment. Second, many studied samples were from a small number of WEIRD (Western, educated, industrial, rich, and democratic) backgrounds (Henrich et al., 2010). That is, many samples were from the United States or the Netherlands with middle-to-high socioeconomic status. To fully understand how parenting processes work within families across a wide variety of families, it is vital for future research to also study families from non-WEIRD backgrounds and to take into account differences between families in terms of ethnicity and socioeconomic status, but also for example sexual orientation (as no information was provided in the included studies). Third, the systematic search focused on peer-reviewed studies. Research that has not been peerreviewed, such as book chapters or dissertations, may present additional insights that are not obtained through the included peer-reviewed studies. Nonetheless, it can be expected that peer-reviewed research often has higher quality standards than non-peer reviewed research and is often the knowledge on which researchers further build upon. Directions for Future Research To support the promising trend and forthcoming studies, three aspects seem important to consider when matching theoretical questions about parenting processes to the design of within-family studies. The first aspect is the timescale of observation (i.e., over-time fluctuation
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