Sibling Differences and Parents' Differential Treatment of Siblings: A Multilevel Meta-Analysis

The current study used meta-analysis to ask whether age differences, sex differences, and family size are linked to differences in parental treatment, as well as whether effect-sizes were moderated by the way parental differential treatment (PDT) was measured, who reported on the PDT, and the domain of PDT. Between August 2015 and November 2020, PsycINFO, Google Scholar, and PubMed Central were searched for articles relating to parental differential treatment; additionally, 13 sibling relationship scholars were contacted to collect unpublished analyses or unused data. Multilevel data came from 13,628 unique participants across 1,388 effect sizes nested within 66 sources (articles/raw data sets), nested within 23 unique samples (74% from North America; 26% from Western Europe). Multilevel meta-analysis conducted in R with the metafor package showed greater differences in parental treatment when siblings were further apart in age or a different sex. The main effects for age spacing, however, were moderated by several factors, particularly the domain of parenting. Specifically, age spacing was linked only to PDT based on control or autonomy granting, meaning when siblings were further apart in age, parents granted more autonomy to one sibling over another. Results were limited, however, by limited sample sizes at the sample level. Overall, findings suggest that parents may, in part, treat siblings differently because they are different to begin with.