Environmental factors work to make siblings in the same family different from one another rather than similar. The Sibling Inventory of Differential Experience (SIDE; Daniels & Plomin, 1985) was constructed to evaluate how differently siblings perceive their experiences to be across three domains: parental treatment, sibling treatment, and peer characteristics. The goals of the study were threefold: to assess the degree to which siblings agree about their differential experiences; to examine the stability of perceived differential treatment across three years; and to assess genetic and environmental contributions to the SIDE using both the twin design and the stepfamily design. These goals were explored using a genetically sensitive sample of 719 same‐sex sibling pairs ranging from 10 to 18 years of age of whom 94% of eligible pairs were retested three years later. Results indicated moderate agreement between siblings, and considerable stability across 3 years. As in previous research on the SIDE, MZ vs. DZ results indicated greater genetic influence than did results for full siblings vs. genetically unrelated siblings. The results suggest that, although intended as a measure of nonshared environment, the SIDE is not impervious to genetic influence as assessed by the twin design.
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Pike et al. (2000) studied this question.