Evolutionary theory predicts that relatedness will affect family relationships. Previous studies on siblings have mainly focused on sibling differentiation, sibling rivalry, and incest avoidance, and very few have examined the impact of genetic relatedness on the sibling relationship. Using a large data set from the Netherlands (Netherlands Kinship Panel Study), I show that relatedness (full vs. half-sibling) independently influences social investments between siblings. Maternal half-siblings, who are raised together like full siblings (FS), were found to show significantly lower levels of investment than FS. This suggests that a psychological mechanism besides childhood proximity regulates investment in the sibling relationship. Yet, levels of investment were overall higher for maternal than paternal half-siblings, suggesting an important role for childhood co-residence. Results are discussed with reference to kin selection theory. © 2007 Elsevier Inc. All rights reserved.
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