Elsevier

Evolution and Human Behavior

Volume 36, Issue 5, September 2015, Pages 379-388
Evolution and Human Behavior

Original Article
Do parents favor their adoptive or biological children? Predictions from kin selection and compensatory models

https://doi.org/10.1016/j.evolhumbehav.2015.03.001Get rights and content

Abstract

Evolutionary reasoning (Kin Selection Theory) predicts less favorable behaviors directed by parents toward their unrelated children, relative to their biologically related children. By extension, it may be argued that parents should also have less favorable perceptions of the intellectual, personality and other behavioral traits of unrelated children, compared with biologically related children. However, recent work has modified this expectation, given the distinction between unrelated adopted children (who are acquired intentionally) and unrelated stepchildren (who are acquired via mating effort). The compensatory model takes into account evolved desires for parenting and the evolutionarily novel availability of unrelated children. It predicts that adopted children may be viewed as favorably, or even more favorably, than biological children due to parents' compensation for the perceived challenges and stigma linked to their exceptional family structure. In the present study, IQ, Adjective Checklist and Child Behavior Checklist scale scores were available for 135 virtual twin pairs (same-age unrelated siblings raised together). Virtual twins included 41 adopted–biological pairs and 94 adopted–adopted pairs, with a mean age of 6.14 years (SD = 3.51). These unique data allowed tests of hypotheses and predictions concerning parenting perceptions, given the matched age and placement of the biological and adoptive siblings. Consistent with prior research, the IQ scores of the biological children exceeded those of the adopted children, both between and within pairs. A between-pair analysis revealed no difference between biological children and members of adopted–adopted pairs in ratings of favorable or unfavorable traits. However, more telling within-family comparisons of adopted–biological pairs revealed higher scores for adoptees on unfavorable traits, consistent with Kin Selection Theory, but no differences between adoptive and biological children on favorable traits, consistent with the compensatory model. These findings refine our understanding of parenting genetically related and unrelated offspring.

Section snippets

Kin Selection Theory

Evolutionary reasoning predicts less favorable treatment of unrelated children, relative to biological children. These expectations derive from Hamilton's (1964) Kin Selection Theory, namely that altruistic interactions between individuals should vary with their genetic relatedness. Hamilton asserted that behaviors incurring cost to the self (e.g., altruism) could evolve if such costs were outweighed by the benefit, multiplied by the coefficient of relatedness. He reasoned that natural

Fullerton virtual twin study

Participating families were enrolled in the Fullerton Virtual Twin Study (FVTS). The FVTS, ongoing since 1991, gathers behavioral, physical and health-related data on virtual twins and their families.

Four strict criteria decided if a given sibling set qualified as a virtual twin pair for the study. These criteria were generated with a view toward replicating ordinary twinship as closely as possible.

  • 1.

    Both children must be in the home by one year of age. This rule is based on the knowledge that

Results

For descriptive statistics, means and standard deviations for IQ and for the selected ACL and CBCL scales were computed for the full sample and for the VT pairs according to relationship status (adopted, biological). These data are displayed in Table 3.

Summary of findings

The present study compared mental ability and externalizing behavior in a unique sample of unrelated sibling pairs, with reference to different, but related theoretical frameworks. The key finding from Part I was that being an adopted or biological child does not predict ability-related or conduct-related behaviors. The exception was IQ in which the biological children significantly outscored the group of adoptees. However, this measure, while known to the parent, was obtained by an independent

Acknowledgments

This work was supported, in part, by the National Science Foundation (SBR-9712875), and CSU Fullerton faculty awards to the first author. The virtual twins and their families are gratefully acknowledged for their participation and support. This work was completed, in part, while the first author was in residence at the Singapore Management University, in Singapore, January 2013. Several colleagues and several anonymous reviewers completed critical readings of the manuscript prior to submission.

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    Currently in the Department of Human Development and Family Sciences, University of Texas, Austin.

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