If stepchildren are mistreated because they are not kin, we should see the same mistreatment of adopted children. To test this hypothesis, Gibson (2009) surveyed parents with at least one genetic and one adopted child over the age of 22, the idea being to compare the two groups of children for total parental investment. Contrary to expectation, the parents invested more in their adopted children than in their own:
This study categorically fails to support the hypothesis that parents bias investment toward genetically related children. Every case of significant differential investment was biased toward adoptees. Parents were more likely to provide preschool, private tutoring, summer school, cars, rent, personal loans and time with sports to adopted children (Gibson, 2009).
Why? One can imagine the parents making no distinction, but why would they discriminate against their own children? The answer seems to be that the adopted siblings made greater demands.
Adoptees were more likely than genetic offspring to have ever received public assistance, been divorced or been arrested. They also completed fewer years of schooling and were more likely to have ever required professional treatment for mental health, alcohol and drug issues.
… The current study may demonstrate cases where “the squeaky wheel gets the grease.” Summer school and private tutors are often remedial, and the fact that adopted children were more likely to receive them suggests they required them more often than genetic ones. The same can be said for rent, treatment and public assistance. Adoptees may have more difficulty establishing themselves relative to genetic children, and the fact that they divorce more often suggests they also have more difficulty staying established. Addiction and divorce may put adoptees in situations that require more parental investment. Parents provide more for adoptees not because they favor them, but because they need the help more often. (Gibson, 2009)
For many behavioral traits, adoptees seem to differ genetically not only from their adoptive parents but also from the general population:
This supports other research showing that, compared to genetic children, American adoptees have a higher overall risk of contact with mental health professionals, specifically for eating disorders, learning disabilities, personality disorders and attention deficit hyperactivity disorder … They also have lower achievement and more problems in school, abuse drugs and alcohol more, and fight with or lie to parents more than genetic children …
… Adoptees may be genetically predisposed to negative outcomes at higher rates than the general population. Genetic factors clearly contribute to alcohol and drug addiction, as well as to some mental disorders like attention deficit hyperactivity disorder and schizophrenia …. An association between nonviolent criminality has been found between European adoptees and their genetic parents … Furthermore, research with Swedish adoptees suggests 55-60% of their educational performance is explained by genetic factors, and that the number of years of school adoptees complete is significantly related to how many years their genetic mothers completed ... (Gibson, 2009).
All of this may explain why parents invest more in adopted children than in their own. But why do any parents adopt? Doesn’t such a decision, in itself, contradict kin selection theory?
The contradiction may be more apparent than real. Most adoptive parents have fertility problems and cannot have children on their own. Their only other option is to remain childless.
It may be that adopting fulfills a common instinct to reproduce and parents do it because it produces positive emotions. When people cannot have children biologically, adoption gives them a way to fulfill the “drive” to parent, maladaptive or not. (Gibson, 2009)
Daly, M., & M. Wilson. (1980). Discriminative parental solicitude: A biological perspective. Journal of Marriage and the Family, 2, 277-288.
Gibson, K. (2009). Differential parental investment in families with both adopted and genetic children, Evolution and Human Behavior, 30, 184-189.