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Adoption and Parental Investment
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It has long been known that children are likelier to be abused, neglected, or murdered by stepparents than by birth parents. This kind of genetic discrimination seems consistent with kin selection theory: parents are expected to care more for children who share kinship with them, as opposed to a purely legal and social relationship (Daly & Wilson, 1980).

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.

(Republished from Evo and Proud by permission of author or representative)
• Category: Science • Tags: Adoption, Kin Selection 
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  1. John Smyth says: • Website

    I'm guessing stepparents end up stepparents less often out of a totally free choice compared to adopting parents. Adopting parents deliberately want a child to take care of, and the same psychological bonds can occur with an adopted child as with a genetic child. A stepparent may be more likely to end up taking care of a child by circumstance, and not had the ability or desire to bond as well (on average). The main difference I see between genetic and adopted is the adopted might have spent less time together, thus less time to bond, especially missing out on pregnancy and the first few years of attachment.

  2. Tod says:

    My impression is that the average SES of those who adopt despite already having a genetic child of their own is especially high. The adopted child is being held to a high standard in comparisons with the genetic child of such adoptive parents I think.

    Women who gave their baby up for adoption would be likely to have been in an unusual emotional state during their pregnancy. Would fetal stress be a non genetic explaination for difficulties as adults ?

    Not realy relevant to this post but I found this surprising

    "The proportion of people who never married and the age at first marriage increased in rural Ireland […] In 1851, 11% of the population were never married at 45–54 years and this percentage increased steadily over time to 34% for men and 25% for women in 1936.
    In rural Ireland wealthy heads of households were more likely to be celibate than occupiers of small holdings".
    Kent 2002. In the moral climate of Ireland at that time choosing to remain unmarried meant most of these men were chooosing to be truly celibate.

  3. John,

    I agree. Another factor is that stepparents tend to be a select group. They are probably more altruistic than parents in general. Stepparents probably represent a more random sample of the general population.


    Fetal stress might be an explanation. There's also alcohol and substance abuse — both of which are common among mothers who give up their children for adoption. But all of these environmental factors are confounded with genetic factors. People who end up in dysfunctional relationships tend to be less discerning than other people. The same goes for people who abuse alcohol. Genetically, they're not a random sample of the general population.

  4. John,

    I should have written 'adoptive parents tend to be a select group'.

  5. Anonymous • Disclaimer says:

    A bit on another topic, but regarding the overrepresentation of adoptees in the juvenile justice system, psychiatric treatment centres, do you feel that this is due to the supposed "genealogical bewilderment" theory (that adoptees are psychologically traumatized by not being raised by their birth parents) or to genetic and prenatal factors? I strongly suspect it's the second (genetic factors), but I like to know others' opinions and explanations.

    • Replies: @Emilia
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  10. Emilia says:

    I strongly suspect genetic (and prenatal) factors are behind the overrepresentation of adoptees in the juvenile justice and psychiatric system. One other powerful argument against the “genealogical bewilderment” theory is that despite their problems, adoptees generally do better than children raised by single and/or teenage mothers.

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