Cinderella's stepmother was cold and wicked. The dangers of being raised by such uncaring non relatives recur in folklore around the world. Yet, there are good reasons for doubting the implication that we can only really love children who are our own progeny. Although remarriage of a father or mother can have bad consequences for children, when a couple adopts a baby unrelated to either of them its prospects are much brighter.
The Cinderella effect is well substantiated in crime data. Children growing up in step families are about 40 times as likely to be abused and 140 times as likely to be murdered as children growing up with both natural parents (murder still being a low probability) .
Adoption is quite different. In Kindness in a Cruel World, I concluded that parents treat their adopted children just as well as biological children.
Two recent studies help to clarify the issue of how well adoptive children are treated. The first, published in 2007, found that children in adoptive households are treated better than children in homes with two genetic parents. Adoptive parents were more likely to provide computers for their children, more likely to eat meals with them, and more involved in sports, science projects, and so forth. This falsifies the fairy-tale claim that parents cannot treat genetically unrelated children as well as their own kin.
Still, adoptive parents are an usual breed, that are carefully chosen by adoption agencies for kindness, and commitment to children, as well as stable employment. Perhaps these factors tip the scales against natural parents.
The second study, by anthropologist Kyle Gibson, was explicitly designed to get around such problems and studied homes having one natural child and one adopted child using records provided by an adoption agency for adoptees over 22 years.
Once again parents invested more in the adopted child than in own offspring. Adopted children were more likely to attend preschool and to receive private tutoring. Adoptees had a better chance of receiving cars and personal loans. Parents also spent more time at their sports events.
The findings raise three key questions. First, why did parents favor adoptees over kin? Second, how could such a tendency have slipped through the net of natural selection? Third, why are stepchildren at so much higher risk of parental abuse than adoptees are?
According to Gibson, the likely reason that parents invested more in the adopted children was that they needed more help - possibly for genetic reasons. Adopted children did worse in school, had more problems with alcohol and drug addiction, had more arrests, and were more likely to receive welfare. Evidently, parents invest more in adopted children not because they favor them but because they need more help.
As to why human parents have no defense against nurturing non relatives, it can be argued that adopting non relatives is an artificial consequence of modern environments. In the distant past, our foraging ancestors were likely to be fairly closely related to any unattached babies they encountered and so evolved no defense against investing in non relatives.
Finally, on the issue of why step-parents behave so differently from adoptive parents, it seems that there are conflicts of interest whereby the step parent cares for unrelated children as a means of wooing the natural parent with a view to having children of their own. Jealousy may ensue as children vie with the step parent for the affection of the biological parent. Step families are thus formed as a result of high mating effort whereas adoption selects parents capable of high investment in children and couples who work better as a team because their loyalties are not divided.
Step families may be formed following divorce and therefore involve persons who are less agreeable or more aggressive. The fact that stepchildren are often first encountered in childhood rather than infancy may also make it harder for the step parent to form a bond with them. Adoption generally involves infants.
Whatever the reasons for the Cinderella effect, it is now quite clear that it is not triggered by a lack of genetic relationship. After all, adoptive parents take better care of children than birth parents do. It seems that caring for children, related or not, involves a deep human need that is better satisfied by adoption than step-parenting.