In evolutionary psychology, the Cinderella effect describes the phenomenon of a higher incidence of child abuse and mistreatment by stepparents than biological parents.
[5] This discrimination against stepchildren does not align with the typical abuse statistics involving the overall population given, "the following additional facts: (1) when child abuse is detected, it is often found that all the children in the home have been victimized; and (2) stepchildren are almost always the eldest children in the home whereas the general (albeit slight) tendency in families of uniform parentage is for the youngest to be the most frequent victims.
They argue that human child rearing is so prolonged and costly that "a parental psychology shaped by natural selection is unlikely to be indiscriminate.
Martin Daly and Margo Wilson follow this second mechanism, and expect that parents "discriminate in favor of their own young"; i.e., their actual close relatives.
Abundant data on the mistreatment of stepchildren have been collected and interpreted by psychologists Martin Daly and Margo Wilson, who study with an emphasis in Neuroscience and Behavior at McMaster University.
[12] In addition, the act of infanticide speeds the return to sexual receptivity in the females, allowing for the male to father his own offspring in a timelier manner.
Thus, according to Daly and Wilson, step-parental investment can be viewed as mating effort to ensure the possibility of future reproduction with the parent of their stepchild.
"If child abuse is a behavioral response influenced by natural selection, then it is more likely to occur when there are reduced inclusive fitness payoffs owing to uncertain or low relatedness".
[18] Daly and Wilson's reports on the over-representation of stepparents in child homicide and abuse statistics support the evolutionary principle of maximizing one's inclusive fitness, formalized under Hamilton's rule, which helps to explain why humans will preferentially invest in close kin.
The results indicate that the only living situation that has a significant correlation to increased child abuse is one natural parent and one stepparent in the same household.
[6] Daly and Wilson examined several potentially confounding variables in their research, including socioeconomic status, family size, and maternal age at childbirth.
[27] Strong support for the Cinderella effect as described by Daly and Wilson comes from a study of unintentional childhood fatal injuries in Australia.
[28] Tooley et al. follow the argument of Daly and Wilson to extend the Cinderella effect from cases of abuse to incidents of unintentional fatalities.
[28] Based on data gathered from the Australia National Coroners' Information System, stepchildren under five years of age are two to fifteen times more likely to experience an unintentional fatal injury, especially drowning, than genetic children.
[28] This higher risk of injury for stepchildren can be attributed to the fact that stepparents occupy the same supervisory role as a genetic parent, yet they have a lower intrinsic commitment to protecting the child and therefore are less likely to be adequately vigilant.
[28] The authors conclude that the Cinderella effect applies not only to purposeful abuse by stepparents, but is also relevant to explaining increased rates of accidental fatalities among stepchildren.
[32] Though the general trend of the data from this study supports the Cinderella effect, Anderson and colleagues note that the observed differences between investment in children and stepchildren might be slightly reduced by a few confounding factors.
[31] For example, the authors point out that step parenting is a self-selective process, and that when all else is equal, men who bond with unrelated children are more likely to become stepfathers, a factor that is likely to be a confounding variable in efforts to study the Cinderella effect.
[33] Additionally, a study of Hadza foragers in Tanzania by Marlowe also finds evidence of decreased care provided by men to stepchildren when compared with genetic children.
[35] The author argues that these results show that in terms of time invested, men favor their children over stepchildren, and this preference is not attributable to the duration of the adult-child relationship, a factor which is sometimes believed to be a confounding variable in the Cinderella effect.
[35] Philosopher of science David Buller, as a part of his general critique of evolutionary psychology,[36] has reviewed Daly and Wilson's data.
Furthermore, they found that when the perpetrator was an "Other unrelated (including boyfriend)" individual, maltreatment was reported on the death certificate 86 percent of the time, significantly higher than for parents.
[39][41] Therefore, as Daly and Wilson respond to Buller's critique, this does not seem to be a significant source of error in studying the Cinderella effect and does not provide evidence for inherent bias in their data.
The study, published in 2000 and conducted by Temrin and colleagues argued that when Daly and Wilson classified homicides according to family situation, they did not account for the genetic relatedness of the parent who actually committed the crime.
After correcting for age differences, the Swedish data set produces results in accordance with the previous findings of Daly and Wilson.
The selection factors theory proposes that individuals who are likely to be stepparents (divorcees) are likely to be inherently more violent due to emotional disturbances, aggressive impulses, and self-esteem issues.
[43] Discussing the implications of this line of research, Australian psychologist Greg Tooley, author of a 2006 study confirming the existence of the effect,[28] confessed that "it is certainly difficult to talk about because it is such a hot issue".