Cinderella effect
In evolutionary psychology, the Cinderella effect describes the phenomenon of a higher incidence of child abuse and mistreatment by stepparents than biological parents. It takes its name from the fairy tale character Cinderella, a girl who is mistreated by her stepmother and stepsisters. Evolutionary psychologists describe this effect as being a byproduct of a bias towards biological family and a conflict between reproductive partners investing in young children that are unrelated to one partner.
Background
In the early 1970s, a theory arose on the connection between stepparents and child maltreatment. In 1973, forensic psychiatrist P. D. Scott summarized information on a sample of "fatal battered-baby cases" perpetrated in anger, and found that 15 of the 29 murders he considered, or 52%, were committed by stepfathers. Although initially there was no analysis of this raw data, empirical evidence has since been collected on what is now called the Cinderella effect through official records and reports.Since the 1970s, scholars have sought data regarding the validity of the Cinderella effect from a variety of sources including official reports of child abuse, clinical data, victim reports, and official homicide data and have found a direct relationship between step-parents and child abuse. Studies have concluded that "stepchildren in Canada, Great Britain, and the United States indeed incur greatly elevated risk of child maltreatment of various sorts, especially lethal beatings."
In circumstances where the family contains both biological children and step-children, studies have found that step-parents generally favor their biological children. In such families, stepchildren were exclusively targeted 9 out of 10 times in one study and in 19 of 22 in another. In addition to displaying higher rates of negative behaviors toward stepchildren, stepparents displayed fewer positive behaviors toward stepchildren compared to the biological parents. For example, on average, stepparents invested less in education, played with stepchildren less, and took stepchildren to the doctor less, among other things. This discrimination against stepchildren does not align with the typical abuse statistics involving the overall population given, "the following additional facts: when child abuse is detected, it is often found that all the children in the home have been victimized; and stepchildren are almost always the eldest children in the home whereas the general tendency in families of uniform parentage is for the youngest to be the most frequent victims."
Evolutionary psychology theory
and Margo Wilson propose that the Cinderella effect is a direct consequence of the modern evolutionary theory of inclusive fitness, especially parental investment theory. They argue that human child rearing is so prolonged and costly that "a parental psychology shaped by natural selection is unlikely to be indiscriminate." According to them, "research concerning animal social behavior provides a rationale for expecting parents to be discriminative in their care and affection, and more specifically, to discriminate in favor of their own young." Inclusive fitness theory proposes a selective criterion for the evolution of social traits, where social behavior that is costly to an individual organism can nevertheless emerge when there is a statistical likelihood that significant benefits of that social behavior accrue to other organisms whom also carry the social trait. Under such conditions, a net overall increase in reproduction of the social trait in future generations can result.The initial presentation of inclusive fitness theory That a social trait might reliably operate straightforwardly via social context in species where close relatives are usually concentrated in a local home area where they were born That genetic detection mechanisms might emerge that go beyond statistical correlations, and reliably detect actual genetic relatedness between the social actors using direct "kin recognition". The relative place of these two broad types of social mechanisms has been debated, but many biologists consider "kin recognition" to be an important possible mechanism. 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.
Daly and Wilson research
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. Their first measure of the validity of the Cinderella effect was based on data from the American Humane Association, an archive of child abuse reports in the United States holding over twenty thousand reports. These records led Wilson and Daly to conclude that "a child under three years of age who lived with one genetic parent and one stepparent in the United States in 1976 was about seven times more likely to become a validated child-abuse case in the records than one who dwelt with two genetic parents." Their overall findings demonstrate that children residing with stepparents have a higher risk of abuse even when other factors are considered.Explanation
All organisms face trade-offs as to how to invest their time, energy, risk, and other resources, so investment in one domain generally takes away from their ability to invest in other domains. Investment in non-genetic children therefore reduces an individual's ability to invest in itself or its genetic children, without directly bringing reproductive benefits. Thus, from an evolutionary biology perspective, one would not expect organisms to regularly and deliberately care for offspring not their own.Daly and Wilson point out that infanticide is an extreme form of biasing parental investment that is widely practiced in the animal world. For example, when an immigrant male lion enters a pride, it is not uncommon for him to kill the cubs fathered by other males. Since the pride can only provide support for a limited number of cubs to survive to adulthood, the killing of the cubs in competition with the new male's potential offspring increases the chances of his progeny surviving to maturity. 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. These observations indicate that in other animals, males employ certain measures to ensure that parental investment is geared specifically toward their own offspring.
Unlike the lion however, humans in a step parenting situation face a more complicated trade-off since they cannot completely disown their partner's offspring from a previous relationship, as they would risk losing sexual access to their partner and any chance of producing potential offspring. 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. This mating effort hypothesis suggests that humans will tend to invest more in their genetic offspring and invest just enough in their stepchildren. It is from this theoretical framework that Daly and Wilson argue that instances of child abuse towards non-biological offspring should be more frequent than towards biological offspring.
One would therefore expect greater parental responsiveness towards one's own offspring than towards the offspring of others, and this will result in more positive outcomes and fewer negative outcomes towards one's own children than towards other children in which one is expected to invest. "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". Owing to these adaptations from natural selection, child abuse is more likely to be committed by stepparents than genetic parents—both are expected to invest heavily in the children, but genetic parents will have greater child-specific parental love that promotes positive care taking and inhibits maltreatment.
Daly and Wilson also note that this parental love can explain why genetic offspring are more immune to lashing out by parents. They assert that, "Child-specific parental love is the emotional mechanism that permits people to tolerate—even to rejoice in—those long years of expensive, unreciprocated parental investment". They point to a study comparing natural father and stepfather families as support for the notion that stepparents do not view their stepchildren the same as their biological children, and likewise, children do not view their stepparents the same as their biological parents. This study, based on a series of questionnaires which were then subjected to statistical analyses, reports that children are less likely to go to their stepfathers for guidance and that stepfathers rate their stepchildren less positively than do natural fathers.
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. Adoption statistics also substantiate this principle, in that non-kin adoptions represent a minority of worldwide adoptions. Research into the high adoption rates of Oceania shows that childlessness is the most common reason for adopting and that, in the eleven populations for which data were available, a large majority of adoptions involved a relative with a coefficient of relatedness greater than or equal to 0.125. It is also observed that parents with both biological and adopted children bias the partitioning of their estates in favor of the biological children, demonstrating again that parental behavior corresponds to the principles of kin selection.