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Forget the Cinderella effect: stepparents are just as likely to kill their biological children as their stepchildren

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By Tim O’Mahony Operations Manager at Kexue Communications, www.kexuecommunications.com
Researchers have known since the early 1970s that children in stepfamilies are at greater risk of child abuse and murder (Fergusson, Fleming & O’Neill, 1972). A team of Swedish researchers has found that parents in stepfamilies are equally likely to kill their biological children as they are to kill their stepchildren. Their findings are published in the journal Current Zoology (Online First).

Evolutionary reasons such as a lack of genetic relatedness and kin selection have previously been used to explain the higher rates of child abuse and homicide observed in stepfamilies (Daly & Wilson, 1988). The research team from Stockholm University and Mid Sweden University investigated whether an evolutionary explanation sufficiently explained this higher prevalence.

The research team analyzed Statistics Sweden records of parental child homicides in Sweden for the period 1965–2009. They included data from two biparental family types in their study: families with two genetic parents and stepfamilies. The group also analyzed the proportion of stepfamilies and families with two genetic parents in the general population for 1987, the midpoint of the study, to determine whether children in stepfamilies were statistically more likely to be murdered.

Dr. Temrin’s team used contingency tables to compare the observed frequencies of parents from the two family types committing child homicide with the frequencies of parents in the two family types in the general population. They found that on average there are 3.2 perpetrators per million parents for stepfamilies, and 1.9 per million parents for families with two genetic parents.

The team also investigated perpetrators in stepfamilies specifically, by finding the ratio of biological parent and stepparent child homicide perpetrators in stepfamilies and comparing it to an expected 1:1 ratio using a Chi-square test.

There were 152 perpetrators of parental child homicide in the two family groups used over the 45 year study period: 125 perpetrators were in families with two genetic parents and 27 were in stepfamilies. Of the 27 stepfamily murderers, 13 killed their genetic children, 13 murdered their stepchildren, and 1 perpetrator killed both. This means that the risk of a stepparent or a genetic parent in a stepfamily murdering a child is not significantly different from a 1:1 ratio (Chi-square = 0, p > 0.99).

“Our study suggests that the risk of being killed is not associated primarily with the non-genetic relation stepparent and stepchild but rather by living in a stepfamily,” said the paper’s lead author, Dr. Hans Temrin from Stockholm University.

Data on the criminal record of all Swedish parents in both genetic parent families and stepfamiles was also taken from the The Swedish National Council for Crime Prevention dataset for the midpoint of the study, 1987.

Dr. Temrin’s team found that rates of crime across Sweden were higher for adults in stepfamilies than in biparental families, with general crime 1.5 times higher (28.2% vs. 17.8%), and violent crime twice is high (4.4% vs. 1.9%). These findings agree with previous studies that found that there is a higher incidence of unemployment, psychiatric problems and anti-social behaviors for parents in stepfamilies than parents in families with both genetic parents (Belsky, 1993; Turner, Finkelhor & Ormrod, 2007).

“The Cinderella effect – the observation that adults are more likely to kill their stepchildren than their biological children – has long been a staple of evolutionary psychology-informed homicide research, and this study suggests that this explanation is likely too simple,” said Dr Damon Muller from the National Centre for Epidemiology and Population Health at the Australian National University.

Dr. Temrin explains that “most parental child homicides are not caused by conflicts with the child but rather by problems that parents have.”

“Giving help to people with psychiatric problems and to families with problems in my opinion is the only way to decrease child maltreatment and the risk of children being killed.”

The research team hopes to replicate their study in other countries to investigate whether their observations hold.

References

Belsky J, 1993. Etiology of child maltreatment: A developmental-ecological analysis. Psychological Bulletin 114: 413-434.

Daly M, Wilson M, 1988. Homicide. New York: Aldine de Gruyter.

Fergusson DM, Fleming J, O’Neill DP, 1972. Child abuse in New Zealand. Wellington, New Zealand: A. R. Shearer, Government Printer.

Turner HA, Finkelhor D, Ormrod R, 2007. Family structure variations in patterns and
predictors of child victimization. American Journal of Orthopsychiatry 77: 282-295.