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Posts Tagged ‘Evolutionary Psychology

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,
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.


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.

Why Evolutionary Psychology is a Valid Approach for Studying Human Behaviour.

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I will first demonstrate that in behavioural science there is no alternative explanation to evolution when exploring the whole causal chain that leads to a specific behaviour. Secondly I will show that the implication of this perspective yield results also in studies that focus on a more narrow subject. Last I hope to refuse the critics who claim that EP is build on unscientific presumptions.

As humans we are drawn to find causality. Our automatic way of thinking even leads us to see connections where none exists. It is usually not obvious that causal relations can be of different quality. Consider this: If I say: “The stone broke the window.” then I have made a proper causal statement. But when I say “The girl threw the stone, that broke the window.” I have somehow explained more, as I went further backwards along the causal chain of events, which resulted in the broken window. In a scientific reality that defines time s a linear function, there must be an ultimate starting point of the chain of causal events. Explanations or hypotheses that cover the whole length of the causal chain from beginning to end can be considered an “ultimate explanation”.

Until the middle of the 19th century the starting point of this causal chain was considered to be God. But in 1859, Darwin proposed a process, natural selection, that could explain how different species could evolve. A little later Popper proposed that every theory must be falsifiable to yield any scientific merit. Therefore the theory of a God as a creator has been discarded as object of scientific research. Within the scientific community today no other processes are known that could explain the evolution as well as Darwin’s theory. Therefore I draw the conclusion that if one wishes to make an ultimate explanation of any process concerning human behaviour, one must adapt the perspective of evolutionary psychology.

A scientific theory can not be verified, but research can accumulate support for specific idea. The theory under investigation is used to generate hypothesis, which then can be falsified. If various parts of the theory hold up against critical research, the theory can be presumed to be correct. Adopting the perspective of Evolutionary Psychology has brought tremendous progress to many areas of scientific research. The study of animals can offer insights into human behaviour. This is not only true for the cognitive and social processes that have been studies in great apes, but also for the more basic biological processes that have been studied in animals with only a few thousands neurons. For example the insights into the neuronal structure of the cockroach has greatly benefited research in artificial intelligence. It is a great advantage that evolution theory can be applied to all processes whether they are mechanical or historical. In this regard, Evolution Theory brings together biology and many social science and might one day serve as framework to incorporate findings from different scientific areas into one theory.

Critics often state the problem that Evolutionary Psychology is based on presumptions about human life, that can not be verified. Although historians and anthropologists are able to offer a vivid picture of human life in a former time they might plainly be wrong on important facts. Of course this would form a problem to all theories that rely on those presumptions, including Evolutionary Psychology. In my view this problem is not a problem at all. In constructing hypotheses about evolution (for example the social structure of the homo neandertalis) the same principles apply to any other hypothesis: Only if enough evidence has been accumulated from different sources and with different tools the hypothesis can be used as possible explanation. Setting up an experiment without a hypothesis and then arguing backwards to fit an evolutionary picture is clearly against proper scientific procedures. So when criticizing experiments of Evolutionary Psychology one should always be clear on what is imperfect: The theory itself, which in my opinion can only be proven by falsifying deducted hypothesis, or the methodology used.

Therefore I conclude that Evolutionary Psychology, when used within the proper scientific guidelines, is a very interesting perspective that entails great potential for all science dedicated to biological life. The most important function of the evolutionary perspective in my opinion is the stimulation of and focussing of research towards an ultimate theory of life.

Written by Martin Metzmacher

September 27, 2007 at 11:09 am