What Does Evolutionary Theory Predict About Step-Parents?

Evolutionary theory predicts that, on average, the more closely someone is genetically related to us, the better we will treat them (i.e. we will be more inclined to divert resources, energy, etc. to them for their benefit and at a cost to ourselves). For example, the theory predicts an individual will act more selflessly towards his/her child (with whom s/he shares 50% of his/her genes) than his niece or nephew (with whom s/he shares only 12.5 % of his/her genes). Of course, human behaviour is far, far more complex than that but these are the ballpark figures evolutionary theory produces when considering the likely behaviour, on average, of all human beings who have ever existed. In other words, according to the theory, on average, and all else being equal, an individual is predicted to behave about 4 times less selfishly towards his/her child than towards his/her nephew. (I remember when I was an undergraduate at university our lecturer chose to illustrate this phenomenon in rather more stark, clinical and chilling terms – he explained that, theoretically, at least, our ancestors would have had a preference to save five of their nephews over one of their children because the maths says one child represents a fifty per cent genetic overlap with said ancestor whereas five nephews represent, collectively, a sixty-two-and-a-half per cent genetic overlap.

Does this theory help to predict the behaviour of step-parents towards their step-children compared to their behaviour towards their own, biological children?

Well, according to the cold calculations evolutionary theory impels us to apply, there is no contest because an individual shares (as we saw above) 50% of his/her genes with his/her biological child but zero percent with an unrelated step-child. We should then expect that step-parents will tend to treat their own children significantly better (e.g. display more altruism towards them) than their step-children.

Or, to put it the other way around, the theory predicts that step-parents will tend to treat their step-children significantly worse than they treat their own biological children. This prediction was put to the test by Daly and Wilson who analyzed statistical data relating to child maltreatment in North America. What they discovered confirmed the prediction as shown below:

  • a child living with one or more substitute parents were about 100 times more likely to be fatally maltreated than a child living with his/her biological parents
  • a study carried out in Canada produced similar results: children aged two years or younger were about 70 times more likely to be killed by a step-parent than by a biological parent.


Daly and Wilson also found that as step-children got older they were less likely to be killed by step-parents from which they inferred men who killed in outbursts of rage were probably new partners who had not had time to develop a bond/attachment with the baby.


The above finding has, understandably, given how emotive the theory is, sparked much controversy. One objection to Daly’s Darwinian theory of stepparenting (sometimes referred to as the ‘Cinderella Effect‘) is that the research upon which Daly drew to support his ideas did not take into consideration potentially contaminating variables such as addiction (to drugs and alcohol) and poverty. However, the idea that people from poor environments are more likely to maltreat children is itself controversial and liable to inflame passions.

Objection That Theory Undermines The Stepfamily:

Pryor (Victoria University) has argued that theories such as the one elucidated above can undermine the conscientious efforts so many stepfathers make to nurture, love, care and provide for their stepchildren.


And, of course, stepparents who kill their stepchildren are exceptionally rare and Daly concedes that most families that incorporate stepparents ‘function pretty well.’


David Hosier BSc Hons; MSc; PGDE(FAHE).