The message from over 20 studies of child killers and abusers over four decades is that stepfathers are many times more dangerous. However, our recent study suggests that this is wrong.
One of the most influential studies that made the case that stepfathers are more dangerous than biological fathers was published in 1994 by Martin Daly and Margo Wilson of McMaster University in Canada. They analysed British child homicide data and reported that stepfathers were over 100 times more likely to beat their young children to death than were biological fathers.
Daly and Wilson’s explanation is evolutionary. They propose that the dramatic disparity in rates of child homicides occurs because stepfathers lack genetic relatedness to their children and so are less invested in them than are biological fathers. As a result, stepfathers are more prone to frustration and irritation with their children and hence lash out at them more often and more violently.
This view from research is consistent with commonly held views about wicked step-parents, some of which stem from fairy tales. Indeed, the greater risk to children by stepfathers is called the “Cinderella effect”.
We found that there was a marked disparity between rates of child homicides by step and biological fathers, although, among fathers who killed, biological fathers now outnumber stepfathers by nearly four to one. The data from the surveys allowed us to calculate far more accurate estimates than had been possible in earlier studies. They indicate that about 1% of children acquire stepfathers each year (so about 5% have a stepdad at age five, 10% at age ten, and so on). And for children up to the age of five (the age group that is relevant to Daly and Wilson’s study), about 2% of children live with their stepfathers.
So the numbers of child deaths perpetrated by stepfathers now seem to be lower, and the proportion of children with stepfathers is higher than Daly and Wilson reported. Together, these figures suggest that the risk to young stepchildren is about 16 times the risk to biological children. While much lower than Daly and Wilson’s estimate, this is still a large difference in rates of child homicide, and it still appears to support the Cinderella effect.
But there are several problems with these figures. First, they concern only young children. We found that older stepchildren are little or no more likely than biological children to be killed by their fathers. This finding is hard to square with the evolutionary explanation because it is unclear why stepfathers’ lack of genetic relatedness should matter before five years, but not after.
Second, stepfathers tend to be much younger than biological fathers, and fathers who kill their children tend to be much younger than fathers who don’t. The implication is that, to some extent, stepfathers are more likely than biological fathers to kill their children not because they are stepfathers but because they are young.
And third, while the population surveys include only cohabiting fathers, according to the Homicide Index almost half of perpetrators did not live with their victims. This means that there are more men who are labelled “stepfathers” out there than the population surveys show, and so the disparity in the rates of child homicides between step and biological fathers must be correspondingly lower.
This last point is closely related to a problem of definition. Common usage of the term “stepfather” refers to a cohabiting partner of the mother who contributes to the child’s upbringing. But it is likely that many short-term and casual non-cohabiting partners of mothers were mislabelled by reporting police officers as stepfathers when actually they hardly knew their victims at all. So the numbers of true stepfather perpetrators might also be much lower than the Homicide Index suggests.
When we took these factors into account, we found that stepfathers were little or no more likely to kill their children than were biological fathers.
When only young children were considered, the risk to stepchildren was about six times that of biological children. But we think this is still an overestimate because other characteristics of some stepfathers that we could not control for – such as poor mental health and greater experience of abuse when they were children – may also account for much of their increased risk to young children. And we still do not know how many so-called stepfathers who kill children are actually stepfathers at all.
The findings from our study suggest that stepfathers are not as dangerous as earlier research has suggested. There are probably many reasons for any increased risk, but if a lack of genetic relatedness of stepfathers is one of them, it is much less influential than evolutionary psychologists have claimed.