Justice For Families

The “Cinderella effect”: Elevated mistreatment of stepchildren
in comparison to those living with genetic parents.
Martin Daly & Margo Wilson
Department of Psychology, Neuroscience & Behaviour
McMaster University
Hamilton, Ontario, Canada L8S 4K1
<daly@mcmaster.ca> <wilson@mcmaster.ca>
Theory
Parents commit a huge amount of time, attention and material resources to the care of their children, as
well as incurring life-threatening risks to defend them and bodily depletion to nourish them. Why are
parents motivated to invest so heavily in their children? From an evolutionary perspective, the answer is
surely that natural selection has favoured intensive parental care in our lineage. Those ancestral
genotypes and phenotypes that best succeeded in raising children to become reproducing adults were
the ones that persisted and proliferated.
If the psychological underpinnings of parental care have indeed evolved by natural selection, we may
furthermore anticipate that parental feeling and action will not typically be elicited by just any random
conspecific juvenile. Instead, care-providing animals may be expected to direct their care selectively
towards young who are (a) their own genetic offspring rather than those of their reproductive rivals, and
(b) able to convert parental investment into increased prospects for survival and reproduction. This is
the kernel of the theory of discriminative parental solicitude, which (notwithstanding some interesting
twists and caveats) has been abundantly verified in a broad range of care-giving species (see Clutton-
Brock 1991; Daly & Wilson 1980, 1988a, 1995).
From this perspective, care of young who are not the caretaker’s own requires explanation. In
nonhuman animals, adoption of unrelated young is usually best interpreted as a failure of discrimination,
which should not be so surprising after all when we consider that there is a sort of “evolutionary arms
race” between discriminative parents and those (of both the same and other species) who might gain
fitness by overcoming parents’ evolved defences and parasitizing their efforts (see, e.g., Davies &
Brooke 1991; Yom-Tov 1980). In the human case, adoption by unrelated persons is a recent cultural
invention rather than a recurrent aspect of ancestral environments, and cannot have been a feature of the
social milieus in which our parental psychology evolved (see Silk 1990).
Stepparental care, unlike modern adoption, is cross-culturally ubiquitous and almost certainly ancient. It
is also not peculiar to human beings, and its distribution in the animal kingdom lends support to the idea
that the reason why such care occurs is because investing resources in a new mate’s young of prior
unions is a part of “mating effort”, confined to species in which suitable mates are scarce and in which
couples, once established, often stay together for longer than just one breeding season (Rohwer et al.
1999). Investing pseudoparental care in a predecessor’s offspring can thus be adaptive and favoured
by selection. However, a stepchild must rarely have been as valuable to a stepparent’s expected fitness
as a child of one’s own would be, and we may therefore anticipate that stepparents will not, in general,
feel such whole-hearted, self-sacrificial love for their wards as genetic parents so often do.
The Cinderella effect - 2
It is on these grounds that we hypothesized, many years ago, that any and all sorts of abuse and
exploitation would be seen to occur at higher rates in steprelationships than in genetic parent-child
relationships, and that the differences would persist when possible confounds such as socio-economic
status were controlled for (see Daly & Wilson 1998). This hypothesis has since been abundantly
supported in our own research and in that of many others. This differential (mis)treatment is what we
refer to as the “Cinderella effect”.
Fatal batterings of small children
This most severe category of child maltreatment exhibits Cinderella effects of the greatest magnitude: in
several countries, stepparents beat very young children to death at per capita rates that are more than
100 times higher than the corresponding rates for genetic parents.
The most thorough analyses are for Canada, where data in a national archive of all homicides known to
police indicate that children under 5 years of age were beaten to death by their putative genetic fathers
at a rate of 2.6 deaths per million child-years at risk (residing with their fathers) in 1974-1990, while the
corresponding rate for stepfathers was over 120 times greater at 321.6 deaths per million child-years at
risk (Daly & Wilson 2001). Note that because few small children have stepfathers, this rate differential
does not, in itself, convey anything about the absolute numbers of victims; what these rates represent are
74 fatal batterings by genetic fathers in 28.3 million child-years at risk, and 55 by stepfathers in 0.17
million child-years at risk.
Estimates of this sort have not been made for other countries, but it is clear that this immense excess risk
to stepchildren is not peculiar to Canada. In England & Wales in 1977-1990, for example, 117
children under five years of age were beaten to death by putative genetic fathers and 103 by stepfathers
(Daly & Wilson 1994). As in Canada, the available population-at-large survey data indicate that fewer
than 1% of British children of the same age as the victims dwelt with stepfathers, while over 90% dwelt
with putative genetic fathers, and so, as in Canada, the difference in per capita rates of such fatal
assaults is well over 100-fold.
Australian data indicate an even larger Cinderella effect. Wallace (1986) reported that perpetrators of
fatal baby batterings in New South Wales in 1968-1981 included 11 putative genetic fathers and 18
stepfathers, even though the victims’ median age was only 12 months. Strang (1996) reported that
comparable cases for the country as a whole in 1989-1993 included 11 children killed by putative
genetic fathers and 12 by stepfathers, although the victims’ median age was in this case less than 1 year.
For both of these samples, the age distribution was such that fewer than 0.5% of a random sample of
same-age children from the population-at-large would be expected to have had a stepfather according
to Australian Family Characteristics Survey data, and the estimated relative risk from stepfathers vs
genetic fathers exceeds 300-fold.
There are no high-quality national data on fatal batterings in the United States, but the available evidence
again indicates a large overrepresentation of stepchildren as victims. According to an analysis of the
FBI’s Supplemetary Homicide Reports (SHR) case data by Weekes-Shackelford & Shackelford
(2004), stepfathers beat children under 5 years old to death at a rate of 55.9 per million children at risk
The Cinderella effect - 3
per annum, compared to 5.6 for genetic fathers. This 10-fold risk differential, albeit substantial, is
surprisingly low in comparison to what has been documented in Canada, Britain and Australia, but there
is good reason to believe that it is an extreme underestimate. The main reason for saying this is that
SHR coders are instructed to restrict the “stepparent” code to persons in registered marriages and to
code “mothers’ boyfriends”, whether they coreside or not, as nonrelatives. (Recently, a specific
“mother’s boyfriend” code has been added to the “incident-based” NIBRS codes that are replacing the
SHR.) In contrast, genetic fathers are coded as “fathers” regardless of marital status, and so Weekes-
Shackelford & Shackelford’s (2004) comparison is effectively one of married stepfathers versus
married and unmarried genetic fathers. Moreover, the SHR data suffer from a substantial incidence of
coding errors; for example, 13 boys under 5 years of age who were beaten to death by adult men were
coded as their killers’ stepfathers rather than stepsons and were therefore omitted from Weekes-
Shackelford & Shackelford’s calculations.
There is considerable direct evidence that U.S. stepparents are more extremely overrepresented as fatal
child abusers than Weekes-Shackelford & Shackelford’s 10-fold estimate. Wilson et al. (1980)
analyzed child abuse data from an archive collating mandated reports from jurisdictions representing
about half the U.S. population, and found 279 cases of “fatal physical abuse” (a broader category than
lethal battering) in 1976; 43% of the victims (whose median age was under 2 years) dwelt with
stepparents, and these data in combination with population-at-large estimates suggest that stepchildren
incurred such deaths at about 100 times the rate for same-age children living with two genetic parents
(Daly & Wilson 1988b). Various small-scale local studies similarly imply a very large Cinderella effect
among murdered U.S. toddlers. Four examples of such studies are these:
(1) 9 fathers and 6 de facto stepfathers (live-in “mothers’ boyfriends”) were identified as killers of small
children (median age 1 year) in a Birmingham, Alabama sample (Lyman et al. 2003);
(2) 4 fathers and 4 “mothers’ boyfriends” were killers of infants (median age 6.5 months) in a Dayton,
Ohio sample (Hicks & Gaughan 1995);
(3) 11 fathers and 15 (registered marriage plus de facto) stepfathers (plus an additional 2 mothers’
boyfriends who did not coreside with their victims) inflicted fatal injuries on preschool-age children in
Missouri over a 3-year period (Stiffman et al. 2002); and
(4) 14 fathers and 9 (registered marriage plus de facto) stepfathers were killers of preschool-age
children in an analysis of such murders by U.S. Air Force personnel (Lucas et al. 2002).
If stepfathers and genetic fathers were equally likely to kill, then in view of the very young ages of the
victims and the household circumstances of children in the U.S. population-at-large, the expected count
of stepfathers in each of these four samples, rounding to the nearest whole number, would be zero.
Swedish data indicate a smaller, but still substantial, Cinderella effect with respect to parental homicides
in toto, (i.e. not just fatal batterings). Temrin et al. (2000) initially reported that there was no excess
risk to Swedish stepchildren whatever, but this claim was based on an analytical error: the researchers
used a very young group of victims to generate the numerators for their homicide rate estimates and a
much older distribution of children’s ages in the population-at-large to generate the denominators.
When the analysis was done correctly, toddlers were found to have been killed by genetic parents at a
The Cinderella effect - 4
rate of 3.8 per million coresiding parent-child dyads per annum, while the corresponding rate for
stepparents was 8.4 times greater at 31.7 deaths per million dyads per annum (Daly & Wilson 2001).
Because these estimates include all parental and stepparental killings, many of which have different
typologies and different risk factors than fatal batterings, they are not strictly comparable to the
Canadian, British and Australian numbers discussed above, but they certainly suggest that the magnitude
of Cinderella effects may vary considerably across countries (see also Temrin et al. 2004).
How and why Cinderella effects vary in magnitude are important questions for future research, and we
are going to need cross-national research that differentiates homicide typologies to get the answers.
Fatal batterings are clearly different, for example, from murder-suicides by depressed parents, who may
even construe the killing of their children as a “rescue”, and in both Canada and Britain, stepparents are
overrepresented as killers to a much lesser extent in murder-suicides and familicidal massacres than in
fatal batterings (Daly & Wilson 1994; Wilson et al. 1995). As regards the specific case of Sweden,
Daly & Wilson (2001: 294) speculate that “it may well be the case that the modern Swedish welfare
state provides a social climate in which stepparents do not experience, and thus do not resent, heavy
pseudoparental obligation”. Whether social policy indeed has such effects on the incidence of family
violence is an important question that will require more sophisticated analyses than have yet been
undertaken.
Nonlethal abuse
The evidence for Cinderella effects in nonlethal abuse is much more extensive than that for homicides.
Numerous studies from a diversity of countries indicate that stepparents perpetrate both nonlethal
physical assaults and sexual abuse at much higher rates than genetic parents.
One sort of evidence comes from the case data collected by child protection agencies, in which
stepfamily households and stepparent perpetrators are greatly overrepresented relative to their
prevalence in the population-at-large (e.g. Creighton 1985; Creighton & Noyes 1989; Craissati &
McClurg 1996; Cyr et al. 2002; Daly & Wilson 1985; Gordon 1989; Gordon & Creighton 1988;
Klevens et al. 2000; Rodney 1999; Sirles & Franke 1999; Trocmé et al. 2000; Wilson et al. 1980).
Another source of evidence is victimization surveys, from which comparisons can be made between the
responses of those who live or formerly lived with stepparents and those raised by genetic parents. The
former routinely report much higher rates of both physical and sexual abuse (e.g. Kim & Ko 1990;
Russell 1984; Sariola & Uutela 1996). Surveys of runaway youth combine the features of the criterion
case study and the victimization survey, and provide further evidence. When runaway and homeless
adolescents are interviewed, a very large proportion report that they have fled stepfamilies in which they
were subject to abuse (e.g. Powers et al. 1990; Tyler & Cauce 2002).
Are Cinderella effects byproducts of other risk factors associated with stepparenthood?
That stepparents abuse and kill children at much higher per capita rates than genetic parents does not
necessarily implicate the steprelationship as a causal factor. It could instead be correlated
(“confounded”, in statistical jargon) with some other factor that is of more direct relevance.
The Cinderella effect - 5
An obvious example of a possible confound is socioeconomic status: one might hypothesize that the
stresses of poverty cause the poor to be especially likely to abuse and kill their children and also to
experience high rates of divorce and remarriage, making steprelationship an incidental correlate of
abuse. This initially plausible hypothesis has been tested and rejected with respect to Cinderella effects
in Canada (Daly & Wilson 1985) and the U.S. (Wilson et al. 1980; Wilson & Daly 1987): in both
countries, poverty is indeed a risk factor for child maltreatment, but it is weakly or not at all associated
with steprelationship, with the result that having a stepparent and being poor are in large measure
independent and additive (“orthogonal”) predictors of the risk that a child will be abused. Other
confound hypotheses that have been tested and rejected are that the differences between stepparent
families and genetic parent families might be byproducts of differences in parental age and/or family size;
such differences are in fact small and make negligible contributions to Cinderella effects (e.g. Daly &
Wilson 1985).
A final confound hypothesis is that there are “personality” differences between parents who reside with
only their own children and people who become stepparents. In principle, the population of adults in
stepfamilies could include disproportionate numbers of disturbed, violent or otherwise abuse-prone
people, elevating victimization rates for those living in such families regardless of how victims and
assailants were related. But although the population of persons who become stepparents may indeed
be atypical of parents in general, one line of evidence speaks against the idea that this could account for
Cinderella effects: abusive stepparents typically spare their own children. In a study of abusive families
in the U.S., for example, only the stepchildren were abused in every one of 10 households containing
both stepchildren and children of the current marital union (Lightcap et al. 1982); similarly, in urban
Canadian samples, the stepchildren were selectively abused in 9 of 10 such families in one study (Daly
& Wilson 1985), and in 19 of 22 in another (Rodney 1999). This tendency for stepchildren to be
targeted is especially striking in light of the following additional facts: (1) when child abuse is detected, it
is often found that all the children in the home have been victimized, and (2) the abused stepchildren
were almost always the eldest children in the home, whereas the general (albeit slight) tendency in
genetic-children-only families was for the youngest to be the most frequent victims (Rodney 1999).
Stepfathers or “mothers’ boyfriends”?
In our own research and in the review above, we typically define a “stepparent” as the coresiding
partner of a (presumed) genetic parent, regardless of marital registration. But marital status may not be
irrelevant, and a large proportion of slain and abused stepchildren were the victims of their mothers’
“live-in boyfriends”. This raises the question of whether Cinderella effects might be due primarily, or
even solely, to abuse by de facto stepparents rather than registered-marriage stepparents. The answer
is that Cinderella effects are large regardless of marital registration.
Both registered-marriage stepfathers and de facto stepfathers (aka. commonlaw stepfathers, mothers’
boyfriends, cohabitees, and, in older literature, “paramours”) are overrepresented as perpetrators of
abuse in many of the studies cited above. Weekes-Shackelford & Shackelford (2004) analyzed U.S.
homicide data using a data base that effectively limits the term “stepparent” to persons in registered
marriages (even though the comparison group of “parents” includes both married and unmarried), and
The Cinderella effect - 6
nevertheless found large Cinderella effects. Creighton & Noyes (1989) estimated rates of child abuse
by married stepfathers versus mothers’ cohabitees in Great Britain, and actually found the former to be
significantly higher than the latter, a unique result that is likely to prove exceptional.
The most thorough examination of the simultaneous relevance of steprelationship and marital registration
is that conducted by Daly & Wilson (2001) with respect to fatal batterings in Canada. What they found
was that both steprelationship and commonlaw status were strong predictors of homicide risk, and that
neither variable’s influence could be explained away as an artifact of the other’s. In other words,
stepfathers were greatly overrepresented as killers within both registered and de facto unions
considered separately, and de facto fathers were greatly overrepresented within both genetic and
stepfathers considered separately.
Stepparents or stepfathers?
Many of the analyses discussed above have focused on homicides and abuse perpetrated by stepfathers
vs (putative) genetic fathers. Can we infer that excess risk is a feature only of stepfather homes and not
stepmother homes? The answer is no. The reason why stepmothers are often omitted from the data
presentation is because small children live with stepmothers so infrequently that in all but the largest data
bases, the cases are usually so few that estimates of abuse risk are unreliable, changing markedly as a
result of the addition or subtraction of a single case. Nevertheless, all available evidence indicates that
excess risk from stepmothers (relative to genetic mothers) is roughly on the same order as excess risk
from stepfathers (relative to genetic fathers).
The best evidence on this question comes from large child abuse data bases such as those analyzed by
Daly & Wilson (1981) and Creighton & Noyes (1989). Both studies included large numbers of
stepmother cases and provided evidence that rates of physical abuse in stepmother and stepfather
households are roughly similar and far in excess of those in two-genetic-parent households.
Stepmothers are also substantially and significantly more likely to kill young children than genetic
mothers according to the analyses of U.S. data by Weekes-Shackelford & Shackelford (2004), despite
the facts that (1) as with stepfathers, the code “stepmother” was restricted to those in registered
marriages, and (2) the genetic mother cases included neonaticides, a distinct category of homicides that
is sometimes quite numerous. We have already mentioned the identical abuse rates in stepmother and
stepfather households in the Korean study by Kim & Ko (1990). Finally, stepmother households tend
to be even more extremely overrepresented than stepfather households among adolescent runaways
who aver that they are fleeing abusive families.
Mundane (non-abusive) discrimination against stepchildren
It is important to stress that although stepchildren incur elevated risks of abuse and homicide, these dire
outcomes are by no means typical. Many, perhaps most, stepparents make positive contributions to the
well-being of their stepchildren, and most stepparents and stepchildren evaluate their relationships at
least somewhat positively. Nevertheless, steprelationships are difficult, and those who make it their
business to help stepfamilies in distress are unanimous in cautioning that it is a mistake to expect that a
The Cinderella effect - 7
stepparent-stepchild relationship is, or will with time become, psychologically equivalent to a
birthparent-child relationship (e.g., Johnson 1980; Turnbull & Turnbull 1983). Research tells the same
story. Duberman (1975), to take a single example, interviewed a select sample of well-established,
“successful”, middle class, registered-marriage U.S. stepfamilies, and reported that only 53% of the
stepfathers and 25% of the stepmothers felt able to say that they had any “parental feeling” (much less
“love”) for their stepchildren. There are literally hundreds of self-help manuals for stepfamily members,
and they have a single focus: how to cope with the characteristic conflicts of stepfamily life.
To an evolutionist, these facts are unsurprising. Assuming the role of stepparent may be a tolerable
price to pay to acquire a desired mate, but how much one should then invest in stepchildren remains
negotiable. The extent to which a new couple’s combined resources will be devoted to children of
former unions is therefore likely to be a source of persistent conflict, an expectation that is abundantly
confirmed by studies of marital discord (see Daly & Wilson 1996; Wilson & Daly 2001, 2004).
Children of former unions enter into (re)marriage negotiations as a cost, not a benefit (e.g. White &
Booth 1985), and their presence therefore reduces the custodial parent’s value on the marriage market.
Moreover, children of former unions increase the marital-duration-specific probability of divorce,
whereas children of the present union reduce it (Becker et al. 1979). Having children of former unions
also elevates the risk that wives will be assaulted (Daly, Singh & Wilson 1993) and killed (Daly,
Wiseman & Wilson 1997; Campbell et al. 2003).
In light of the theoretical ideas that we espoused at the beginning of this review and facts like those
recounted above, we long ago proposed that violence against stepchildren would prove to be the
atypical and extreme “tip of the iceberg” of a more ubiquitous discrimination. A wide variety of recent
research in diverse disciplines has now demonstrated that this is indeed the case.
Econometric analyses of large data bases such as the U.S. Panel Study of Income Dynamics provide
one sort of evidence: controlling for the family’s economic means, U.S. stepchildren receive reduced
investment in the form of support for higher education, routine medical and dental care, and even food
(e.g. Case et al. 2000; Case & Paxson 2001; Zvoch 1999). Surveys that ask people directly about
parental support tell the same story: according to both the parents and the children, stepparents withhold
investment relative to genetic parents (e.g. Anderson et al. 1999a,b; White 1994). Also of interest in
this context is Ferri’s (1984) finding that both the mothers and stepfathers in British stepfamily homes
expressed low aspirations for the children’s education, lower even than those of single mothers of lesser
means.
Another sort of evidence comes from anthropological studies using observational sampling methods. In
one such study of Trinidadian villagers, Flinn (1988) found that stepfathers spent significantly less time
with their children than genetic fathers, and that a significantly higher proportion of their interactions were
“agonistic”. In another such study of Hadza hunter-gatherers in Tanzania, Marlowe (1999) reported
that although stepfathers mind their stepchildren in camp, they are unlike genetic fathers in their
behaviour towards them; for example, they never play with them. Stepchildren also suffer elevated rates
of accidental injury, both lethal and nonlethal, from infancy onwards, apparently because they are less
assiduously monitored and protected (e.g. Fergusson et al. 1972; Wadsworth et al. 1985 ), and they
The Cinderella effect - 8
suffer elevated mortality in general, not just from assaults (e.g. Hill & Kaplan 1988; Voland 1988).
In view of all the above, it is no surprise to learn that stepchildren find their home life stressful. Many
studies have reported that they leave home at a substantially younger age than children from intact birth
families (e.g. Aquilino 1991; Davis & Daly 1997; Kiernan 1992; White & Booth 1985), and not only
do they leave earlier, but they are far more likely to cite family conflict as the reason (Kiernan 1992).
The last findings that we will cite are from a study of child health in Dominica: stepchildren exhibit
reduced growth (Flinn et al. 1999) and have chronically higher circulating levels of the stress hormone
cortisol (Flinn & England 1995; Flinn et al. 1996) than their age mates living with only their genetic
parents under similar material circumstances in the same village.
Let us stress again that most stepparents try hard to treat their stepchildren fairly, and extreme negative
outcomes, despite being much more prevalent than in genetic-parent homes, are infrequent. That said,
however, it is also important to recognize that Cinderella is no fairy tale.
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