シンデレラがいじめられるほんとうの理由 (進化論の現在) 単行本 – 2002/10/17
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It is a chilling example of the narrow line between civilization and savagery. In this case, the book asserts there is a "natural" tendency of males to kill the small children of their mate if they were sired by another father. Natural? It's a common occurrence in nature, and a reminder that "civilization" is truly a veneer, not an evolved quality.
In brief, stepchildren don't have it easy. "Civilized" societies share a consensus that all children deserve respect, protection, support and love. This book cites numerous studies to prove the opposite is true when stepchildren are involved; it cites numerous studies of real and sometimes deadly violence against stepchildren. In other words, compared to the fate of real children in real life, Cinderella had an easy life with her cruel stepmother.
The book deals with violence against stepchildren. It's why the Cinderella stories exist, to warn small children that life can be miserable. Unfortunately, there is no way to assess the impact of a quiet bias against stepchildren, the last of support compared to that received by a natural child. This book doesn't speculate on such intangibles. It does something far more valuable; it raises the fundamental question of "Why?" such bias exists, and the natural reasons for it. It offers a "Darwinian" view, even though Darwin may have been too optimistic in his faith in the results of evolution.
It shows "Be yourself" is a formula for disaster. If it's "natural" to kill stepchildren, then civilization requires everyone to be better than their natural selves. John Dryden defined the freedom of Nature as "when wild in the woods the noble savage ran"; such freedom likely included a natural instinct to kill stepchildren. Dryden's "noble savage" was more savage than noble. Darwin said species evolve; this book says our evolution may be relatively minor. Some very primitive instincts remain, and are the opposite of civilized morals.
At heart we may still be close to animal instincts. Killing babies is an example. How many other old habits exist, hidden from polite society except in "children's stories" such as Cinderella? Read it, and you'll wonder how much "savage" lurks in the heart of everyone . . . . . even today.
The massive increase in the risk of child abuse that is associated with step-parenting has been a major theme of the research work done by Professors Martin Daly and the late Margo Wilson, his wife. They expounded on their work in this 1998 book, and defended its genetic / evolutionary arguments at length in a review article [Daly and Wilson, 2008]. They explain the relatively poor treatment of step-children in terms of the often-unconscious tendency to make choices that increase the likelihood of the perpetuation of our genetic heritage. Other explanations they show to be less compelling.
A slender book, fairly expensive, still well worth reading, THE TRUTH ABOUT CINDERELLA awakens the reader to the problems of re-marriage, step-parenting, and being a step-child.
My opinion: Awareness of the problem can help those involved in step-families. The solution requires a greater reluctance to set up a blended family and improved behavior if in one.
In 'The Truth about Cinderella', long-term scholarly collaborators (and married couple) Martin Daly and Margo Wilson, review the evidence for this claim.
Stereotypes and Folk Tales
The idea that stepparents are especially likely to be abusive is, of course, hardly original to evolutionary psychology. Indeed, it accords with popular stereotypes, as reflected in folk tales (e.g. Cinderella, Hansel and Gretel, Snow White) that Daly and Wilson briefly survey in their opening chapter.
Such stereotypes not confined to the West. The Cinderella story has analogues all around the world, including China ('Ye Xian') and Japan ('Benizara': p2) and “the official Japanese botanical name of the wickedly spiney broad-leaved plant Polygunuum senticosum is 'Mamako-no-shiri-nugui: the stepchild’s bottom-wiper” (p5).
Whether these stories share a common ancestry or were invented independently, the fact they have spread all around the world suggests they reflect a universally recognised phenomenon.
However, hard data to confirm this stereotype was, until recently, surprisingly scant.
All that existed was some suggestive data, collected by researchers studying child abuse in general, plus various studies of stepfamilies, many designed specifically to debunk “Cinderella 'myths'”, but which typically confirmed the truth behind them, but attributed the difficulties experienced to the existence of the negative stereotypes (p22; p56). This, however, fails to explain the existence of the stereotype in the first place and seems to have the causation precisely backwards.
From Fairy Stories to Hard Data
The first researchers to directly test whether there was any truth behind the stereotypes were Daly and Wilson themselves. The magnitude of the disparity they uncovered was enormous.
Thus, in the US, cases of fatal child abuse were one hundred times more common in stepfamilies than among intact nuclear families (p28).
Neither was the phenomenon restricted to the West. In addition to Daly and Wilson’s data for the US, UK and Canada, similar effects have been found in Korea, Hong Kong, Japan, Nigeria, Trinidad and among the indigenous Ache of Paraguay (p35-6).
Langurs and Lions
Daly and Wilson seem to have been inspired to conduct this research by observations of the behaviour of certain non-human species. For example, where a male lion (or group of male lions) fight and drive away the male (or males) previously resident among a pride of lionesses, they then systematically kill the dependent offspring sired by the previous male.
Whereas such behaviour was previously dismissed as pathological, primatologist Sarah Blaffer Hrdy, studying analogous behaviour among langurs, theorised that, by killing nursing offspring, mothers return to estrus (sexual receptivity and fertility) earlier. This would enable the newly incumbent males to sire their own offspring with their new mates as soon as possible, maximising their Darwinian fitness.
Of course, in humans, as among lions, langurs and other mammals, lactation is known to suppress female fertility. This is, of course, itself an evolutionary adaptation to prevent mothers from becoming overburdened with multiple offspring.
However, Daly and Wilson reject any notion of a direct analogy between the systematic infanticide practiced by species such as langurs and lions and the disproportionate levels of abuse perpetrated by human stepparents.
Homicide as an Index of Conflict – But Not Itself Adaptive
Instead, Daly and Wilson focus on fatal child abuse, not because they see homicide as adaptive, but rather because homicide is the form of abuse least likely to go undetected. In contrast, minor forms of abuse, neglect or maltreatment often go unreported, as they typically occur 'behind closed doors' within the family home.
Thus, they discuss the possibility that the disproportionate levels of reported abuse among stepfamilies might simply reflect the negative stereotypes surrounding such families. For example, the authorities might be more investigate a stepparent for abuse because of their preconceptions regarding the likelihood or abuse. Or third-party witnesses may be more likely to report suspicious bruises that would otherwise be dismissed as innocuous if they know a child lives with a stepparent.
However, they reason that “as the severity of child abuse increases, up to the extreme of lethal battery, it should be increasingly unequivocal, so distortions due to biased detection and reportage should diminish” (p28). In other words, a murdered child is unlikely to go unreported/undetected.
Actually, however, as the severity of abuse increases, so does the overrepresentation of stepfamilies among the families in which abuse occurs (p28; p32; p33).
The form of homicide also varies. A relatively higher proportion of biological parents killed their children as part of a suicide, and psychiatric illnesses were common among those who killed their biological children, but not those who murdered stepchildren (p34-5).
In contrast, eighty percent of stepparents killed their stepchildren by literally beating them to death (p34) and “on a sample of 'fatal battered-baby cases perpetrated in anger by British men in loco paternis… fifty-two per cent [of the killers] were stepfathers” despite the fact that “fewer than one per cent of a sample of children with the same age distribution as the fatally battered babies would be expected to have had a stepfather… the odds ratio for this particular kind of lethal assault by stepfathers versus genetic fathers [coming to] approximately 150” (p33).
“Discriminative Parental Solicitude”
Instead, Daly and Wilson explain the overrepresentation of stepparents among abusers, not by claiming that abuse is itself adaptive, but rather that it is a maladaptive by-product the a more general evolved psychological mechanism that they refer to as “discriminative parental solicitude” (p38; Daly and Wilson 1980).
In particular, Daly and Wilson contend that, while abuse is not adaptive, it is adaptive for stepparents to care less for the well-being of stepchildren than for that of their own biological children – and that abuse occasionally (but disproportionately) manifests itself as a maladaptive by-product of this adaptive disposition.
However, differences in levels of parental investment that are not sufficient to qualify as 'child abuse' are not easily measurable, as parental investment, being not merely pecuniary, is not easily quantified, and often occurs, like abuse, 'behind closed doors'.
Thus, they conclude, “enormous differentials in the risk of violence are just one, particularly dramatic, consequence of this predictable difference in feelings” (p38).
Thus, they propose that stepparents may be more likely to temporarily lose their temper and strike misbehaving stepchildren than they would when their biological offspring behave similarly (p33-4).
Is/was the Killing of Stepchildren by Stepparents Ever Biologically Adaptive in Humans?
Thus, Daly and Wilson explicitly reject the notion that the killing of stepchildren is, or was, ever adaptive in humans, as it is in langurs and lions, writing “we see little reason to imagine that the average reproductive benefits of killing stepchildren would ever have outweighed the average costs enough to select for specifically infanticidal inclinations” (p38).
Certainly, murdering one's stepchildren is unlikely to be adaptive in modern Western societies, where elaborate criminal justice procedures exist to punish such behaviour, including imprisonment in single-sex prisons where future reproductive opportunities are curtailed.
However, most of our evolutionary history was spent, not in modern western societies, but in what evolutionary psychologists refer to as the 'Environment of Evolutionary Adaptedness' (EEA) – i.e. the ancestral environment in which human psychological adaptations were shaped.
Here, the payoffs may have been rather different – as Daly and Wilson themselves show.
Thus, they report that among the Tikopia people “a man who acquired a wife who was already a mother would… demand that the child be either fostered out or destroyed”, while, among the Yanomamö, “men who abducted the mothers of infants reportedly insisted that those children be killed” (p23).
Similarly, they report that in a further “fifteen societies, adulterous conception was offered as [legitimate] grounds for infanticide” (Homicide: p47).
This suggests that in what anthropologists formerly termed 'primitive cultures' (now referred to as 'traditional societies'), namely the precise sorts of societies in which we spent much of our species’ evolution and in which our behavioural adaptations were presumably shaped, the murder of stepchildren may sometimes, not merely have gone undetected or unpunished, but been tacitly or openly condoned.
This suggests that, without the accompanying costs of punishment, it may also have been adaptive.
Perhaps Daly and Wilson were reluctant to reach the conclusion that the killing of stepchildren could ever be adaptive, lest they be misinterpreted as thereby somehow condoning such behaviour. However, such a misinterpretation would represent a version of the 'naturalistic fallacy'.
Why do Stepparents Care for Their Stepchildren At All?
The analogy with the infanticide observed among langurs and lions has led to one common objection to Daly and Wilson’s theory – namely the observation that (as Daly and Wilson are themselves at pains to acknowledge) the vast majority of stepparents do not abuse or maltreat their stepchildren and many are excellent parents.
However, since Daly and Wilson never actually claim that the abuse of stepchildren is actually biologically adaptive, this observation does not contradict their theory.
Nevertheless, we must still explain why stepparents do indeed often care for their stepchildren, despite the lack of a biological relationship with the children in question.
Daly and Wilson explain this by reference to reciprocal obligations entailed in romantic relationships (p64).
Thus, a stepfather may provide parental care for his partner’s offspring from a previous relationship in return for sexual access to the mother.
On this view, rather than a misdirected form of 'parenting effort', care provided by stepparents is better conceptualized as a form of 'mating effort' designed to gain sexual access to the mother (or in the case of a stepmother, access to male resources/provisioning).
Some evidence for this theory comes from the fact that men and women with dependent children from a previous relationship are perceived as less attractive as romantic partners than men and women of otherwise similar 'mate-value' but without dependent offspring.
Thus, one study analysing 'lonely hearts ads' found that men and women who described themselves as having dependent children demanded fewer qualities in their prospective partner than did men and women who did not so describe themselves (Waynforth and Dunbar 1995). Another study found that stepfathers have, on average, lower mate-value than males in relationships with women without children from a previous relationship (Anderson 2000).
Daly and Wilson therefore conclude that “many, perhaps most, stepchildren are better off than if their parents had remained single” thanks to the care provided by the substitute parent (p38).
However, just a few pages previously, they report the results of a study conducted by Eckhart Voland that casts doubt on this rosy conclusion. Voland, analysing data from seventeenth to nineteenth century Europe, found that, not only, unsurprisingly, was “the age-specific mortality of pre-modern Friesian children, elevated in the aftermath of the death of either parent” but also, more surprisingly, "the risk of death was further elevated if the surviving parent remarried” (p36).
This suggests that, even before the introduction of welfare assistance for single mothers, children who have lost a biological parent were paradoxically better off if their remaining parent remained single than if they 'benefited' from the additional 'investment' provided by a substitute mate.
'Evil Stepmothers' vs 'Evil Stepfathers'
In modern Western societies, where death during childbirth is rare, but divorce and remarriage are increasingly common, with mothers retaining custody of offspring in most cases, a stepfather is much more common in a child's household than a stepmother. For this reason, discussions of abuse have typically focused on the former.
However, the figure of the 'evil stepmother' is more common in folk tales (e.g. Cinderella, Hansel and Gretel, Snow white – though Daly and Wilson report that, even for stepfathers, the two “motifs” recognised by Stith Thompson were “'cruel' and 'lustful'”: p4.).
Daly and Wilson explain this by reference, first, to the fact that, with higher rates of death during childbirth and lower rates of divorce/separation, stepmothers may have been more common historically (p61). They also observe that such tales are typically taught to children by mothers, who are unlikely to want to impede their own chances of remarriage in the event of the death or desertion of their husbands, or their own elopement, but who have fewer qualms about poisoning their offspring against any potential future partners with whom their father might pair up (p62).
The more interesting question, however, is whether stepmothers or stepfathers represent a greater risk factor for abuse.
On the one hand, males are generally more physically violent than females. This might lead us to expect greater levels of abuse from stepfathers.
However, mothers usually spend more time with children. They therefore have more opportunity to mistreat children, more time over which to momentarily lose their temper, and more incentive to neglect a child, given that they may expend more effort caring for that child.
This probably explains why, perhaps surprisingly, the US Children’s Bureau report on 'Child Maltreatment 2015' reports that women were responsible for an overall majority (54.1%) of recorded cases of child maltreatment in the US in 2015 (Children’s Bureau 2017: p65).
Interestingly, one survey of stepparents found that only 25% of stepmothers reported having “parental feeling” towards their stepchildren, as compared to 53% of stepfathers (Duberman 1975: quoted in Homicide: p84).
This might reflect the fact that males are less easily able to identify their biological offspring than are women. Thus, males can more easily 'fool' their evolved psychologies into treating stepchildren as biological offspring.
Alternatively, it might reflect the fact that maternal feelings are more intense than those of fathers. Thus, the phrase “parental feeling” may have more intense connotations for women.
Daly and Wilson admit that the rarity of stepmothers is such that there is not yet sufficient data to reach a definitive conclusion. However, they conclude, “our best estimate is that the hazards are roughly comparable” but that “in the large data archives of the American Human Association, the odds ratio by which homes with stepmothers exceed chance expectation is actually greater than the corresponding excess of homes with stepfathers, and the same is true for the families of origin of homeless adolescents in New York” (p61).
Other Correlates of Infanticide
While critics sometimes accuse Daly and Wilson of promoting an over-simplistic understanding of the causes of abuse, what is actually over-simplistic is not Daly and Wilson's theories, but rather their critics incomplete understanding of them.
In their longer work, Homicide, they identify several other correlates of abuse by applying evolutionary theory. Thus, they hypothesise that young, unmarried mothers, lacking a long-term mate, would be particularly likely to practice infanticide, because they lack sufficient resources to raise a child (especially in the EEA), yet, being young, have much of their reproductive careers ahead of them, such that it is rational to 'cut their losses' and defer reproduction until conditions are more propitious.
“As menopause approaches," they write, "investing in the children you already have has less and less negative impact on your expected future reproduction” and “we… anticipated that abuse risk would decline steadily as a function of the mother’s age at the child’s birth” (p31)
They also hypothesised that disabled offspring would be overrepresented among the victims of abuse, due to their lower chance of surviving and reproducing.
Therefore, far from being simplistic, Daly and Wilson’s work is more complex than previous treatments of the topic because, in addition to identifying at least one previously unrecognised correlate of abuse (i.e. stepparenthood), the authors also provide an ultimate evolutionary explanation for several previously recognised correlates of abuse (e.g. maternal age, poverty, marital status).
Proximate vs. Ultimate Explanations
Often, as alternatives to the sociobiological interpretation offered by Daly and Wilson, critics offer proximate explanations for the higher rates of abuse observed in stepfamilies. However, proximate and ultimate explanations are not alternative or competing explanations, but rather complementary.
Thus, one theory has it that, since stepparents usually enter the family at a later stage of the child’s life, this makes bonding more problematic. However, this merely begs the question as to why bonding is more difficult with older children.
If indeed true, this could reflect an (admittedly crude) evolved mechanism in parents designed to detect offspring that are not their own. Thus, whereas a male might have difficulty determining which of his partner’s babies he has fathered, he can nevertheless be sure that any child who is already past infancy when he begins a sexual relationship with the mother is certainly not his own.
Alternatively, an unwillingness to bond with a substitute parent whom one first meets when one is already well into one’s childhood could reflect an evolved adaptation in the child to exercise caution in the presence of a substitute parent, given the potential risk to the child a stepparent represents.
At any rate, such mechanisms cannot be the whole story. In fact, the disparity in rates of abuse in intact versus stepfamilies is actually greater in respect of infants and declines with the age of the offspring (p30). This is compatible with sociobiological theory, since the younger the child, the greater the potential parental investment that the substitute parent might be expected to provide if he remains in a relationship with the mother throughout the child's dependent years.
Abuse of Stepchildren by Biological Parents
Other critics point to the fact that, while rates of abuse may indeed be higher in stepfamilies, often the abuser is, not the stepparent themselves, but rather the remaining biological parent.
However, Daly and Wilson report, “analysing [rates of abuse] by perpetrator actually produces much greater step-parent/genetic parent odds ratios than our more conservative analysis by household” (p53).
Moreover, the presence of a stepparent could surely increase the general level of hostility within the family. Thus, for example, a mother may resent her biological offspring for poisoning her relationship with her partner (i.e. the stepparent), and this could motivate her to mistreat that child.
Similarly, Daly and Wilson report that “in several ethnographies, a category of infanticide was described in which there was no direct male coercion, and yet the mother’s decision seemed clearly related to men’s reluctance to invest in other men’s children”, such that, among the Ojibwa, women sometimes “abort or do away with an illegitimate child” in order to improve their marriage prospects (Homicide: p47).
Interestingly, an analogous physiological mechanism operates in rodents – the so-called 'Bruce Effect', whereby pregnant females respond to pheromones emitted in the urine of unfamiliar males by terminating their existing pregnancy, a mechanism designed to forestall the probability that the resultant offspring would be victims of infanticide by the unfamiliar nonpaternal male, with all the wasted investment this would entail for the gestating mother.
Parental Personality as a Confounding Factor
Another proffered explanation for the higher rates of abuse in stepparents is simply that parents in stepfamilies have, on average, different personality profiles from those in intact nuclear families.
In particular, there is reason to believe that stepparents may be more impulsive that parents who remain in an intact family unit. This impulsiveness may have motivated them to have a child from a relationship that was destined to be fleeting, or to impulsively leave their previous relationship and begin a new one – and this same impulsiveness may render them liable to lash out at their offspring when stressed.
However, Daly and Wilson observe that “the idea that abuse differences might reflect an excess of indiscriminately violent personalities in remarriages [has] been disposed of by demonstrations that mistreatment in stepfamilies [is] usually discriminatively targeted at the stepchild while the abuser’s own children in the same household were well treated” (p50).
At any rate, it is doubtful whether differences in the personality profiles of parents who find themselves in reconstituted families are capable of explaining the magnitude of the effect uncovered by Daly and Wilson, especially since, today, divorce, illegitimacy, remarriage and step-parentage are increasingly common.
What of Adopted Children?
Another common objection to Daly and Wilson’s theory seems, however, in my view, to have rather more force – namely: Why is it that adopted children seem to fare so much better?
Like stepchildren, adopted children are (usually) biologically unrelated to their adoptive parents. However, there is little evidence that adopted children are disproportionately overrepresented among victims of abuse, mistreatment or infanticide.
Indeed, one recent study found that, on average, adoptive parents provide more financial support for their adoptive children than those same parents do for their biological offspring (Gibson 2009).
Gibson explains perhaps surprising finding by reference to the greater demands such offspring often place on their parents.
Thus, adoptive children often come from troubled backgrounds, having been abandoned by their biological parents or taken from them by social services. Such children may inherit the dysfunctional personality attributes of their parents, and may also have suffered abuse prior to their placement. Presumably as a result, adopted children, on average, impose greater demands on their adoptive parents than the latter’s biological children, and may therefore demand greater investment.
Nevertheless, this still fails to explain why adoptive parents are willing to provide this care in the first place, for children to whom they are not biologically related, a paradox from the perspective of evolutionary psychology.
Daly and Wilson provide several explanations for this apparent anomaly (p45-6).
First, adoptive parents typically actively seek to become adoptive parents (p45). In contrast, stepparents may only reluctantly tolerate the prospect of becoming a stepparent as a necessary cost of entering a relationship with the child’s parent.
Secondly, prospective adoptive parents are “screened for suitability” by adoption agencies (p45). Those deemed unsuitable as adoptive parents (e.g. drug addicts, habitual criminals, known or suspected child-abusers) may be barred from adopting.
In contrast, virtually anyone who forms a romantic relationship with a child’s parent is permitted to become a stepparent. Likewise, there are no legal restrictions on who can become a biological parent. Only in relatively serious circumstances will the social services intervene to remove a child from their biological parent.
Thus, adoptive parents are a highly unrepresentative sample of the general population, being both self-selected (for desire to adopt), and screened by adoption agencies.
Finally, Daly and Wilson observe that step-parenthood was likely a familiar phenomenon in our ancestral environment (the 'EEA'), not least due to the more common occurrence in ancestral environments of early death by one or other of a child’s biological parents.
In contrast, “the contemporary practice of adoption by unrelated persons attempting to simulate the experience of genetic families is a modern novelty” (p45-6).
Indeed, where adoption has been documented among 'primitive' peoples, it usually involves the adoption of close relatives (e.g. Silk 1980).
This suggests that, whereas we have had sufficient time to have evolved psychological mechanisms designed specifically to deal with the prospect of step-parenthood, the same is not true of adoption – at least in its modern form. This means that adoption may represent an 'evolutionary novelty' in respect of which we have yet to evolve adaptive behaviours.
However, this explanation is not entirely convincing.
While adoption agencies surely didn’t exist in the EEA, abandoned or orphaned infants surely did, probably in greater numbers than they do today – and, if the abandoned or orphaned infants were not biologically related to oneself, it would have been highly maladaptive to invest parental effort in caring for such children.
We should therefore have evolved strong evolutionary defences against any such behaviour.
There is indeed evidence that humans have evolved mechanisms of distinguishing kin from non-kin (what biologists refer to as Kin Recognition).
Thus, Daly and Wilson hypothesize that males may be more willing to invest in offspring who physically resemble themselves (p44) – and, in the recent years, evidence has emerged supporting this supposition (e.g. Apicella & Marlow 2004; Alvergne et al 2010; Wu et al 2013).
Research on mate-preferences and, in particular, incest avoidance (e.g. the ''Westermarck Effect'') also shows evidence of kin recognition mechanisms operating in humans.
Curiously, Daly and Wilson report that “no non-human animal has yet been shown capable of using… cues of resemblance for the purpose of adaptively modulating parental investment” (p44). However, they fail to note that, while there is indeed no evidence of animals using physical appearance to recognise relatives, the use of heritable similarity in scent for the same purpose (i.e. what Dawkins hypothetically christened the 'Armpit Effect') has been documented in Belding's ground squirrels (Holmes 1986).
The fact that significant numbers of people are willing, and often eager, to adopt and care for biologically unrelated children and, moreover, apparently willing to provide levels of investment greater than that that they provide for their own offspring (Gibson 2009), itself demands an evolutionary explanation.
Therefore, whereas step-parenthood is eminently explicable in Darwinian terms, adoption, at least as practised in the contemporary West, remains, for the time being, an evolutionary paradox.
Anderson (2000) 'The life histories of American stepfathers in evolutionary perspective' Human Nature 11(4):307-33
Alvergne, Faurie & Raymond (2010) 'Are parents' perceptions of offspring facial resemblance consistent with actual resemblance? Effects on parental investment' Evolution and Human Behavior 31(1):7–15
Apicella & Marlow (2004) 'Perceived mate fidelity and paternal resemblance predict men's investment in children' Evolution and Human Behavior 25(6):371–378
Daly and Wilson (1980) 'Discriminative Parental Solicitude: A Biological Perspective' Journal of Marriage and Family 42(2): 277-288
Duberman (1975) The Reconstituted Family: A Study of Remarried Couples and Their Children
Gibson (2009) 'Differential parental investment in families with both adopted and genetic children' Evolution and Human Behavior 30(3):184–189
Holmes (1986) 'Kin recognition by phenotype matching in female Belding's ground squirrels' Animal Behaviour 34:38-47
Silk (1980) 'Adoption and Kinship in Oceana American Anthropologist' 82(4):799-820
Children’s Bureau (2017) 'Child Maltreatment 2015' (U.S. Department of Health & Human Services, Administration for Children and Families, Administration on Children, Youth and Families).
Waynforth & Dunbar (1995) 'Conditional Mate Choice Strategies in Humans: Evidence From 'Lonely Hearts' Advertisements', Behaviour 132(9):755-779
Wu, Yang, Sun, Liu & Luo (2013) 'The male advantage in child facial resemblance detection: behavioral and ERP evidence' Social Neuroscience 8(6):555-567.