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(Co)parenting of a partner’s child which is not one’s biological child.
Step-parenting takes on many different forms and may have different causes. Historically and across societies, there is variation in what constitutes the most common causes for stepfamily formation: Whereas in modern industrial societies increasing rates of separation and divorce along with re-partnering and multiple-partner fertility have been major drivers for the formation of stepfamilies (Steinbach 2010), historically stepfamilies were mostly formed after the death of one biological parent, often out of economic necessity (Hill and Kopp 2013, p. 33).
The presence of a stepparent does not necessarily contribute to the well-being of the child to the same degree as the presence of two biological parents. Research shows overwhelming evidence for the existence of a so-called stepgap (Delongis and Preece 2002), that is, a mean difference in outcomes between children living with and without stepparents on a range of indicators, in both contemporary as well as historical societies. This includes a range of subjective outcome indicators like children’s assessment of their relationship with their parents and a range of objective measures like parental investment of time and money, children’s academic achievement, psychological adjustment, child abuse and mistreatment, behavioral problems, and even cortisol levels as indicator of social stress (Anderson et al. 1999a, b; Brown and Rinelli 2010; Coleman et al. 2000; Daly and Wilson 1985; Flinn and England 1995; Hamilton et al. 2007). The direction of the stepgap is usually such that, on average, children living in a stepfamily have worse outcomes than children living with two-biological-parent families. Although part of the stepgap may be attributed to between-family differences (e.g., Buller 2005), Schnettler and Steinbach (2011) show that a stepgap persists after taking differences between families into account. In their study, the stepgap in subjective assessments of the child-parent relationship even roughly follows a gradient along the lines of genetic similarity in sibling dyads, based on pairs of monozygotic and dizygotic twins, full-, half-, and stepsiblings from the US Adolescent Health Study.
Nevertheless, the persistence of a stepgap in studies using a within-family research design is no proof that the difference is biologically hardwired (Schnettler and Steinbach 2011). Theories in evolutionary biology provide reasoning for why parents would be biased in their investment toward biological versus stepchildren. One central such reason would be (the lack of) genetic relatedness that – according to kin selection theory – should play a decisive role in steering parental investment patterns and other forms of altruistic behavior. Yet, there are also conditions under which, even from an evolutionary perspective, parents may strategically provide investment to their stepchildren. Furthermore, as evolutionary psychology has amply demonstrated regarding other adaptive problems in human behavior, psychological mechanisms that have evolved in an ancestral environment may prove maladaptive in the modern environment (Barkow et al. 1992). There may exist social contextual conditions in which we would expect smaller or nonexisting differences in stepparent- versus biological parent-child ties, even though evolutionary biology predicts that preferential treatment of biological kin is adaptive. Genetic relatedness is but the ultimate evolutionary explanation, relating to underlying mechanisms. More direct, proximate mechanisms may involve co-residence duration, similarity in looks (phenotypical similarity), or similarity in smell (olfactory cues) that all affect how children are treated and which can result in different treatment of biological vis-à-vis stepchildren. In this interplay of multiple potential mechanisms lies the possibility that the size of the stepgap is malleable by contextual differences, such as differences across time and space in the likelihood that stepparents and stepchildren live together in one household. Such differences provide potential leverage for policy-making that aims at improving parent-child relationships in stepfamilies.
Previous research has investigated different potential mechanisms for this stepgap: The majority of studies with a background in the social sciences as well as in psychology has focused on stressors associated with parental separation or death and with life in (complex) post-separation families, and on stepfamily selectivity. Evolutionary research, on the other hand, has emphasized the evolved motivational and behavioral patterns which may play an important role in producing the stepgap.
Step-Parenting in the (Standard) Social Sciences
Family research offers two major hypotheses on potential causes of the stepgap: the selection hypothesis and the stress-mediation hypothesis (Coleman et al. 2000; Hadfield et al. 2018). According to the selection hypothesis, “multiple transitions and negative child outcomes may be associated with each other through common causal factors reflected in the parents’ antecedent behaviors and attributes” (Fomby and Cherlin 2007, p. 181; cited in Hadfield et al. 2018). Multiple transitions are (repeated) parental separation and re-partnering events that produce more or less complex types of stepfamilies. In this view, it is unobserved heterogeneity between different family types which is also responsible for the stepgap in developmental outcomes. Better data availability in the form of repeated measures over time and for siblings in the same families has made it possible to test this hypothesis empirically. One study using longitudinal data, for instance, shows that parental attention after the birth of a new biological child shifts from older children to the newborn roughly to the same degree in two-biological-parent families and stepfamilies (Stewart 2005). This seems to imply that the stepgap may partially be driven by a birth-order or age effect. Yet, in a within-family comparison of siblings living in the same families in the USA, Schnettler and Steinbach (2011) find that the stepgap in adolescents’ perceived parental closeness and care persists even after taking into account children’s age, other relevant factors that can vary within families, and, most importantly, after applying family fixed effects to control for unobserved heterogeneity between families. Arránz-Becker et al. (2013) also find a persisting stepgap after inclusion of family fixed effects in their statistical models, in this case applied to parents’ emotional closeness to their mostly adult children in Germany.
Both studies also reveal a number of moderating factors that narrow the stepgap. These are, most importantly, relationship duration, the amount of family resources, and the importance of norms of familism. Yet, in both studies, a considerable stepgap persists even after taking these moderating factors into account (Arránz-Becker et al. 2013; Schnettler and Steinbach 2011). Together, these studies thus provide evidence that unobserved heterogeneity seems not to be the sole explanation for the stepgap, at least with regard to the particular indicators studied. This is consistent with studies using fixed-effects models to control for unobservable factors across time, siblings, or families with regard to the effects of divorce. Here, too, a central conclusion is that effect sizes decrease after taking into account unobserved heterogeneity, but moderate associations between divorce and outcomes persist on a range of measures (Amato and Anthony 2014).
Different from the selection hypothesis, the stress mediation hypothesis attributes a causal role of family transitions (separation, re-partnering) to developmental outcomes for children and adolescents growing up in stepfamilies. Living in a stepfamily implies that the parents of a focal child or adolescent have at least once separated or divorced and formed a new relationship. According to the hypothesis, each family transition creates stress which can accumulate across multiple transitions and negatively affect developmental outcomes in those affected by these (multiple) stressors (Hadfield et al. 2018). An enormous amount of literature on the consequences of divorce shows that relationship instability and divorce can harm development in manifold ways and with persistent, adverse effects into adulthood (Amato 2001; Amato and Anthony 2014; Patterson 2001). A recent review that covers 39 articles published over the preceding decade that specifically test the instability hypothesis also generally shows support for the instability hypothesis. But mixed results with regard to certain transitions, groups, and outcomes point toward additional explanatory factors that may be at play but are not yet well understood (Hadfield et al. 2018, p. 20).
Recently, researchers in the social sciences have increasingly focused on the complexity of family constellations across households at a given time as one such overlooked factor (cf. Schnettler and Steinbach 2011, p. 190). One might argue that the specific family and household structure of a child adds another set of potential stressors that contributes to the link between family type and developmental outcomes. Navigating family relationships across households may be stressful merely because it is more demanding to organize life across two more or less geographically distant households than it would be to organize daily life in one household. In addition, communicating and negotiating the demands of the parent living in either household may be emotionally taxing, a challenge that arguably becomes more difficult as additional relationships are involved, e.g., toward stepparents and step-siblings in the main and secondary parental home. This reasoning may be used to extend the instability hypothesis toward an instability-complexity hypothesis: multiple family transitions and the degree of complexity of cross-household family constellations both provide independent and additive sources of stress that may cumulate and affect developmental outcomes of children and adolescents.
The Evolutionary Approach to Understanding Stepparental Behavior
In the standard social science approach, the stepgap is interpreted mainly as the consequence of various stressors stemming from characteristics of the family environment (see above). In contrast, sciences that explicitly draw on Darwinian evolutionary theory see evolved anthropological inclinations as the driving forces behind the stepgap. Kin selection theory (“Kin Selection”) explains why biological children usually receive significant parental investment and why parents truly exhibit an interest in their children’s prosperity. For the same ultimate causes, that is, the absence of (personal or inclusive) fitness benefits, stepparents do not have kin-altruistic interests in their non-genetically related stepchildren. In general, parental investment can be understood as any kind of resource transfer from parents to their offspring. Parents usually do not receive economic or social net-benefits for taking over the tasks of parenting (Lee and Kramer 2004), but when investment occurs for biological offspring, altruistic behavior can increase direct fitness, also known as personal fitness.
In contemporary industrialized countries, step-parenting occurs most commonly due to parental separation or divorce. However, in historical Europe as well as in societies in Sub-Saharan Africa, stepparental relationships are mostly due to parental death (Bock and Johnson 2008). The tasks of parenting are then often taken up by adult kin, like grandparents, biological aunts, and uncles who adopt their grandchildren, nieces, or nephews. For these cases, inclusive fitness theory predicts that stepchildren receive more care and attention than stepchildren would do on average. This is because they are biologically related to their stepparents. However, due to their lower genetic relatedness and the absence of direct fitness benefits, the theory still predicts them to receive less care and attention, on average, than children would usually receive from their biological parents.
Whereas kin selection theory argues on the level of ultimate causality, there is also research on the proximate mechanisms that may be involved in mediating parental investment: It is expected that natural selection has favored such mechanisms that (a) increase the likelihood of having biological offspring, e.g., by means of sexual desire or an emotional desire to have children (cf. Foster 2000), (b) make adults attend and respond to the needs of infants (see “John Bowlby and Attachment Theory”), and (c) allow individuals to reliably recognize and to direct investment toward close biological kin (“Kin Recognition”). The evolved proximate mechanisms therefore will reduce the likelihood that stepparental altruistic behavior is exhibited toward non-genetically related children.
When it comes to stepparental investment specifically, one central point of debate with regard to potential mechanisms has been the question as to whether the stepgap is merely the result of higher (step-)parental investment in favor of biological over stepchildren or whether stepparents actively disadvantage their stepchildren in the household. Research on this topic has been inspired by Daly and Wilson (1985, 1999) who were among the first to make an argument for the latter and to name the tendency toward disadvantaging stepchildren the “Cinderella effect,” alluding to Cinderella’s bad treatment by her stepmother in the fairy tale of the same name by Brothers Grimm. They supported their argument empirically with a study using data from the American Humane Association. Here, they compared the risk of fatal child abuse in households with and without the presence of a stepparent and showed that the risk was indeed higher in households where a stepparent was present. Although their foundational work in this area has provoked many, partly justified, methodological criticisms (see, e.g., Buller 2005), further research around the globe replicated Daly’s and Wilson’s finding, even after considering a variety of relevant context factors (e.g., Anderson et al. 1999a, b). In this regard, the sequence of events resembles that in the social scientific literature cited above. Yet, the interpretation of results has been different: Whereas social scientists have tended to see the remaining stepgap as the result of yet unexplained contextual factors (e.g., hitherto unmeasured forms of stress in complex family and household constellations), evolutionary psychologists have interpreted the residual stepgap as a potential result of the “Cinderella effect,” and there has been a heated debate on whether this effect can be understood as an unconditional (“hardwired”) human trait.
The discussion of the “Cinderella effect” in Darwinian psychology has largely ignored potential variability in the origins of stepfamilies across time, place, and population segments. However, as we have already pointed out above, stepfamilies can be the result of divorce and remarriage. Or they can be the result of parental death and remarriage. In that case, the formation of a stepfamily can indicate normalization after a severe family crisis (Anderson et al. 1996), whereas step-parenting due to divorce might indicate a family environment in which parental motivation to care is reduced and in which parents and children are exposed to a set of potential stressors (see above). Willführ and Gagnon (2013) illustrate the conditionality of parental motivations to invest in or even actively disadvantage their stepchildren. They investigated the consequences of parental death and remarriage in the historical Krummhörn region in Germany (1720–1874) and in the historical St. Lawrence Valley (1670–1799). In the Krummhörn region, the presence of a stepmother was associated with increased offspring mortality to a similar extent as the death of a mother, whereas the remarriage of the father was a neutral event for his children from a previous marriage. The differential effect of the stepmother can be explained by the different socioeconomic contexts: Although the population of the St. Lawrence Valley was expanding, individuals were competing less for an inheritance (e.g., the parental farm), as land was abundantly available for the French settlers who benefitted from large family sizes. In contrast, the Krummhörn region was demographically saturated and there is evidence for increased sibling competition. By disadvantaging the children of her husband’s former marriage, the stepmother intervened for the benefit of her own children. The conclusion of this study is that a stepparent’s behavior is conditional on socioeconomic context and depends on whether reproductive interests are narrowed by one partner’s former reproduction.
Overall, we can understand the treatment of stepchildren as an expression of an opportunistic strategy which is characterized by a trade-off (cost-benefit calculation). This trade-off is dependent on the socio-environmental context: If the benefits for the stepparent prevail over the costs, stepchildren can expect to receive a certain amount of stepparental investment even if the stepparent does not have an altruistic interest in a stepchild’s prosperity. An illustration for this trade-off in modern stepfamilies would be a case of overdue alimony payments by the absent biological parent. Depending on the amount of financial resources in the newly formed stepfamily, overdue or withheld alimony payments by the absent biological parent may to a higher or lesser degree mean a financial burden for the stepparent and thus enhance conflict potential within this newly formed stepfamily.
Whereas much of the literature on parental investment in stepfamilies focuses on disadvantages, it is worth noting that stepparental investment is – under certain conditions – consistent with evolutionary theory. Although kin-selection theory predicts that stepparents are not altruistically interested in their genetically unrelated stepchildren because of the absence of fitness benefits, there may be other evolved motivations why stepparents display parental investment toward their stepchildren. One such case is when the motivation to invest may be part of a mating or mate-retention effort (Anderson et al. 1999a, b). Another case could be stepparental investment as a signaling effort, e.g., to earn reputation in one’s social network in social contexts where stepparental investment is highly encouraged by social norms. Finally, if one considers that many individuals display a desire to have and care for children – a desire that itself may be an evolved motivation (Foster 2000; Rotkirch 2007) – step-parenting may be one way to fulfill this desire, especially for individuals unable to have biological children.
From this review of the literature on step-parenting, it becomes apparent that the (evolutionary) biological, psychological, and social scientific literature focus on different aspects of step-parenting. Whereas both the evolutionary and social sciences have empirically established the stepgap and then shown that it gets smaller with better methodological designs and better data, they have highlighted different potential explanations for the remaining stepgap. Evolutionary biology and psychology aim at explaining different motivations for parents to invest in their biological children and stepchildren. The social scientific literature, however, has focused mainly on explaining the stepgap by means of contextual factors like family resources and stressors across different family types. We argue that these approaches are not necessarily at odds with each other but complement each other in central aspects. In order to integrate them we need to engage with these various theories on a more fine-grained level and build new, interdisciplinary models that integrate motivational and contextual factors. Also, new types of data collection will be necessary that combine contextual variables along with biologically relevant variables. Human life courses are neither solely affected by evolved inclinations and individual motivations nor are these anthropological inclinations hardwired to a degree that makes them unmalleable to environmental influences.
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