Evolutionary Perspectives on Step Parental Investment

This entry was written by Gabrielle Eyer as part of a project done in BIAN 2133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.


In present-day WEIRD societies, the presence of a stepparent in nuclear families appears to be more and more common. By evolutionary standards, the existence of alternative family configurations in modern Western societies is very recent (Hamilton et al., 2007), resulting in the emergence of studies highlighting the impacts of different family compositions on children. These studies suggest that children living in a household with one biological parent and one non-biological parent receive lower levels of parental care and tend to be at a higher risk of neglect or abuse (Temrin et al, 2011; Hamilton et al, 2007). Evolutionary theories are applied to specific situations to seek an explanation for the discrepancy in parental care towards a child between stepparents and biological parents. These theories interpret the variation in level of care as being due to the lack of genetic relatedness, leading to discriminative parenting and increased parent-offspring conflict between a stepparent and a child within a household.

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Non-Genetic Relatedness

Genes and the genetic ties between two individuals allow for the opportunity of gene transfer onto future generations (De Baca et al., 2012). Based on parental investment theory, a parent is likely to invest more in an offspring when the child will provide an advantage to the parent’s fitness, such as that of passing on their genes (Temrin et al., 2011). Due to the lack of genetic ties between a stepparent and a child within the same nuclear family, that evolutionary advantage of intergenerational gene transmission is absent. It is proposed by Tooley and colleagues (2006) that the evolution of increased degrees of parental investment have been influenced by psychological mechanisms that encourage nurturing and protective behaviours towards offspring, particularly at a young age. These mechanisms are only partially, if at all, triggered in stepparents. This study provides an explanation for the increased number of reported unintentional childhood injuries, due to lack of supervision in the presence of a stepparent (Tooley et al., 2006). Parental investment and care for a child, as a whole, is a substantial cost to an individual (Temrin et al., 2011). Without the evolutionary benefit of genetic transmission between a stepparent and a child, there is a greater chance for decreased parental investment often reported as neglect or abuse.

Discriminative Parenting

Kin selection theory explains the justification behind observed discriminative parenting within stepfamilies. Discriminative parenting occurs when a stepparent invests more in one offspring or child over another (Hamilton et al., 2011). Evolutionarily, parents are genetically predisposed to invest more in biological offspring, than non-biological children, which can be tested by evaluating resource allocation, such as time, money and care, within different family configurations (Hamilton et al., 2007; Zvoch, 1999). This idea explains the degree of energy spent towards the upbringing and care of one’s own biological offspring and the resistance for parental effort expenditure on stepchildren, based on the fitness value that a child represents to a stepparent (De Baca et al., 2012; Temrin et al., 2011). For example, studies have demonstrated that stepfathers are less likely to provide direct care, financial support, quality time and homework assistance to their stepchildren than biological fathers towards their own children (Hamilton et al., 2011). Discriminative parenting sheds further light on the discrepancies of parental investment and care towards a child between a biological parent and a stepparent.

Stepparent-Child Conflicts

Parent-child conflict occurs when a parent’s fitness and needs are dissimilar to those of the offspring (Zvoch, 1999). Based on a study conducted by Temrin and colleagues (2011), results showed that child abuse and neglect resulted from stepparent-child conflicts, whereas child homicide was more often due to between parent conflicts. This increased conflict within a household is believed to derive from a reduction of shared genetic interest between a stepparent and a child, as well as the stepparent and the biological parent (Zvoch, 1999). Additionally, there is an inherent cost-benefit imbalance between a stepparent and a child. The degree of parental investment to a stepparent is costly to the individual, all while the non-biological child does not represent a genetic benefit (Temrin et al., 2011). As a result, a stepparent is thought to not gain the same emotional benefit from a child that is not genetically related to them (Zvoch, 1999). Lastly, the role of a stepparent within an alternative family configuration causes ambiguity (Hamilton et al., 2007). As a stepparent, placed a parental role, that ambiguity combined with a lower level of intrinsic commitment compared to a biological parent results in decreased parental investment towards a child (Tooley et al., 2011). Overall, within family genetic conflict seems to increase in alternative family configuration, which further supports past studies’ findings that step parental investment towards a child is lower compared to the investment from a biological parent.


All in all, the discrepancy in step parental investment levels compared to those of genetic parents can be explained using evolutionary theories such as Parental investment theory, Kin selection theory, and Parent-child conflict. The overarching argument for this discrepancy is the lack of genetic relatedness between a child and a stepparent which leads to neglect, discriminative parenting and increased within-family conflict. Along with the lack of genetic relatedness, other factors, such as life histories, influence the level of investment of a stepparent. While there are increased rates of child neglect and abuse in stepfamilies, based on life-history theory, a child will benefit from the quality of total parental investment, whether from a biological parent, or a stepparent (De Baca et al., 2012).

Literature Cited

Cabeza De Baca, T., Figueredo, A., & Ellis, B. (2012). An Evolutionary Analysis of Variation in Parental Effort: Determinants and Assessment. Parenting, 12(2-3), 94-104.

Hamilton, L., Cheng, S., & Powell, B. (2007). Adoptive Parents, Adaptive Parents: Evaluating the Importance of Biological Ties for Parental Investment. American Sociological Review, 72(1), 95-116.

Temrin, H., Nordlund, J., Rying, M., & Tullberg, B. (2011). Is the higher rate of parental child homicide in stepfamilies an effect of non-genetic relatedness?. Current Zoology, 57(3), 253-259.

Tooley, G., Karakis, M., Stokes, M., & Ozanne-Smith, J. (2006). Generalising the Cinderella Effect to unintentional childhood fatalities. Evolution And Human Behavior, 27(3), 224-230.

Zvoch, K. (1999). Family Type and Investment in Education A Comparison of Genetic and Stepparent Families. Evolution And Human Behavior, 20(6), 453-464.

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