The Cinderella Effect is a term in evolutionary psychology that refers to higher incidence of maltreatment and/or abuse in children by step-parents  compared to biological parents. 

From an evolutionary perspective, natural selection has favored intensive parental care in humans. Thus, parents have to commit a lot of time and resources to raise children. Moreover, parents also have to be able to protect and defend their investment. 

According to Daly and Wilson (click title for full article), if the psychological underpinnings of parental care have evolved by natural selection, care-providing animals may be expected to direct their care selectively towards young that are a) their own genetic offspring and b) able to convert parental investment into increased prospects for survival and reproduction. This notion is known as the theory of discriminative parental solicitude and has been described and verified in a broad range of care-giving species. From this perspective, adoption of unrelated young has been interpreted as a failure of discrimination. In humans, adoption by unrelated caretakers is a recent cultural invention than repeated aspect of ancestral environments, meaning that it could not have been a feature of parental psychology as it evolved.

However, step-parental care is ubiquitous across cultures and species, while also being present throughout history. The main explanation as to why this occurs is thought to be that investing pseudoparental care in a new mate’s offspring is adaptive and favored by natural selection. In humans, for example, suitable mates are scarce may be scarce and established couples usually stay together for longer than one breeding season. 

On these grounds, Daly and Wilson hypothesized that any and all sorts of child abuse and exploitation would occur at elevated rates in steprelationships than in genetic parent-child relationships. This differential mistreatment is what the authors refer to as the “Cinderella Effect.” 

Considerable support has been found for the Cinderella Effect, but the theory does not come without controversy. Confounds such as socieconomic status and personality differences between parents that live with their own children and parents who become parents have been brought up although studies in Canada and the US have assessed these factors and found them to be non-plausible. 

Findings supporting the Cinderella Effect include: Stepparents beat very young children to death at per capita rates over 100 times higher than the corresponding rates for genetic parents. Stepparents also perpetrate both nonlethal physical assaults and sexual abuse at much higher rates than genetic parents. Abused stepchildren were almost always the eldest in the home. Cinderella effects are large regardless of marital registration (abuse can happen by unrelated live-in boyfriends, not necessarily a spouse). 

Sources: 

Daly and Wilson. The Cinderella effect is not fairy tale. TRENDS in Cognitive Sciences (2005). 9 (11): 507-8. 

Daly and Wilson. (2008). Is the “Cinderella Effect Controversial? A Case Study of Evolution-Minded Research and Critiques Thereof. Foundations of Evolutionary Psychology (pp. 383-400). Psychology Press.