In this blog post, we will examine the debate over whether rape is an evolutionary adaptation or a byproduct of another adaptation and evaluate its validity.
The argument that rape is an adaptation was first formally put forward in ‘A Natural History of Rape: The Biological Basis of Sexual Coercion’, published in 2000 by Professors Randy Thornhill and Craig T. Palmer, and has since sparked significant criticism and debate not only among scholars in the humanities and social sciences but also among biologists. For the purposes of this article, we will define rape as the act of forcing sexual intercourse against another person’s will, and adaptation as a trait that aids an individual’s survival and reproduction and is thus maintained through natural selection. According to these definitions, the claim that rape is an adaptation implies that the act of rape once aided an individual’s reproduction, and as a result, psychological tendencies related to rape evolved through natural selection. Proponents of this argument cite evidence such as the fact that a significant proportion of rape victims are women of childbearing age and that forced mating has been observed in some animal species. On the other hand, scholars in the humanities and social sciences, who have traditionally explained rape as an expression of violence, counter that victims are not limited to women of childbearing age, and that cases such as murder following rape are difficult to explain as behavior aimed at reproduction. Furthermore, some evolutionary biologists suggest that rape may not be the product of an independent adaptation but rather a byproduct of other adaptive processes.
I believe that rape is a byproduct of other adaptive processes rather than a cognitive adaptation that evolved directly through natural selection. While there may be cases where reproduction is successful through rape, it is difficult to conclude from this alone that the psychological drive to rape evolved through natural selection. Rather, I view the sexual desire to engage in sexual activity and the motivation to reproduce as having evolved as a result of adaptation; when combined with the development of physical strength and aggression resulting from the competition for survival, this can lead to the behavior of rape in certain situations. In other words, while various adaptations that evolved for different reasons may combine in specific situations to trigger the behavior of rape, it is difficult to view rape itself as a single adaptation.
The claim that rape is an adaptation implies the premise that the psychological drive to rape is also an adaptation carefully selected through natural selection. In ‘Darwin’s Table’, Stephen Jay Gould explained that a “spandrel” is not a structure intentionally designed to express Christian symbolism, but rather a structural byproduct inevitably created in the process of installing a dome atop an arch. Richard Dawkins, on the other hand, argued that since spandrels were selected even though other structures—such as “corbels”—could have been used to support the roof, many traits that appear to be byproducts on the surface may in fact be adaptations. From this perspective, rape, too, may appear to be a byproduct but could in reality be an adaptation.
However, it is difficult to view rape as an adaptation specifically selected through natural selection from among various possible behaviors. The reason it is hard to view rape as having been subject to natural selection as a reproductive behavior is that normal mating behavior is overwhelmingly common. The fact that rape occurs in certain situations where it is difficult to find a mate is too limited in scope to conclude that it is an independent adaptation. Furthermore, among the various ways of expressing aggression, behaviors that directly aid survival—such as hunting or competition—are far more common. Therefore, it is also difficult to view rape as having been selected by natural selection as a representative adaptation for expressing aggression.
Furthermore, cases that are difficult to explain if rape is viewed as an adaptation can be explained relatively naturally from the perspective that they are byproducts of two adaptations: sexual desire and aggression. Even acknowledging that a significant number of rape victims are women of childbearing age, it remains a fact that sexual crimes against children, same-sex sexual violence, and spousal rape exist. In the first two cases, reproduction is unlikely, and spousal rape is also difficult to view as a behavior intended to increase reproductive success; yet these are crimes that actually occur. If rape is viewed as a single adaptation, it is difficult to explain all these diverse cases consistently. On the other hand, if it is viewed as a byproduct of two adaptations—sexual desire and aggression—it can be explained more broadly. Sexual crimes against children and same-sex sexual violence can be understood as the result of a combination of sexual desire and aggression rather than as behaviors aimed at reproduction, and spousal rape can also be interpreted in the same context. Furthermore, in cases where sexual violence is used as a means to kill the victim after the rape or to inflict extreme humiliation and suffering on the other party, the explanation that this is a byproduct of a combination of aggression and dominance behaviors—rather than an adaptation for reproduction—is relatively more persuasive.
Since Stephen Jay Gould and Richard Lewontin criticized adaptationism in “The Spandrels of San Marco and the Panglossian Paradigm,” extreme adaptationism—which attempts to explain nearly every trait as an adaptation—has not received significant support in academic circles. In evolutionary biology today, research is being conducted to distinguish whether a specific trait is an adaptation, a byproduct of another adaptation, or the result of multiple factors acting in combination. From this perspective, rather than categorically asserting that rape is an independent adaptation, it is a more cautious interpretation to also consider the possibility that it is a byproduct arising from other adaptive processes.
