Is Rape a Biological Adaptation? What Does the Adaptation Debate in Evolutionary Biology Tell Us?

In this blog post, we examine the evolutionary biological debate over whether rape is a biological adaptation and the evidence supporting it.

Prominent evolutionary biologists engaged in a heated debate on the topic, “Is Rape an Adaptation?” Adaptationists argue that rape is an adaptive behavior that helps males achieve greater reproductive success. In contrast, anti-adaptationists contend that rape is merely a byproduct of sexual desire and not an adaptation. The reason for this divergence in opinion stems from differing perspectives on the concept of “adaptation.”
According to Dawkins, a proponent of the adaptationist view, for a trait to be recognized as an adaptation rather than a mere byproduct, it must possess a certain level of complexity. He also argued that there are many cases where traits previously considered byproducts should actually be classified as adaptations. Applying this to rape, if the behavioral trait of rape—which was once viewed as a byproduct of sexual desire—possesses a certain level of complexity, it can be regarded as an adaptation.
In response, opponents question the very criterion of complexity required to classify a specific trait as an adaptation. Gould cites the spandrels of a cathedral as an example, arguing that a “byproduct that arose inevitably” should not be viewed as a “specifically designed” structure—that is, an adaptation. Furthermore, Coine, in his book ‘The Natural History of Rape’, which addresses the topic of this debate, criticized the claim that male rape behavior is an adaptation, arguing that the evidence presented is insufficient. These arguments can ultimately be summarized as follows: “Rape does not meet the level of complexity required to be considered an adaptation, and since the evidence presented is also insufficient, it is reasonable to view it as a byproduct.”
Before discussing whether rape is an adaptation, there is a point that must be clarified. This discussion does not address whether rape is right or wrong from an ethical or social perspective, but rather whether it is an adaptation from a biological perspective. In other words, arguing that rape is an adaptation does not mean justifying or encouraging rape. Nor does calling it an adaptation imply that all men commit rape. As noted in the view cited by the original author, just as humans can avoid obesity, rape is also entirely avoidable.
Adaptation is a product of natural selection. Therefore, the claim that rape is an adaptation can be seen as equivalent to the claim that rape is a behavior influenced by natural selection. At the heart of natural selection lies the instinct to pass one’s genes on to offspring, and that instinct manifests as sexual desire. Rape can be described as an extreme expression of sexual desire. Of course, no one commits rape with the conscious intention of passing on “good genes” to their offspring. According to this perspective, this instinct exists in the unconscious and surfaces as a conscious desire when encountering an attractive member of the opposite sex.
However, there is no clear standard for defining “good genes.” Nevertheless, this perspective holds that when a man encounters a member of the opposite sex whom he perceives as possessing “good genes,” sexual desire—that is, the instinct to pass on better genes to offspring—arises. Of course, this instinct is heavily restricted by social, ethical, and legal norms. It could be argued that in a society where such norms did not exist, people would act more in accordance with the instincts shaped by natural selection. Furthermore, the term “rape” implies “coercion,” and this coercion is a concept defined by social, ethical, and legal standards. Therefore, the logic of the proponents is that if this aspect is excluded, rape can be viewed as a mere sexual act, and the fact that sexual acts are products of natural selection is difficult to deny.
In response to this, it could be argued that even if one considers only the evolutionary biological factors of rape, non-biological factors—such as social and ethical elements—inevitably influence the biological outcomes of rape and must therefore be fully taken into account. For example, rape generally involves violence, which results in social punishment that ultimately works against reproduction. This raises the question of whether such behavior can be called a biological adaptation. However, this article argues that social punishment is a product of the legal systems established in modern civilization, and that in primitive societies where social punishment did not exist, those who committed rape may have had a reproductive advantage over those who did not.
Even if social punishment for rape did not exist in primitive societies, it is entirely possible that primitive forms of sanctions were in place. If we consider the existence of morality to be one of the criteria distinguishing humans from animals, morality can be divided into civilized morality—which emerged after the formation of civilization—and primitive morality from pre-civilized societies. In this case, it is also possible to argue that the primitive morality of prehistoric societies would have been anti-rape in nature, that certain punishments were imposed on those who committed rape, and that this would have been disadvantageous for reproduction. However, such arguments are, after all, merely speculation. Furthermore, since explaining rape solely through the lens of morality makes it difficult to account for rape behavior observed in animals, this alone does not provide sufficient grounds to conclude that rape is not an adaptation.
Another argument from the opposing side is based on examples presented by Gould. He refutes the claim that rape is an adaptation by citing cases such as male-on-male rape, incestuous rape, and child rape. These cases can be viewed as special instances distinct from general rape; an examination of each is as follows.
First, cases of male-on-male rape can be understood as instances related to same-sex sexual behavior. However, modern medicine and psychology distinguish between coercive sexual violence and sexual orientation as separate concepts. Sexual orientation refers to the gender toward which an individual feels a persistent emotional, social, and sexual attraction, and homosexuality is one form of this sexual orientation. According to current research, sexual orientation is known to be formed through the complex interaction of various factors, including genetic factors, the hormonal environment during fetal development, and the developmental process; there is no scientific evidence to support the claim that specific parenting styles or sexual abuse determine sexual orientation.
Incest is considered a strong social taboo. One reason for this is that when close blood relatives have children, there is a higher probability of both parents passing on the same recessive gene, thereby increasing the risk of genetic disorders manifesting. Conversely, when parents are not blood relatives, even if a specific recessive gene is present, its effects are relatively less likely to manifest because they are masked by the dominant gene from the other parent.
Finally, pedophilia refers to a psychiatric concept characterized by a persistent and intense sexual interest in or attraction to children who have not yet reached puberty. Modern medicine treats this as a distinct concept separate from sexual orientation, and all sexual acts involving children are defined as clear crimes and serious human rights violations.
The common thread among these three cases is that sexual desire is directed toward a specific target, and from the perspective of reproduction, such relationships are either impossible or place the individuals in a highly disadvantageous position. However, as mentioned earlier, people generally do not engage in sexual relations with the conscious intention of passing on superior genes to their offspring; rather, they do so as part of the process of satisfying their instinctual sexual desire. Furthermore, this sexual desire can be viewed as part of an instinctive system formed by natural selection over the course of the human species’ long evolutionary history. While this system may be altered by various factors—such as social and ethical influences or mental illness—this article argues that such factors alone do not render sexual desire itself unrelated to natural selection. Therefore, I believe it is not valid to argue that rape is not an adaptation based on sexual orientation or the existence of specific sexual objects.
Since a trait must meet a certain level of complexity to be considered an adaptation, it could be argued that rape is not an adaptation but merely a byproduct. However, there is still no consensus within the field of evolutionary biology regarding the exact threshold for this complexity. This difference in perspective is also discussed in ‘Darwin’s Table’, mentioned in the original text, and Dawkins’ ‘The Selfish Gene’ likewise does not provide a clear standard for complexity. Therefore, this paper takes the position that it is premature to conclusively determine whether rape is an adaptation based solely on standards that have not yet been established.
The core argument of this paper is that rape can be viewed as a process through which sexual desire is expressed, regardless of the moral and social issues surrounding the act or the identity of the victim, and that sexual desire is the driving force behind natural selection. Non-biological factors, such as social and ethical elements, cannot be excluded from rape, and counterarguments have been raised—based on various characteristics of rape—that it is merely a byproduct of sexual desire. However, this paper argues that such counterarguments do not sufficiently refute the adaptationist claim that rape may be advantageous for reproduction. Ultimately, it concludes that rape is a behavior in which sexual desire—the driving force of natural selection—is expressed, and if adaptation is the result of natural selection, then rape can also be viewed as an adaptation resulting from natural selection.