In this blog post, we will examine the pros and cons of whether rape qualifies as an “adaptation” in evolutionary biology, focusing on the reasons why it is difficult to view rape as an adaptation.
Through his theory of evolution, Darwin proposed that evolution occurs through natural selection. Individuals within a population possess different traits, and because of these differences, their ability to respond to the environment also varies. As a result, individuals who are better adapted to the environment have a survival advantage, and natural selection occurs when these traits are passed on to the next generation. An adaptation is the product of this natural selection. Many traits of living organisms can be explained by the concept of adaptation. However, there remains significant debate over whether the human mind and behavior have also evolved through natural selection. In particular, the debate over whether male rape behavior can be viewed as an adaptation is highly contentious, compounded by ethical objections. In this article, I intend to support the position that rape is not an adaptation, as opposed to the claim that it is. Before doing so, I will first summarize the arguments for and against the notion that rape is an adaptation.
Those who argue that rape is an adaptation view it as an adaptive behavior that increases a man’s chances of reproductive success. In particular, they suggest that this behavior may have evolved as a strategy for some men who face difficulties in finding a mate to pass on their genes to the next generation. Although this argument provokes ethical aversion and has faced significant criticism, proponents maintain that the question of whether rape is an adaptation is not an ethical issue but a scientific one aimed at explaining facts. They also argue that understanding rape from an evolutionary biological perspective may actually help prevent rape crimes.
Opponents of the claim that rape is an adaptation point out that there is insufficient evidence to support this view. If rape were an adaptation resulting from a trait advantageous for reproduction, it would be difficult to explain the fact that a significant proportion of rape victims are women who cannot become pregnant or are of advanced age. Furthermore, cases of male-on-male sexual assault, incestuous sexual violence, and sexual violence against children cannot be fully explained by the adaptation hypothesis alone. Therefore, it is more likely that rape is a byproduct of the combination of human sexual behavior and aggression rather than an adaptation for reproduction. Furthermore, the argument that rape is not an adaptation is supported by the fact that various biological traits can arise through evolutionary processes other than natural selection.
Both sides of the debate are discussing whether rape is an adaptation, citing various examples related to adaptation. However, the criteria for determining which phenomena should be viewed as adaptations and which should not are unclear. Although some argue that a trait must possess a certain level of complexity to be considered an adaptation, this too is difficult to regard as a clear criterion. Therefore, more persuasive criteria are needed to determine whether rape is an adaptation or not. I believe it is reasonable to use the generally accepted definition of adaptation as such a criterion.
Evolutionary biologist Dobzhansky, who contributed significantly to linking Darwin’s theory of evolution with Mendel’s laws of inheritance, defined adaptation as an evolutionary process that enables organisms to survive and reproduce more effectively in their environment, and as a characteristic of genetic traits formed in a direction favorable to survival and reproduction.
Let us examine the act of rape in light of this definition of adaptation. If rape were an adaptation, humans would have to survive better in their environment and achieve favorable reproductive outcomes through rape. However, throughout human history, this claim lacks persuasiveness. As societies and cultures have developed, rape has been defined as a serious crime in most societies, and those who commit rape have faced legal punishment and social sanctions. In modern society, these penalties have become even more severe. Therefore, it is difficult to view rape as a strategy that benefits survival and reproduction for humans living in society. Consequently, rape is difficult to explain solely through the generally accepted concept of adaptation.
Meanwhile, the argument that rape has become an adaptation because it benefits survival and reproduction has another flaw. If the trait of rape were adapted through natural selection, that process of adaptation would have had to occur over a very long period of time. However, assuming that rape has been adapted as a reproductive strategy for some men who find it difficult to secure a mate, it would be difficult for such a trait to persist over the long term.
In the short term, rape may provide reproductive opportunities for some individuals. However, for the trait of rape to spread throughout the male population via natural selection, it must yield a higher reproductive success rate than other reproductive strategies; consequently, the associated trait must become widespread across generations. In reality, however, reproduction through rape occurs very rarely compared to reproduction based on normal mate selection and consent. In other words, assuming that traits associated with rape and those not associated with it coexist, in the long run, the latter are far more likely to produce offspring.
Furthermore, difficulty in securing a mate compared to other males is generally associated with the individual’s traits being relatively unattractive to the opposite sex. Therefore, even if reproduction through rape is successful, assuming that such traits are passed on to offspring, the same disadvantage is likely to recur in subsequent generations. Ultimately, it is difficult for such traits to spread continuously through the long-term process of natural selection. Consequently, it is difficult to conclude that the act of rape has been adapted through natural selection.
In addition to these logical problems, there is also biological evidence that makes it difficult to definitively support the adaptation hypothesis—namely, that traits not directly advantageous for survival and reproduction can still be passed on to the next generation in conjunction with other genetic factors. For example, suppose gene A is frequently passed on to offspring through natural selection. However, if gene B is located very close to gene A and is inherited together with it, gene B can continue to be passed on to the next generation even if it does not directly contribute to adaptation. Furthermore, when a single gene influences multiple traits, traits that aid adaptation and those that do not may be inherited together.
This possibility was confirmed in a study on the evolution of insecticide resistance in fruit flies conducted by Paul Ehrlich and Robert Sokal. The researchers conducted an experiment on fruit flies in which they gradually increased the concentration of insecticide in the culture medium, allowing individuals with high resistance to survive. As a result, the surviving fruit flies exhibited high levels of insecticide resistance. Interestingly, these fruit flies also differed in where they formed their pupae. While typical fruit flies form pupae randomly on the surface of the culture medium, the fruit flies selected for insecticide resistance tended to form pupae along the edges of the medium. In other words, even though only insecticide resistance was selected for, the behavior of forming pupae at the edges also emerged.
Conversely, when individuals that formed pupae along the edges of the culture medium—rather than those selected for insecticide resistance—were selected as parents in each generation and bred, the resulting fruit fly population also developed insecticide resistance. This suggests the possibility that insecticide resistance and the behavior of forming pupae along the edges are genetically linked.
These experiments demonstrate that traits advantageous for survival through natural selection are not necessarily inherited independently. Therefore, it is difficult to conclude that all human behavioral traits are direct results of natural selection. There is also insufficient evidence to conclude that rape is an adaptation. These research findings show that, based on current knowledge alone, it is difficult to clearly distinguish whether the behavior of rape is a directly adapted trait or a byproduct that emerged alongside the selection of other traits.
To summarize the points discussed so far: First, when applying the generally accepted definition of an adaptation to determine whether rape is an adaptation, we have confirmed that rape cannot be fully explained by the concept of adaptation alone. Second, considering that adaptation is a process formed over a very long period of time through natural selection, there are several logical limitations to viewing rape as a trait adapted through natural selection. Third, based on biological research findings showing that seemingly unrelated traits can be inherited together, we have examined that, given the current evidence alone, it is difficult to clearly distinguish whether the behavior of rape is a direct product of natural selection or a byproduct that appears alongside other traits. Therefore, we can conclude that the claim that rape is an adaptation lacks sufficient evidence and involves several logical leaps.
This conclusion does not negate the attempt to understand rape biologically. Biologically studying various human behaviors—including aggression and sexual behavior—can make a meaningful contribution to understanding the causes of sexual violence and developing prevention strategies. However, based on a synthesis of research findings in evolutionary biology and behavioral science accumulated to date, there is insufficient evidence to conclusively assert that rape is a biological adaptation. It is possible that new evidence will emerge as related research advances in the future. However, based on current scientific knowledge, it is difficult to conclude that rape is an adaptation shaped by natural selection.
