In this blog post, we will examine the view that rape is an evolutionary adaptation, along with the counterarguments against it, and explore whether rape can be understood as a byproduct of sexual desire.
Although we cannot know exactly when rape first began, various historical records and texts confirm that it has existed for a very long time in human history. Even today, rape remains a serious crime, and several theories have been proposed to explain why this behavior has persisted over such a long period. To explain this, some evolutionary psychologists—including Randy Thornhill, known as an “adaptationist”—have argued that “rape is an adaptation.” Here, “adaptation” refers to the process by which organisms change to better suit their environment; it is a fundamental biological concept in which traits advantageous for survival and reproduction are maintained across generations through natural selection. Adaptationists have argued that rape behavior was selected by natural selection as a strategy to help men achieve greater reproductive success, and that it has persisted to this day as a result. In particular, Thornhill developed this argument in ‘The Natural History of Rape’, drawing on statistical data regarding rape victims.
However, this claim is currently regarded as a highly controversial hypothesis even within the fields of evolutionary psychology and behavioral ecology, and it faces criticism for lacking sufficient empirical evidence. I believe that it is not rape but sexual desire that has been adapted. All living organisms possess an instinct to pass on their genes to the next generation, and this instinct can be fulfilled through sexual activity. However, if sexual activity itself had been an excessively arduous or aversive behavior, the opportunities to pass on genes would have been greatly reduced. To solve this problem, the mechanism of sexual desire evolved, and as a result, sexual activity was able to continue.
Over time, however, sexual activity ceased to be merely a means of reproduction and came to serve the additional function of satisfying sexual desire. Considering that most sexual activity in modern society does not have pregnancy as its direct purpose, sexual desire itself can be understood as an adaptation. Therefore, it is the author’s view that rape should be seen not as an adaptation for reproduction, but rather as a byproduct of the distorted manifestation of the adapted mechanism of sexual desire. In other words, the author argues that “rape is a byproduct of sexual desire.”
However, adaptationists might refute this claim based on the following data. Gottschall published statistical data based on a study of pregnancy rates among female rape victims. According to the study, which surveyed 405 female rape victims aged 12 to 45, the pregnancy rate following a single instance of rape was 6.4%, and when cases of contraception were excluded, it rose to approximately 8%. In contrast, for couples engaging in regular sexual intercourse with the intent to conceive, the pregnancy rate per sexual encounter was found to be approximately 3.1%. Based solely on these figures, rape might appear to be a behavior more advantageous for reproduction than consensual sex, and one could therefore interpret this as supporting the claim that rape is an adaptation.
However, there are two problems with using this data to refute the claim that rape is a byproduct of sexual desire. The first is that the comparison of pregnancy probabilities was based solely on a single act of intercourse. By its very nature, rape is mostly a one-time event, whereas consensual sex occurs repeatedly throughout a woman’s fertile period. Therefore, to compare actual success rates from a reproductive perspective, it is more appropriate to consider the entire fertile period. In fact, the reality that the number of children born through consensual sex in society as a whole far exceeds the number born as a result of rape also supports this point. Therefore, comparing pregnancy probabilities based solely on a single sexual encounter has limitations as an indicator for evaluating the efficiency of reproductive strategies.
The second problem lies in the very direction of the rebuttal. To refute the claim that rape is a byproduct of sexual desire, one must prove the logic that “the motive for rape is not sexual desire” or “the motive for rape is reproduction.” This is because the key lies in the motive behind the behavior, not the outcome. However, the data presented earlier only shows the probability of pregnancy—the result of rape—and does not explain the motive behind the act of rape itself. Therefore, based on this data alone, it is difficult to conclude that the claim that rape is a byproduct of sexual desire has been directly refuted.
However, regarding the claim that the direction of the rebuttal is flawed, the following counter-rebuttal is possible: In evolutionary theory, natural selection is a process driven by outcomes rather than an individual’s will or purpose; therefore, to determine whether rape is a behavior selected by natural selection, one must evaluate it based on the outcome—reproduction—rather than on motives or purposes. Consequently, it can be argued that basing the discussion on the probability of pregnancy—the outcome of rape—is entirely valid.
In that case, even if we set aside motives and purposes for the moment, we need to examine—from a results-oriented perspective—whether rape was actually a behavior that increased the likelihood of reproduction. In other words, we must examine whether rape was a behavior advantageous enough for reproduction to be maintained through natural selection. However, a closer look at the statistical data presented by Thornhill reveals problems with this claim as well. According to Thornhill’s study, approximately 30% of rape victims were women who were not in their fertile period. If rape were a behavior that evolved to increase reproductive success, it would be difficult to explain why a group incapable of reproduction accounts for such a high proportion of victims.
Furthermore, cases of sexual violence targeting not only women but also men and animals have been reported in various studies and criminal cases. If the behavior were intended to increase reproductive success, it would be difficult to explain why sexual violence repeatedly occurs against targets who are incapable of reproduction in the first place. Therefore, the position that claims rape is an adaptation has the limitation of failing to adequately explain the actual distribution of victims.
On the other hand, if rape is understood as a byproduct of sexual desire, this phenomenon can be explained relatively naturally. Because the fundamental motivation of sexual desire exists, reproductive potential itself does not become a key factor in determining the behavior. In other words, since the perpetrator does not perceive the victim as a reproductive partner but rather as an object to satisfy their own sexual desires, the crime can occur regardless of the victim’s gender or fertility status.
However, while some past survey results suggested that a significant number of sex offenders suffered from mental illness, current research and clinical opinions are more cautious. While it is true that some sex offenders have mental illnesses, there is insufficient evidence to suggest that most sex crimes can be explained by mental illness alone. In fact, sexual violence is understood to result from a complex interplay of various factors, including the desire for power and control, distorted perceptions of sexuality, antisocial tendencies, impulse control issues, and sociocultural factors. Therefore, while there may be a certain correlation between rape and sexual desire, it is difficult to explain this phenomenon solely through mental illness or any single factor.
To summarize, while traditional adaptationists argued that rape itself is a naturally selected adaptation, the author views sexual desire—not rape—as the adapted mechanism. Accordingly, the discussion was developed from the perspective that rape is a byproduct of the distortion of this adapted mechanism of sexual desire. Regarding Gottschall’s statistical data, I pointed out that it compared pregnancy probabilities based solely on a single sexual encounter and attempted to refute my argument based solely on outcomes rather than the motives behind the behavior. Furthermore, even when interpreting natural selection from a results-oriented perspective, I examined the fact that Thornhill’s statistical data included a significant number of victims—such as women outside their fertile period and men—who were incapable of reproduction; thus, this data lacks sufficient explanatory power to support the view that rape is an adaptation designed to increase reproductive success.
Ultimately, the author argues that there are several limitations to explaining rape as an adaptation, and that understanding rape as a byproduct of the distortion of an adapted mechanism—sexual desire—is more appropriate for explaining certain phenomena. Of course, in today’s academic community, the view that rape is the result of a complex interplay of biological, psychological, and sociocultural factors—rather than being explained by a single evolutionary hypothesis—is more widely accepted. Therefore, to understand the causes of rape and prevent it, it is necessary to approach the issue from diverse perspectives rather than relying on a single specific theory.
