Exploring Human Sexual Diversity: Is It an Evolutionary Outcome and What Is the Anthropology of Sex?

Human sexuality is diverse. Many other animals engage in sex only during fixed reproductive cycles, but humans do so year-round—or not at all. Moreover, the purpose is not necessarily reproduction. The partner is not always of the opposite sex; sometimes, it may not even be another human.

Because it is an act usually hidden in daily life and not typically shared with others, the ways in which it is practiced vary from person to person. Looking at human society as a whole, there are also diverse ways shaped by distinct cultures.

Focusing on this diversity of sexuality in human societies, the anthropology of sex seeks to document and analyze how people engage in sexual practices, capturing even the nuances of lived experience through fieldwork. However, according to anthropologist Katsumi Okuno—professor at the College of Intercultural Communication at Rikkyo University and author of “Lessons on Sex Taught by an Anthropologist” (Hayakawa Shinsho), published in December 2025—his approach goes beyond that.

The book introduces the wide variety of sexual practices that anthropologists have encountered through fieldwork around the world: societies where a pregnant woman has sex with men other than her husband, who then take on responsibilities as a “second father” to the child; societies that believe non-reproductive sex brings misfortune; and societies where boys experience homosexuality or bisexuality as part of initiation rites. However, Okuno says that merely listing these diverse practices like a catalog only tells half the story of the anthropology of sex.

“Humans have developed cultures in diverse ways depending on their environments, creating various forms of sexual practices. However, if we trace things back, humans are not the only ones that engage in sexual activity—animals and even plants do as well. By going back 1.5 billion years to when sex first emerged and examining its history, we can gain rich insights into understanding our own sexual practices,” Okuno explains.

In “Lessons on Sex Taught by an Anthropologist”, sex is explored through a vertical axis that traces its history within biological evolution from the past to the present, and a horizontal axis that compares the various sexual cultures practiced across different regions of the world.

Before the emergence of sexual reproduction, life forms reproduced asexually, copying their own genes to pass on to the next generation. However, since all individuals had identical genes, a change in the environment or a viral attack could wipe out the entire species. In contrast, individuals produced through sexual reproduction inherit half of their genes from each parent, resulting in unique genetic compositions for each individual. This diversity means that even if the environment changes, some individuals can survive. It is said that sex may have emerged as a result of such evolutionary processes.

Reproduction through sexual means was advantageous for evolution because it allowed organisms to better adapt to their environment. Living beings engaged in various sexual behaviors to ensure the survival and prosperity of their species. In primates, for example, a pattern evolved where males court females, females accept, and this drives the evolution of larger male bodies over time.

Typically, males are stimulated by female sexual signals, such as reddening of the buttocks during estrus, but Okuno explains that in species closer to humans, these signals gradually disappear. The difficulty in detecting estrus leads to a certain change.

“All of this is hypothetical, but when the pattern of females showing estrus signals and males responding sexually breaks down, males must look for other cues. As a result, the target of sexual excitement no longer has to be a female, and in gorillas and orangutans, sexual partners often cross conventional boundaries of sex.

The disappearance of estrus signals also provides the basis for fetishes. Sexual arousal begins to focus on subtle differences, such as gestures, voice, or specific body parts. In humans, the features that trigger sexual attraction are highly varied—voice, gestures, or parts of the body like the nape, chest, or legs; even clothing, underwear, or objects like a bicycle saddle can become objects of sexual interest.

By understanding humans as biological beings, we can see human sexual behavior as part of a larger evolutionary trajectory,” Okuno explains.

Through evolution, humans lost obvious estrus signals, which allowed sexual behavior to develop greater diversity. From there, sex further branched into various forms influenced by culture and environment. According to Okuno, sexual activity broadly serves three main functions: ① reproduction, ② pleasure, and ③ social regulation.

“For example, bonobos, a species of large primates, are said to exhibit all three functions. They are highly sexual and engage in sex without limiting partners by sex, engaging in multi-directional sexual behavior. Sex is used not just for reproduction but also for pleasure and as a tool to resolve social conflicts.

Their sexual acts are not as intense as human intercourse, but include touching the partner’s genitals, males rubbing their testicles together—called ‘rump rubbing’—and females rubbing their genitals together in what is called ‘GG rubbing.’

Through these behaviors, they release tension, express affection, or even greet each other, showing that sex functions as a social regulator. With these three purposes as a foundation, human sexual behavior becomes even more complex depending on environmental and cultural contexts,” Okuno explains.

One example highlighted in the book is the Sambia people of highland New Guinea. The Sambia were studied by American anthropologist Gilbert Herdt, who conducted fieldwork starting in 1947 and introduced them in his book Ritualized Homosexuality (translated as Culture of Homosexuality). To avoid misrepresentation of their striking culture, Herdt used a pseudonym.

Among the Sambia, males are homosexual in boyhood, bisexual after marrying women as young adults, and later heterosexual as fathers, living multiple sexualities over the course of their lives.

“In their culture, although biologically male, people are considered to be born female. ‘Masculinity’ is acquired by the infusion of semen, and from boyhood to adulthood, all males engage in homosexual acts with older men to receive this semen.

The Sambia are in constant conflict with other tribes, and warriors are expected to be masculine. This system, which ensures the acquisition of masculinity, is likely the basis for what is called ‘ritualized homosexuality.’ In other words, their culture developed this sexual system as an adaptation to an environment where warfare was frequent,” Okuno explains.

Human sexual behavior has evolved in remarkably diverse ways across regions and cultures. Yet the world is by no means fully tolerant of this diversity. For example, in cultures that practice FGM (female genital mutilation), Western feminists often denounce it as control over women’s sexuality and a violation of human rights, calling for its abolition. At the same time, women from countries in Africa and elsewhere where FGM is practiced have pushed back, saying, “We don’t want our culture criticized.” On a more familiar level, prejudice against LGBTQ individuals remains deeply entrenched in many places.

“Things that we consider not normal are often labeled as abnormal and are rejected. There is also a kind of phobia or aversion toward things like LGBTQ identities. This stems from being trapped by preconceived notions. By studying the anthropology of sex, one can become aware of what they are bound by, and at some point realize, ‘Ah, that way of expressing sexuality exists too,’ and come to accept it.

By learning about the diverse sexual practices that humanity has developed, we can understand that our own sexual behavior is just one among many. What we may think is abnormal—such as the practices of the Sambia—is also valid. In other words, it is important to flip your perspective, recognize your own biases, and see that there are other legitimate ways of being,” Okuno explains.

Conversely, the Sambia people do not understand terms like gay or lesbian In our society, these words refer to the sexual orientation of specific individuals, but for the Sambia, such experiences are a temporary phase in everyone’s life, so the concept is meaningless to them.

Indeed, when we flip our perspective, we realize that even our own sexual norms can appear abnormal.