Sex among humans is very different than it is among other species, although tree-dwelling species (anthropoid apes and monkey share some of the differences. Jared Diamond started his book with his idea of a dog’s view of human sexuality:

If your dog had a brain, and could speak . .  .

“If your dog had your brain and could speak, and if you asked it what it thought of your sex life, you might be surprised by its response.  It would be something like: ‘Those disgusting humans have sex any day of the month!  Barbara proposes sex even when she knows perfectly well that she isn’t fertile – like just after her period.  John is eager for sex all the time, without caring whether his efforts could result in a baby or not.  But if you want to hear something really gross – Barbara and John kept on having sex while she was pregnant!  That’s as bad as all the times when John’s parents come for a visit, and I can hear them too having sex, although John’s mother went through this thing they call menopause years ago.  Now she can’t have babies anymore, but she still wants sex, and John’s father obliges her.  What a waste of effort!  Here’s the weirdest thing of all: Barbara and John, and John’s parents, close the bedroom door and have sex in private, instead of doing it in front of their friends like any self-respecting dog!”

All sexual species, except humans, engage in sexual activity order to reproduce, i.e. to have babies. Pleasure is not necessary, and the process may be violent. Females typically signal their readiness (ovulation) in some way that triggers a response by males. The signal is usually chemical, an odor. Among insects and arachnids, the females and males may not actually have physical contact. The female lays  her eggs and the male fertilizes them outside her body. (Sometimes the female eats the male.)

Among mammals physical contact is necessary. The procreation mechanism consists of a chemical signal in the form of an odor emitted by the female when she is ovulating. The male, receiving this signal, prepares to impregnate the female, by mounting her from the rear, and ejaculating sperm into her vagina. The whole process is instinctive, quick and sometimes violent. The female usually has no choice among competing males. The males fight for priority. The winner of the battle becomes the next progenitor, based on his age, strength, health and perhaps experience. In many species the losing males are ejected from the group, leaving only one dominant male and a herd. Females are more inclined to stay together, for protection of the young. Only the females of the largest fiercest species, such as big cats and bears, can afford to live and protect their young, alone i the wild, without a group (pack or herd) for support.

For evolutionary reasons (involving communication) the anthropoid apes have evolved larger brains, requiring larger skulls. They also started walking on two legs. Animals standing upright on two legs have another option for copulation, namely penetration from the front. This gives the females much more ability to resist, if she wants to, and thus more choice of mate. As brain and skull size increased during the last several million years, without comparable increase in body size, it became increasingly  necessary for the mother to give birth before the fetus is full-grown and self-sufficient. This, in turn, has forced the parents to care for their young outside the womb for an increasingly long period of time.

This necessity for prolonged care and nurture outside the womb probably explains why anthropoid apes, and especially humans, have sexual activity independent of ovulation, and even after menopause. Moreover, the sexual activity among humans tends to be monogamous, i.e. confined to one couple. This is strongly supported in most societies by the institution of marriage and the creation of taboos against sex outside of marriage.

However, the tendency toward monogamy has a natural basis. It arises from the biochemistry of interaction between males and females. The initial selection of possible sex-partner is made on the basis of a combination of physical, aesthetic, and social criteria: appearance, movement, voice, conversation, culture. The next stage is physical contact, such as touching, hand-holding, dancing, kissing, or massage. During this stage there is an exchange of pheromones, by inhalation or kissing. The exchange of pheromones may then increase the sensitivity of lips, tongue, nipples and genitalia to touching. The increased sensitivity may stimulate more touching and, ultimately, intercourse.

Selection of a sex-partner among humans tends to be based primarily on healthy appearance, so as to maximize the chances of healthy (viable) offspring. The criteria for healthy appearance varies between cultures, since viability after birth also depends upon acceptability to the tribe or race. Thus skin color becomes a factor in selection because offspring of couples of mixed races can suffer discrimination. (Children of Korean or Japanese women with American soldiers had difficulty integrating in Korean or Japanese society). Today attributes like wide hips and large breasts – once valued as indications of easy childbirth – are no longer considered “beautiful” in Western countries where childbirth is no longer dangerous but social acceptability after birth is strongly linked to physical appearance.

It is evident that other selection criteria have become more important in recent years. Women seek males likely to be good providers. Evidence of wealth helps, as does secondary evidence of likely future success in business or professional career (education, accent, vocabulary). A related criterion is socio-economic likeness. “Birds of a feather flock together”. Unfortunately, the most important single selection criterion for most younger women, today, is glamour and fame. Now that sex is no longer tightly restricted to marriage, it is a sad fact that large numbers of young women literally throw themselves at certain categories males, such as rock stars, movie stars and famous athletes. This may not be socially harmful in itself. But when young women choose to marry (or have children by) males based on apparent similarity to those glamorous male models, the stage is set for marital – and parental – dysfunction.

There is another selection problem that deserves consideration, namely the “lottery” phenomenon. Lotteries are enormously popular because the cost of entry is small – perhaps negligible – but the imaginary benefits of a big win are life-changing. People asked by interviewers what they would do with the money almost always say that they would pay off their debts and quit their jobs, presumable to live a life of hedonistic pleasure.  In fact, big winners typically become targets of needy relatives and former friends, not to mention promoters of dubious investments and outright frauds. Much the same motivation seems to operate among young women looking for a mate. Among the young there is a phrase “friend-zone” describing a status to avoid. The point is that friends seldom morph into lovers. The reason is, probably, that people who know each other well may become aware of each other’s weaknesses or character flaws, without seeing comparable pluses, whereas with strangers there is the outside – but exciting – possibility of a big win.  Many failed marriages probably result from selection based on ignorance – learning about problems too late.

The process of “having sex” is instinctive at the simplest level, but there is a lot of feedback and the pleasure experienced by each partner is highly dependent on the other. Hence, it can be learned and improved by practice. By the same token, it can be spoiled by inappropriate behavior, especially by the male. One of the common causes of “bad sex” is the habit of many males to reach climax too quickly, and then disengage without regard to the female’s degree of excitement.   However, once a pattern of “good sex” is achieved between partners, it tends to stimulate a desire for more sex. This is what keeps the partners in a successful relationship together. By the same token “bad sex” will drive partners apart.


In the case of Homo sapiens, during the tens of thousands of years before written records, the mother probably carried the baby in her arms or in a basket of some sort. For two years she probably fed the baby with milk from her breasts, while the child’s teeth were emerging, before it could masticate solid food. Within that period most females would have become pregnant again, so the prehistoric human female was totally consumed by child-care and local food gathering. This continued from puberty to menopause. In the prehistoric “state of nature”, few females (or males) lived that long.

The circumstances strongly supported the existence of communities, where a number of females and their children can live together, both for better protection and to enable some sharing of tasks. Naturally the males (typically being larger and stronger) took over the job of protecting the female child-bearers and providing food by hunting. (Females and children did some local gathering of nuts, berries and edible leaves and tubers.) Since protection of the females and the young is a paramount requirement for survival of the group, the young human males were not ejected from the community because they were needed for protection and for hunting. Hunting tends to be a group activity unless or until the hunters have weapons (such as a bow and arrows) capable of killing reliably at a distance.

However mammalian females are only “ready” for impregnation during a few days of “ovulation” in each monthly cycle. The rest of the time, the females are less interested in sexual activity or not interested at all. But young males after puberty tend to be interested all of the time. Any female may be impregnated by several males, willingly or not, unless there is a social structure to prevent it. Human behavior and social structures have been strongly determined by these facts.

The social system that has been most widely adopted is the monogamous “nuclear family”, headed by a single dominant male with one mate. (The family structure in some human societies – notably Muslims — allows one dominant male to have several females, but then the surplus males need to be gelded, like the Turkish Janissaries, otherwise disposed of.) A family may encompass several generations, all living together.

After agriculture developed, around 10,000 years ago, tribes became localized and families gradually became land-owners. Land was the main source of wealth. It was necessarily allocated among land-owners to prevent constant conflict over boundaries. But when all the arable land is allocated, either the farms get smaller and smaller as time goes on – eventually too small to feed a family (this happened in India) – or the farm is kept intact by passing it to the eldest son.  This pattern, called primogeniture, leaves the younger sons and all of the daughters without property. The landless boys may became soldiers,  share-croppers, camel drivers, bandits or (when cities grew larger) factory workers. This was the beginning of the global two class system consisting of upper class of male “owners” – the aristocracy – and lower class “commoners” or “proletariat” consisting of everybody else.

In most societies around the world girls and women became disposable, thanks to the tradition of primogeniture and land-ownership restricted to males. A boy child in India was desirable, if only as free labor, whereas a girl had to be “married off” accompanied by a dowry. Thus boys were financial assets to the parents and girls were financial liabilities. No wonder large numbers of girl babies were abandoned and allowed to die, or were sold into slavery as prostitutes. Even in the top layers of society, girls were pawns, “owned” by their fathers” with no personal rights. It was a man’s world, with a male emperor or a king on the top of the social pyramid with a small group of male aristocrats owning all of the property, making and enforcing all of the laws in their own interest.


 Fast forward.

The “enlightenment” starting in the 17th century, Europe,  followed by the industrial revolution in the 18th century, changed many things. For the first time in history aggregate wealth began to increase rapidly in those countries, thanks to the exploitation fossil fuels after the invention of coking, and the invention of machines to do work that had previously been done by human or animal muscles. One consequence was that human muscular strength became gradually less and less important while communication skills became increasingly important. Weapons also became more powerful. This gave the industrial countries with advanced weapons power over the less developed countries of the world. Colonialism followed.

Increasingly wealth in the 17th and 18th centuries in Europe was based more on manufacturing and trade than on land ownership. Thus a new class of people with specialized knowledge and skills evolved, the “middle class” or bourgeoisie. This group emerged between the workers who tilled the land and harvested the crops and the “upper class” who still owned the land and the weapons, but not the knowledge or skills needed to keep the system going. Universities have a fairly long history, but public education (for boys) began in the 19th century, to supply that knowledge and those skills. Girls also began to get educated and to find employment outside the home (usually as servants, but sometimes as teachers or nurses). Only in the 19th century did women get the right to own property.  Only in the 20th century did women get the right to vote and to divorce their husbands.

As knowledge spread starting in the 14th century, the “divine right” of kings, based on titular inheritance, was increasingly challenged by intellectuals and “philosophes”. Traditional rule by inherited “divine right” ended in Europe between the mid-seventeenth and mid-nineteenth centuries and in most of the world since then. It was replaced by elected governments of one kind or another. Meanwhile the notion of “human rights” has spread widely since the creation of the United States of America and its constitutional “Bill of Rights”. The end of slavery in the early 19th century was the start, though the civil rights movement is still far from final victory.

For most of human history, human females have used sexual attraction as a means of influencing males in the mating game.  Men, usually fathers, always had the last word and made the final choice. That is still true in much of the world, but less and less in the “advanced” countries. Today, in the most advanced (most powerful) countries, it has been widely accepted in principle that women deserve and should have equal rights with men in every domain. That includes choice of mates.

This equality has not yet been achieved in practice, but it is coming. Lacking physical strength or legal power, women have always used sexual attraction – enhanced in a variety of ways – as a weapon in the contest between males and females. In the past, some legendary beauties – such as Helen of Troy and Jeanne d’Arc — changed history. Some women, such as Queen Elizabeth the first of England and Catherine the Great of Russia, inherited the rule of a nation, by virtue of the established inheritance laws. But the overall dominance of men has not changed significantly until the last quarter century.

Since the middle of the twentieth century, thanks to “birth control” and “the pill”—and changes in the laws — most younger women now have effective control over their bodies, for the first time in history. They cannot be forced to bear children they do not want. They can – and many do — refuse to have children at all. They can, and do, copulate with a number of sex-partners, without any implication of long-term commitment. They can, and do, break up a marriage as easily as a man. They also compete in virtually every field of activity, including the military forces. This fact changes the fundamental relationship between men and women. It is no longer a man’s world, but most men have yet to recognize this fact.

Women still use various means to enhance sexual attraction, during the “selection” phase, which is mainly based on appearance. The weapons include “sexy” clothes, perfume, “beauty” products, elaborate coiffures — even plastic surgery (“botox?”) in the competition for mates. Contrary to the legends of “love at first sight”, sexual attraction between humans – after the initial selection — is based on bio-chemical interactions that accompany physical contact.

The question arises: Are men still necessary? Will boys in the distant future have any useful function in a world where machines and computers do all the work, where eggs and sperm can be stored, genetically modified, where physical impregnation is not necessary and lesbian relationships proliferate? Will surplus boys be treated the way girls were treated in the past? I wonder.

  * Illustration. Battle of the Amazons, Anselm Feuerbach

Museum: Staatstheater Nürnber

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About the author:

robert-ayresRobert U. Ayres is a physicist and economist, currently Novartis professor emeritus of economics, political science and technology management at INSEAD. He is also Institute Scholar at the International Institute for Applied Systems Analysis (IIASA) in Austria, and a King’s Professor in Sweden.   He has previously taught at Carnegie-Mellon University, and as a visiting Professor at Chalmers Institute of Technology. He is noted for his work on technological forecasting, life cycle assessment, mass-balance accounting, energy efficiency and the role of thermodynamics in economic growth. He originated the concept of “industrial metabolism”, known today as “industrial ecology” with its own journal. He has conducted pioneering studies of materials/energy flows in the global economy. Ayres is author or co-author of 21 books and more than 200 journal articles and book chapters.  The most recent books are Energy, Complexity and Wealth Maximization (Springer, 2016), The Bubble Economy (MIT Press, 2014)  “Crossing the Energy Divide” with Edward Ayres (Wharton Press, 2010) and The Economic Growth Engine with Benjamin Warr (Edward Elgar, 2009).

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