Traits of humans, who produce many fertile offspring, who then also reach reproductive age and get children will over the long run remain in the gene pool. Whose traits remain in the genepool only to a limited extent depends on characteristics that are commonly regarded as good and fitness-indicating. Only if characteristics are really associated to the probability to produce offspring before death, they can be regarded as “evolutiuonary relevant” fitness charcteristics. In the intro sequence of the satirical film “Idiocracy” described in the prologue of this book, characteristics that may commonly be regarded as rather negative were associated with a lot of offspring. Mankind dumbed down as stupid people reproduced much stronger than smart people.

Different Incentive Systems for promiscuitive Behaviour for Men and Women

As women carry out babies during pregnancies and give birth, they usually can not withdraw from the duty to parent and bring up the child. A successful reproduction strategy for women is nearly always associated with high investment of time and effort. Many children are no guarantee for continuous reproduction over several generations, if they do not reach reproductive age. The probability for children to reach reproductive age increases if the woman that has delivered the child does not have to care for the child all alone, but gets support from her husband or from other members of the society.

For men an evolutionary successful reproduction strategy does not mandatorily require big investments in time and effort. A man who impregnates numerous women is evolutionary successful if many or all children reach the reproductive age nonregarding, if the father contributed or not. Consequently, we have to assume that we all originate from more different female than male individuals. In harem cultures, with one man having several women, the ratio of female to male ancestors should be particulary high. Even in monogamous cultures, reproductive success concentrates on fewer men than women, be it due to reproduction when being unfaithful or during sequences of serial monogamy. In principle, after having impregnated a woman, a man can immediately move on to the next woman. Once a woman is pregnant, the next pregnancy is not possible before nine months later, in reality usually 2-3 years later, when the previous child has overcome the most critical period of life. Some indigenous people, who live as hunter-gatherers in modern times practice infanticid after twin or triplet births or if the previous child is still too young to be weaned (40).

Mechanisms of sexual Attraction

Freely modified after a non-published draft from Hagen Frickmann

Sexual activity belongs to behavioural repertoire of humans and its absence is considered to be of pathological significance. In women the phenomenon of so-called alibidemia has been described to occur more frequently than in men (41). Alibidemia gained attention in sexual sciences, especially in the second half of the twentieth century. The department for sexual science research of the University of Hamburg described an increase of patients with the diagnosis sexual inappetency from 10% to 60% in women and from 5% to 15% in men since the mid-1970s until the beginning of the 1990s (42). If this increase reflects a real increase of alibidemia in the population, or if people with alibidemia more often perceive it as a disease for which to seak medical care, is hard to tell from these data. For the reproductive success, alibidemia is only relevant if it is a primary alibidemia (exists from the beginning) that prevents sexual acts that would have otherwise led to pregnancy. Alibidemia in a postmenopausal woman can induce individual suffering and hamper the partnership, however it has not influence on the reproductive perspective anymore.

When talking about sexual inappetency we distinguish primary from secondary alibidemia (41, 43). Primary alibedemia means that an individual does never and towards no human develop sexual desires. More frequent are secondary alibidemia, which means that a previously existing sexual interest vanishes. Apart from the normal decline of sexual desire for the always same partner over time, secondary inappetency can also be induced by conflicts in a partnership, health problems or general insatisfaction with regards to professional or private life. Already in the 1970s, monotonia, boredom, weariness and disinterest within monogameous relations were recognized as important determinants of sexual alibidemia (44). Volkmar Sigusch, a representative of the critical sexual sciences, views an unconscious fetish or a “small perversion” as essential for a couple to keep the sexual appetency alive. He sustained his position with the bonmot “Long love relationships without a fetish start with orgasms and and end with friendship kisses” (45). To what extent, fetishes played a role for sexual stimulation in prehistoric ancient cultures, is something we can only divine. The focus of the following effectuation lies in the roundup of the neurobiological and neurochemical fundaments of sexual affinity, supplemented by sociocultural views an evolutionary view.

Neurobiological Background

Sexual activity is a phylogenetically old programme, so that underlying neurobiological mechanisms can be studied on the non-human mammal-brain. Certainly, data from animal experiments only provide models, but they set the frame for understanding the neuronal processes that control human sexuality. After the extinction of the dinosaurs 66 million years ago, the age of the mammals dawned. The ancestors of the monkeys were similar to rodents, so animal models with rodents are somewhat plausible. The Diencephalon, which is a relatively old brain structure from an evolutionary development point of view, contains cyclic oszillatiors generating excitations and activity. We call this internal or endogenous excitement. A prominent example of such oscillators is the so-called Nucleus suprachiasmaticus, that keeps up our day-night-rhythms with waking and sleep periods (46).

Exogenous excitating stimuli are also conveyed by our sense organs. The excitation generated by the oscillators and the sense organs is transmitted via complex synaptic connections and modified in an inhibitory or excitatory way. The cortex is a relatively new brain structure from an evolutionary development point of view and is regarded as the part of the brain, which contains “higher brain functions”.  The neuronal cells of our cortex (Telencephalon) act mainly by giving inhibitory signals. Excitations are thus modified and dimmed down by the cortex and the resulting patterns of electrochemical activities determine our conscious impressions and our interaction with the environment

Examples of Neurobiological Mechanisms that underly Sexuality

Different neurobiological mechanisms connected to sexual behaviour are regarded as well characterised by now. Following are three examples. Example 1 describes the control of the mechanical component of the copulation act. Example 2 describes the intrinsic motivation for sexual activities. Example 3 explains the neurochemical basics of sexual affinity along the sexual meaning of olfactory stimuli.

Example 1: Control of the Mechanical Components of the Copulation Act

The medial pre-optic region, the ventro-medial pre-optic region and the ventro-medial nucleus of the Hypothalamus

The Hypothalamus lies in the Diencephalon and right in front of it towards the face in the medial pre-optic region lies the “Sexual Dimorphial Nucleus” (SDN). The SDN controls the movements of the copulation act in male mammals. It contains an abundancy of testosterone receptors and measures double the size compared to the SDN in female mammals (47).

For controlling the copulation movements in female rats, for example the lordosis posture (bent spine with hind quarters lifted up) that facilitates the entrance of the male penis, the ventro-medial nucleus of the Hypothalamus is important. These regions are larger in females compared to males and female sexual hormons such as estradiol and progesterone are more abundant in female than in male mammals (47).

Example 2: Assuring the intrinsic Motivation for Sexual Activity

The medial forebrain and the Nucleus accumbens

For enhancing evolutionary successful patterns of behaviour, the mammal brain contains intrinsic reward mechanisms in the brain stem and in the limbic system, which for example make an orgasm a pleasurable experience. Given the oportunity to trigger this intrinsic reward system electrically or pharmacologically, mammals or humans tend to make use of this trigger excessively until complete exhaustion. Stimulation with non-natural triggers, such as drugs can lead to dependency and addiction. Dopamine is an important element of the positive enhancement system of the mammal brain. Dopamine transmits pleasure sensations unspecifically in the sexual act as when consuming damaging drugs. Dopamine-agonists, such as amphetamine and cocaine are very strong drugs with a high potential for addiction. Endogenous (produced by the brain itself) just as exogenous opioids stimulate dopamine-releasing neuronal cells that convey the addictive effect of these drugs (47, 48).

Example 3: Importance of olfactory Senses for Sexual Affinity

The olfactory pathway influences directly the affective centres of the brain, which is also of importance for sexuality (49). The release of dopamine, which triggers pleasurable sexual reactions, can be triggered by external stimuli. Sexual reactions, for example are favoured by symmetrical, evenly shaped faces (average faces). Partners with a rather non-similar immune-system are prefered. Here the olfactory sense controls the choice: The genetically based inherent smell of each human leads to a distinction between intrinsic like smell and different smell that is conveyed via the Major Histocompatibility Complexes (MHC). This influences the partner choice and acts as incest-barrier as high genetic relatedness is associated with a higher rate of stillbirths (49).

Unconsciously our sexual responsiveness is controlled via pheromones, scents that act on the hormones. Androstenone, a scent component of male underarm perspiration is usually perceived as unpleasant, but as pleasant to women in the cycle phase of ovulation. This scent also synchronises the female cycle. The scent of female underarm perspiration and secretion can modify heart and respiratory rates in sleeping men and influence dream content in a positive way (49). Anecdotically, after wars with depletion of men, women were applying their vaginal secretions behind their ear to attract men, when going to social events.

However, one should not overestimate the importance of olfactory information in humans. Anosmia, the genetic inability to discern components of smell is very common in humans. Such “olfactory blindness phenomena reach from 2% for isovaleric-acid, the main component of sweat to 40$ for the abovementioned androstenone. Effective functioning of human reproduction, in spite of these genetic limitations, indicate that the importance of olfactory information in humans can at least not be critical.

Hormons

The rewarding effect of sexual interaction is also conveyed by hormons such as oxytocine, vasopressine and prolactine. Oxytocine in combination with androgens favours reproductive bahaviour, in combination with opiods desire for body proximity (47). The influence of sexual hormones on sexual development already starts in the mother’s womb. During the first weeks of pregnancy the development is still bisexual or sex-indifferent. Without male sexual hormones, so-called androgens, the developing organism remains female. If the sexual chromosomes are in the XY-constellation, testicles start to grow, beginning from the 6.-7^th week of pregnancy, and androgen production triggers the masculinisation. Masculinisation goes along with the development of neuronal structures in body and brain that control male sexual behaviour; at the same time the development of neuronal structures for female sexual behaviour is inhibited (defiminization). Before and immediately after birth, androgens, such as testosterone have an organising effect, in puberty testosterone acts activating. The organising effect of androgens is especially important for the sexual orientation. Psychological influences and upbringing do not play a role (or a marginal role) for primary homosexuality of man or woman. The impact of hormons during pregnancy seems to be much more important. Even in late pregnancy, for example an unusual increase of androgens produced by the adrenal glands, can lead to androgenisation of the female foetus. The androgenisation of the developing brain leads to defiminization of partner choice criteria and with increasing masculinisation, the probability for a homosexual orientation of the woman on female sexual partners increases. In male homosexuality we assume a reduced masculinisation with weakened defiminization. Strong psychological stress of the mother during pregnancy can lead to reduced masculinisation. Reduced testosterone levels in middle and late pregnancy can influence sexual orientation, although testosterone levels of heterosexual men are not different from homosexual men (47). Sexual appetency can be attenuated by hormones. The synthetic hormone cyproteronacetate strongly attenuates male sexual appetence and works like a chemical castration (50).

Sociocultural Reasons for Restrictive Sexuality

Humans have an excellent physiological configuration for pleasurable sexual experiences. Easy-accessible contraception have uncoupled such sexual experiences from reproduction (wanted or unwanted) and even the risk for sexual infectious diseases can be minimised and controlled. Nevertheless, hardly any human being lives up to the physiological possibilities, when it comes to a fulfilled and pleasurable sex life. According to a representative study in the sexually active population from 2017, only 2% of all interrogated persons had an open relationship and around 1% lived relationships with involvement of a third person, while serial monogamy with or without bits on the side were by far the most common form of sexual arrangement (51).

The uncoupling of sexuality from reproduction would principally allow for a society, in which consensual copulation could become a form of amusement that is regarded not much different than a non-commital meeting for a coffee or going to the cinema together. The reality in our society keeps sexual contacts a taboo, making them unattainable for many humans.

Presumably, our sexual behaviour is being dominated by our long developmental history, which played out most of the time without contraception. Sexuality and reproduction were not uncoupled and the survival perspectives for children of isolated mothers were rather bad. Unconsciously, evolutionoary factors determine the choice of partners and the motivation for copulation, partly hidden behind romantic feeling and individual vanity. Non-selective heterosexual copulations are rare and sexual interactions are embedded into complex personal relations.

Proponents of a free(er) sexual culture see interindividual fidelity claims as colonization and appropriation desires for the heterosexual partner, which are pursued jealously (52). Behaviour that obligates the sexual partner is plausible in evolutionary history, especially for women. In case of a pregnancy (contraception is available for less than a hundred years) the child was at disadvantage, if the woman had to raise the child all by herself. A highly promiscuous, non-selective, sexual pleasure-driven sexual behaviour was coupled with high risks for the woman and her offspring for most of evolutionary history leading to negative selection of such behaviour.

Women are less active when it comes to initiating sexual activities and probably also less “needy”. The demand for compensation through commercially offered sex is high. According to the “Statistische Bundesamt” (German bureau of statistics), the annual turnover of German sex workers, actually mainly female sex workers, in 2016 was around 15 billion Euros. Main clients are heterosexual men, of whom every tenth makes use of commercial sexual services at least once in a lifetime (45).

The affirmative sexual medicine  postulates a so-called Libido-coefficient for quantifying a smaller female motivation for sexual activities compared to men and a higher sexual tension in men (53). However this position is being disputed (45).

A liberal, non-commercial sexual culture beyond canonical codizes seem mainly be a domain of the MSM-scene (Men, who have sex with men), thus a domain of a non-generative sexuality that remains evolutionary ineffective.

In permanent relationships, the frequency of sexual interactions decreases over time and this decrease of interest sets in quicker in women than in men. This brings about a real problem for men, whose sexual desires are hardly compatible with those of women. This leads to a supply shortage on the sexual market on the female side and brings women (especially in their young and reproductive years) into a strong negotiation position.

While men (knowing about the relatively feeble female interest and in spite of pathetic prospects of success) tend to offer their sexuality relaitively inspecific, women can choose from a broad supply. This oversupply of men with regards to (young) women is also visible in traditional lonely heart columns and even more in internet-based partner-agencies. While men send out hundreds of requests for getting maybe a single answer, attractive and young women are easily overwhelmed by the number of requests. The evolutionary and economic advantage of young, attractive (fertile) women on the sexual market is obvious.

In the belletristic literature the submission of heterosexual relations under the capitalistic (evolutionary) laws of the market with supply and demand  was taken up by the French author Michel Houellebecq in his novels „Extension de la domain du lutte“ (54) and „Les Particules Elementaires“ (55) with zynic knowingness.  

According to Houellebecq, the capitalistic competitive fight between heterosexual men for sexual opportunities is being fought with the same determination as the fight for lucrative jobs. Similar to the labour market, the sexual fighting arena also produces few winners and many losers. The winners are a very small group that attracts nearly the entire interest of young fertile women, from which these winners can pick. The large majority group of the losers do not have sex or have to pay for it. This payment can be direct (e.g. prostitution) or indirect, e.g. expensive maintenance of permanent relationships. Along the slogan “neoliberal man forges his own destiny” (56), losers (who for example lack charm, looks and money) are even made feel responsible for their own failure.

The increased economic independancy of women and the propagandised western „Gentleman culture“ with a suggested ideal that ostracizes male macho-like behaviour has not led to an increase of female libertine-sexual activity or promiscuity (45). The vernacular slogan “be kind, if you want to fuck” does not really carry truth and being kind does not really pay back to the cultivated western postmodern man.

Tightening of Sexual Supply as an Economic and Evolutionary Strategy

Apart from the evolutionary explanation, tightening of sexual supply seem plausible. The resulting economic advantages of such behaviour for women are obvious, as it makes men invest into relationships that do not make economic sense for them. The tightening of sexual supply makes it hard for men to acquire sexual satisfaction outside costy relationships. For women it means that men are pushed into permanent relationship, which is advantageous from an evolutionary point of view.

If we regard the tightening of sexual supply as a female business model, defaming all those who undermine this tightening of supply makes sense. This also includes primitive name-calling of sexually generous women as “sluts”. A similarly dismissive expression for sexually open men does not come to mind. Sex workers also face discriminating-patronising behaviour from people with virtue-signaled good intentions wanting to protect them from exploitation (not denying the fact that commercial sex can also take place in exploitative constellations) (53) (57).

In 1978, Walter Bräutigam wrote that for some men, hiring a prostitute is cheaper and less time consuming than dating a women (44). The Irish dramaturge Brendan Behan coined the sarcastic bonmot: „The big difference between sex for money and sex for free is that sex for money usually costs a lot less.” It is obvious that comercial compensation possibilities undermine the strong evolutionary female position on the sexual market. Prohibition measures against prostitution are established in numerous would-be liberal countries such as France and Sweden, where suitors are prosecuted and in countries, where prostitution is widespread such in Russia, where prostitutes are prosecuted.

Density stress endangering the Eros?

The increase of alibidemia described above (42) seems to be a widespread and international phenomenon. With regards to global overpopulation and the contemporary discussion about anthropogen distructions of our habitats (58) the following passage reflects alibidemia observations made on rats and mice when exposed to increasing population density. Depending on the overall size of the habitat and the available natural ressources, rat populations can grow uninhibited and exponentially over years resulting in population overcrowding. Once exceeding a certain density level, social stress arises. Due to this stress, the female rats become infertile and bite away approaching males. Reproduction wanes. Fighting for the remaining ressources, the animals attack each other presumably triggered by infavourable environmental conditions, physical and psychological stress. In extreme cases the animals kill each other. Gradually the population declines and ultimately collapses. In a natural environment, the animals restart a new population (59).

Corresponding experimental settings were created in the late 1960’s and became known as  “Mouse Utopia” or “Rat Utopia”. Starting with small populations in a setting with food supply always guaranteed the exponential population growth soon led to overcrowding.  Overcrowding with density stress was leading to population collaps, although the food supply was always secured (60, 61). Female mice neglected their offspring so that they died and aggressively fended off mating approaches of male mice.

But also, male animals lost their interest in females or indiscriminately mated with any close animal. Although, there was no competition for food, the animals attacked each other extremly aggressively. Some animals entirely withdrew from the rodent society, stayed alone in higher boxes of the facility and concentrated on sleeping, eating and grroming (body and fure care). As these animals remained free of scratches and biting-wounds or scars and were well nourished with an immaculate shiny fur, the researchers named them “the beautiful ones”. These “beautiful ones” had suspended all social interaction with other animals; they did not show any signs of sexual interest, anymore. After the rodent population had collapsed, reproduction did not start again and the populations went entirely extinct.

Translating observations made on animals to human societies is surely a bit problematic, Nevertheless, the question, if density stress also affects human sexual behaviour is a legitimate one. Japanese cities are extremely well organized and food supply is always secured, but they have a very high population density – in a way like in a human analogon to the rodent utopia scenarios. Actually, the dwindling sexual interest in both sexes in Japan is already well confirmed (62). The model of reproduction abstinence seems milder than the mutual biting to death, that you could observe in rats and mice. Maybe the dwindling desire to reproduce is just reasonable regulatory mechanism. Exponential population growth will reach limits and affect culture, plague dynamics, and competition for resources with the possibility of war, violence and environmental destruction (13).

Certainly, there is no proof for the dwindling sexual desires in modern big city societies being due to density stress. In western cities like Hamburg, where the above mentioned data were gathered and which compared to Tokio is small and not very densely populated, a good quality of life seems possible (42). Nevertheless, psychological abnormalities, such as affective disorders are increasing. In poor countries, birth rates are usually higher. Universal access to contraception, sexual education and self-determination can decrease birth rates. Iran demonstrated that population growth can be controlled without mandatory measures. Here, fertility could be lowered from 6.5 deliveries per woman in 1983 to 1.7 nowadays. This was achieved by individual, voluntary, easily accessible family planning support offers and good school education for boys and girls. The population was controlled by an education-explosion.

The density-stress hypothesis surely is not excessively researched, yet. Maybe it is the guaranteed provision of primary needs (food) that leads to a dwindling sexual desire? How would a rodent utopia with induced shortage conditions play out? Would sexual reproduction and collapse of society and population take place just like in the rodent utopias with enough food? Maybe it is the affluency that triggers alibidemia under density stress.