Body odour and sexual attraction

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Odour is sensory stimulation of the olfactory membrane of the nose [1] by a group of molecules. [2] Certain body odours are connected to human sexual attraction. [3] [4] Humans can make use of body odour subconsciously to identify whether a potential mate will pass on favourable traits to their offspring. Body odour may provide significant cues about the genetic quality, health [5] and reproductive success of a potential mate. [6] [7]

Contents

Body odour affects sexual attraction in a number of ways including through human biology, the menstrual cycle and fluctuating asymmetry. The olfactory membrane plays a role in smelling and subconsciously assessing another human's pheromones. [8] It also affects the sexual attraction of insects and mammals. The major histocompatibility complex genes are important for the immune system, and appear to play a role in sexual attraction via body odour. Studies have shown that body odor is strongly connected with attraction in heterosexual females. The women in one study ranked body odor as more important for attraction than "looks". Humans may not simply depend on visual and verbal senses to be attracted to a possible partner/mate. [9]

Olfactory epithelium

The olfactory epithelium is a thick yellow/brown structure, about one inch square, located in the upper nasal cavity of the human nose. Made up of olfactory receptors and glands, the epithelium is used as a tool to smell others' body odor and pheromones. [10] Chemicals that produce odour pass through the olfactory epithelium to the olfactory bulbs, which contain biological receptors that detect the chemicals, and respond with an electrical signal transmitted to the brain by the olfactory nerves. [11] The olfactory epithelium plays a large role in why humans are attracted to persons biologically rather than physically; this relates directly to the sense of smell and not physical appearance. Olfactory communication is common in all animals and recent studies have shown that humans have this communication trait as well. This kind of communication happens subconsciously, and often influences a person's attraction to another. [8]

Pheromones

Pheromones are chemical messengers produced and emitted by the body that contribute significantly to interpersonal attraction. [8] The two types of pheromones include signal and primer, each playing a distinct role in human behavior. Signal pheromones act as attractants and repellents; they are classified as short-term behavioral pheromones. Primer pheromones produce long term changes in human behavior and hormone production. The vomeronasal organ is used to detect the pheromones of others. Pheromones emitted from sweat glands play a role in sexual attraction, sexual repulsion, mother–infant bonding, and menstrual cycles. [8]

Biology

There is a relationship between various aspects of human biology and genetics with sexual attraction. This includes the role of the major histocompatibility complex (MHC), the human leukocyte antigen (HLA) and their different heterozygotic variations. Such genetic factors may play a role in sexual selection. [12] Signalling odours in reproduction are called attractants; their function is to bring about successful mating. [2]

The MHC (major histocompatibility complex) is a group of genes essential for the immune system, playing an important role in immunological recognition. [13] These olfactory cues are involved in mate choice and preferences. HLA refers to the human form of MHC, [13] and is a gene complex which encodes the MHC.

MHC

There is a correlation between mate choice, odour preference and genetic similarity at the MHC. [13] Unique body smells are heavily influenced by MHC; these olfactory cues are probably involved in mate choice and preferences. [14]

The Major Histocompatibility Complex (MHC) shown presenting peptides to the immunological defence cells Antigen presentation.jpg
The Major Histocompatibility Complex (MHC) shown presenting peptides to the immunological defence cells

MHC is expressed codominantly; a more diverse set of MHC genes leads to a stronger immune system. [15] Women prefer male partners with differing MHC genes from themselves. An evolutionary explanation is that females are attracted to males with MHC alleles different from their own, to provide their offspring with a stronger immune system. [16]

Females not using hormonal contraceptives were more attracted to the scent of males with dissimilar MHCs. Females currently using hormonal contraceptives preferred the scent of men with MHCs similar to their own. [17] The scent of an individual with low fluctuating asymmetry is universally more attractive. [18]

The increased attraction between people of dissimilar MHCs may help to prevent incest and subsequent birth defects. [19] This inbreeding avoidance hypothesis proposes that biases towards heterozygotic alleles prevent harmful genetic consequences which can arise from mating with genetic family members. Olfactory cues can be used to recognise kin. [20]

Couples who have many failed attempts at conception share a considerably larger amount of genes than those who are able to conceive without problems. [21] If, for these genetically similar couples, there is a successful conception, the babies are often born early or underweight. [22]

A third hypothesis explaining the function of dissimilar MHC mate preferences, the parasite hypothesis, suggests that MHC heterozygotes may be resistant to rapidly evolving parasites. [16]

HLA

The human leukocyte antigen system is a protein complex which is encoded by MHC in humans. Over thirty olfactory receptor genes have been located at the HLA class I region, [23] which presents peptides from inside the cell to be destroyed by the immune system. HLA- linked olfactory receptor genes can therefore provide a possible mechanism for detecting HLA- specific odours. [24]

Androstenol, found in fresh male sweat, is attractive to women. 3a-androstenol.svg
Androstenol, found in fresh male sweat, is attractive to women.

Men and women are attracted to the pheromones they produce and HLA is related to the perceptions of other peoples' odours. [25] Males produce androstenol and androstenone. Androstenol is produced by fresh male sweat and is most attractive to women, while androstenone is produced once the sweat is exposed to oxygen and is highly unpleasant to women.

Studies have shown that women who are at the most fertile stage of their menstrual cycle prefer the smell of men that have higher testosterone levels. [26] Heterozygosity of HLA can also be detected through scent: in this case, heterozygosity confers greater ability to recognize a wider variety of antigens. Females, especially when not using hormonal contraceptives, are more attracted to the scent of males heterozygous for HLA.

Androstenone, from stale male sweat, is unattractive. Androstenone.svg
Androstenone, from stale male sweat, is unattractive.

However, the same attraction and mate preferences are not held by males for heterozygous females. Males are, however, more attracted to the scent of females with rare HLA alleles. [27] Men's arousal can be influenced by a variety of odours. For example; pumpkin pie, liquorice, doughnuts, and lavender can increase penile blood flow consequently causing arousal. [28]

Human pheromones and facial attraction

Two types of male pheromones: 5α-androst-16-en-3-one (MP1, androstenone) and 4,16-androstadien-3-one (MP2, androstadienone) and one type of female pheromone: 1,3,5(10),16-estratetrael-3-ol (FP, estratetraenol) are used as signals of mating quality. Studies [29] showed that MP2 has the strongest response produced by female vomeronasal organ (VNO), which is the first stage of the olfactory system. Sex-difference processing in the hypothalamus has been found between female and male pheromone, including in the VNO, where opposite-sex pheromones have different surface potential. [29] [30] [31]

Studies have illustrated a relationship between human facial attraction and pheromones. [32] Individuals rated those of the opposite sex as more attractive if they preferred the individual's pheromones' odour. It was also demonstrated that partners in a long-term relationship would have higher concordance between two types of signals of mating quality, vision and olfaction. In addition, no difference was found between two types of male pheromones in women's preference aside from the location of pheromones production.

Body odour and menstrual cycle changes

Women's fertility levels shift dramatically throughout the menstrual cycle, so the period surrounding ovulation is extremely important because it represents the peak period of reproductive fertility. [33] As conception is most likely to occur during a woman's brief fertile period, evolutionary theories suggest that men possess adaptations designed to maximize their reproductive success during this period. [33] Women's fertility shapes male mating behaviour, many studies have shown that being exposed to the scent of women's fertility led men to display greater implicit accessibility to mating-related concepts, males also judged the odours of women during the follicular phase as more pleasant and 'sexy' than odours during the luteal (non-ovulatory) phase. [4] [33] Olfaction, therefore, transmits information relevant to human mate selection, through which men are capable of detecting shifts in women's fertility. [4] [33]

There is now also considerable evidence from psychological studies that women's preferences for various male traits change throughout the menstrual cycle. [34] Hormonal fluctuation across the menstrual cycle explains temporal variation in women's judgment of the attractiveness of members of the opposite sex. This is due to the psychological processes that shape the formation and maintenance of human romantic relationships, which are influenced by variation in hormonal levels. [35]

The menstrual cycle highlighting the most fertile period, and the follicular and luteal phase MenstrualCycle en.svg
The menstrual cycle highlighting the most fertile period, and the follicular and luteal phase

Due to their high hormone content, oral contraceptives have the potential to alter women's partner preference for a range of male traits, which could have important consequences on sexual relationships, as it alters women's attraction to their partner and, potentially, to other men. [34] If a woman's use of oral contraceptives is congruent, meaning she was using oral contraceptives when she met her partner, her current preference will more closely match the preference that shaped her partner choice in the first place, and the desire is higher than that of a woman whose use of oral contraceptives is incongruent. [34] The resulting factor is that women's partner preferences are influenced by oral contraceptives use, meaning that attraction towards an existing partner changes over time if a woman initiates or discontinues oral contraceptive use. [34]

Studies have shown that men are more attracted to women when they are more fertile and/or on their menstrual cycle. Women gave samples of when they were fertile and less fertile. The men in the study would smell and choose which sample between the two they were more attracted to. Women give off a more favorable smell the more fertile they are; in other words, men notice this and choose the more fertile sample rather than the less fertile sample. [36]

Body odour as a cue for ovulation

In contrast to previous results reported in the literature, it has also been found that odours perceived from a female in follicular phase were actually considered more pleasant and sexier by males than odours perceived from that female in the luteal phase. Researchers discovered that the persistence of the menstrual cycle-dependent olfactory identification was extensive. However, the possibility of odours in the living environment overwhelming the menstrual cycle-dependent odours was not ruled out. [4] Repeated exposure to the specific odours increases the threshold level of odour detection and therefore reduces the stimulation in humans. [37]

Additionally, it has been proposed through research that male in a relationship would be more sensitive to their partner's onset of ovulation-linked odour, and resultantly become desensitised after repeated exposure. On the other hand, non-paired males were continuously sensitive to the odour. [4] This demonstrates the persistence of the odour as an evolutionary or adaptation process, which could ensure the offspring reproduction success of female by keeping constant exposure to males during their menstrual cycle. During this period, the most fertile females tend to have more extra-pair copulations. [4] [38] Males testosterone levels are found to be associated with a females' reproduction fertility, such that males exposed to scents of females during the peak period of their fertility, had significantly higher testosterone levels than their baseline levels. [39] Evolution suggests this is an adaptation due to the physical changes inside the male body, although more research must be conducted to ensure these high levels of testosterone lead to reproductive behaviours.

Body odour and fluctuating asymmetry

Fluctuating Asymmetry (FA) is a type of biological asymmetry, referring to the extent to which small random deviations occur from expected perfect symmetry in different populations of organisms. [40] In humans, for example, FA can be demonstrated through the unequal sizes of bilateral features such as the eyes, ears and breasts. FA acts as an index for measuring developmental instability as it provides a clear indicator of the possible environmental and genetic stressors affecting development. [41] It is thought that having a preference for a symmetrical face offers some adaptive value as such symmetry may signal an individual's ability to cope with environmental challenges. [42] FA shares an inverse relationship with certain desired traits; a low FA is correlated with higher stress tolerance, larger body size in males, smaller body size in females, and higher facial attractiveness. [43] FA is detectable through the olfactory senses and it has a measurable effect on sexual attraction. [27] Significant cues may be found through body odour relating to a potential mate's health, reproductive status and genetic quality and FA is one such cue as it is considered to be a marker of genetic and developmental stability. [44]

During their fertile phase, females have repeatedly been shown as being more attracted to the body odours of more symmetrical males [45] and of males whose faces they rate as attractive. [46] The scent of symmetrical males provides an honest indicator of the male's phenotypic and genetic quality. [45] This may explain why women who are highly fertile find the scent of low FA as attractive and yet this scent is not necessarily as attractive to other women. [27] For example, it has been found that normally cycling women near their peak fertility tended to prefer the odour of shirts worn by symmetrical men and yet women at low fertility in their menstrual cycle or those using the contraceptive pill showed no preference for the odour of shirts of symmetrical men compared to those of asymmetrical men. [38] These findings support the good genes hypothesis such that when women are in the fertile phase of their menstrual cycle, they should prefer markers of genetic benefits or 'good genes'. [26] In other words, fertile women who prefer the scent of men with low FA are demonstrating a preference for the genetic benefits associated with those symmetric men.

It is widely accepted that men prefer the scent of women in her most fertile period. For example, in one study, men rated the body odours of T-shirts worn by women during their most fertile phase as more sexy and pleasant than T-shirts worn during their least fertile phase. [4] Despite this, much of the research in the area concludes that the effect of the scent of symmetry appears to be sex specific such that men do not find the scent of symmetrical women more attractive than the scent of asymmetrical women. [27] [45] It therefore appears that attraction to symmetric body odour of the opposite sex appears to apply exclusively to women, and specifically fertile women, as non-fertile women and men do not display this preference.

Body odour in selection

Sex differences

The oral hormone contraceptive pill Combined oral contraceptive pill (3).JPG
The oral hormone contraceptive pill

Whilst a vast number of studies have been conducted to investigate body odour and mating, research is now shifting in particular towards the effect of male scent to female sexual attraction. This is largely due to the effects of the menstrual cycle and hormone contraceptives, which directly affect women's partner preference. Past research has highlighted the importance of a male's scent to females, such that smell was rated significantly more important for women than men. Furthermore, smell and body odour were rated as the most important physical factor for females, compared to looks for males. [47] Further studies have aimed to understand these sex differences. Using questionnaires and self-report data, a greater reliance was found for females on olfactory cues than males. This reliance was valued for females in both sexual and non-sexual contexts. [48] The research strongly supports the hypothesis that whilst men use more physical and visual cues, women rely more heavily on olfactory cues such as body odour that men tend to overlook.

Body odour enhancement

It has been established that women tend to rely more heavily on olfactory cues than males, rating those with pleasant body odour as more attractive than those with less pleasant body odour. Moreover, body odour and sexual attraction can be enhanced using artificial fragrances and dietary habits. Whilst males are influenced by body odour during selection, past research has demonstrated a significantly greater reliance on such cues by females.

A dependence on such olfactory cues has led to the enhancement of male body odour to influence female attraction. Past research has shown that men using fragranced antiperspirants in videos were rated as more attractive by women than those in a placebo control condition. In addition to this, females gave higher ratings of masculinity than males, particularly in the fragranced condition. [48] This provides evidence that females are more attentive to olfactory cues during partner selection. Aside from artificial fragrances, researchers have also begun to focus on more natural odour enhancements such as diet quality. A recent study aimed to investigate whether a dietary fruit and vegetable intake would influence female mate preferences. Its results showed that subjects rated as most pleasant smelling were those with a higher fruit and vegetable intake, suggesting male body odour can be enhanced by diet to appeal to females. [49]

Both sexes commonly use artificial scents to enhance their perceived sexual appeal. Many of these (musk compounds) share a similar chemical profile with naturally produced body chemicals. [50] Research has found a significant correlation between an individual's MHC genotype and the ratings they assigned to certain perfumes labelled to use "for self". [50] This therefore supports the hypothesis that perfumes can personally enhance body odours that indicate an individual's immunogenetics. However, several infertility issues may arise when people use perfumes or scented body washes that erase their natural scent, hindering women in particular from being able to detect if their partner is genetically compatible.

In animals

Insects

Insects use extremely sensitive receptors to detect pheromone signals. Each pheromone signal can elicit a distinct response based on the gender and social status of the recipient. [51] In insects, sex pheromones can be detected in very minute concentrations in the environment. Insect sex pheromones, usually released by the female to lure a male, are vital in the process by which insects locate each other for mating. [51] The main purpose of releasing these sex pheromones is to attract a partner from a distance, however the sex pheromones also serve to evoke a courtship response and sexually excite the male prior to copulation. [52] Male insects can also release sex pheromones, but this is only for the purpose of sexually exciting the female, making her more receptive to the male's advances. Generally, the majority of insects are sensitive and selective to the sex pheromone of their own species.

Insects make use of two classes of pheromone signals; the pheromones that induce immediate or releaser effects (for example, aggression or mating behaviours) and those that elicit long-lasting or ‘primer' effects, such as physiological and hormonal changes. [51]

A pair of mosquitos during mating season CDC-Gathany-Aedes-albopictus-4409.jpg
A pair of mosquitos during mating season

There is a significant amount of research supporting body odour and sexual attraction in insects. Observations and laboratory experiments identified a chemical substance involved in the mating behavior of Culiseta inornata ; when exposed to this scent the male mosquitoes were found to attempt sex with dead females and when exposed to the scent of virgin females, the males showed increased sexual activity through excited flight, searching and attempts to copulate with other males. [53] Further evidence comes from research on the commercial silkworm moth, Bombyx mori . A chemical produced in the abdominal sac of the female adult moth is released shortly after its emergence from the cocoon; male moths were found to be immediately attracted to this scent, demonstrated by a characteristic wing flutter and attempts to copulate. [54] The sex pheromones of the silkworm moths can elicit responses in the male antenna at concentrations of only a few hundred molecules per square centimeter. [55]

Vertebrates

For vertebrates, aquatic environments are an ideal medium for dispersing chemical signals over large areas. Aquatic vertebrates use chemical signals for a wide range of purposes, from attracting mates to distant nesting sites during spawning, to signalling reproductive readiness and regulating predator/prey interactions. [51] Research on goldfish has identified that the fish release hormones in various combinations, depending on the reproductive status of the releaser, and these different combinations can elicit varying degrees of male courtship in the recipient. [56]

In terrestrial environments, chemosignals can be either volatile or non-volatile. [51] Accordingly, terrestrial vertebrates have two functionally and anatomically distinct olfactory systems: the main olfactory system, which is receptive to volatile cues, and the vomeronasal system, which is thought to process mostly non-volatile pheromones. [57]

Mammals

When it comes to sex, mammals use chemical signals (pheromones) to convey information to one another. Mammal's pheromones are air-borne chemical substances released in the urine or feces of animals or secreted from sweat glands that are perceived by the olfactory system and that elicit both behavioral and endocrine responses in conspecifics. [58] Mammals use sex pheromones to arouse, attract, and elicit specific behavioral responses from the opposite sex. [59] In mammals, chemical signals and the scent glands that secrete them have many features in common, for example, expression in only one sex, development only in adults, often only secreted in breeding season and used exclusively in mating. [60] For an odour to stimulate sexual behavior exclusively, it must not only be perceived and preferred, but when absent there should be a decrease or complete elimination of sexual activity. This exclusivity has only been shown in golden hamsters [61] and the rhesus monkey. [62]

Mammalian pheromones can elicit both long-lasting effects that alter the hormone levels of the recipient animal, and short-term effects on its behavior. [63] For example, detection of male pheromones by female mice has been found to encourage onset of puberty, however the detection of female pheromones have been found to delay the onset of puberty. [64]

Odour can influence mammalian mating both directly and indirectly. Odour may act as a direct benefit to females, for example by avoiding contagious diseases by using odour cues to choose a healthy mate. [60] Odour can also act as an indicator mechanism, a form of indirect benefit, for example when a male displays a particular trait such as strength of odour which is in proportion to their heritable viability, females choosing males with strong odours will gain genes for high viability to pass to their offspring. [65]

There is vast evidence for the use of pheromones in mating behaviors. For example, when boars become sexually aroused, they salivate profusely dispersing pheromones into the air. These pheromones attract receptive sows, causing it to adopt a specific mating posture, known as standing, which allows the male boar to mount it and therefore copulate. [66]

Species specificity

Regardless of the species, sex pheromones are often structurally similar and for that reason different species need to be able to respond to the correct pheromone. It is the variation in the ratios of each compound within a pheromone that yields species specificity. [67] The use of mixtures of compounds as pheromones is well documented in insects, research into male orchid bees demonstrates that specific odours mediate exclusive attraction within a species. [68]

See also

Related Research Articles

<span class="mw-page-title-main">Pheromone</span> Secreted or excreted chemical factor that triggers a social response in members of the same species

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavior of the receiving individuals. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Pheromones are used by many organisms, from basic unicellular prokaryotes to complex multicellular eukaryotes. Their use among insects has been particularly well documented. In addition, some vertebrates, plants and ciliates communicate by using pheromones. The ecological functions and evolution of pheromones are a major topic of research in the field of chemical ecology.

<span class="mw-page-title-main">Sexual attraction</span> Attraction on the basis of sexual desire

Sexual attraction is attraction on the basis of sexual desire or the quality of arousing such interest. Sexual attractiveness or sex appeal is an individual's ability to attract other people sexually, and is a factor in sexual selection or mate choice. The attraction can be to the physical or other qualities or traits of a person, or to such qualities in the context where they appear. The attraction may be to a person's aesthetics, movements, voice, or smell, among other things. The attraction may be enhanced by a person's adornments, clothing, perfume or hair style. It can be influenced by individual genetic, psychological, or cultural factors, or to other, more amorphous qualities. Sexual attraction is also a response to another person that depends on a combination of the person possessing the traits and on the criteria of the person who is attracted.

Body odor or body odour (BO) is present in all animals and its intensity can be influenced by many factors. Body odor has a strong genetic basis, but can also be strongly influenced by various factors, such as sex, diet, health, and medication. The body odor of human males plays an important role in human sexual attraction, as a powerful indicator of MHC/HLA heterozygosity. Significant evidence suggests that women are attracted to men whose body odor is different from theirs, indicating that they have immune genes that are different from their own, which may produce healthier offspring.

<span class="mw-page-title-main">Concealed ovulation</span> Lack of visible changes in ovulating females

Concealed ovulation or hidden estrus in a species is the lack of any perceptible change in an adult female when she is fertile and near ovulation. Some examples of perceptible changes are swelling and redness of the vulva in baboons and bonobos, and pheromone release in the feline family. In contrast, the females of humans and a few other species that undergo hidden estrus have few external signs of fecundity, making it difficult for a mate to consciously deduce, by means of external signs only, whether or not a female is near ovulation.

<span class="mw-page-title-main">Physical attractiveness</span> Aesthetic assessment of physical traits

Physical attractiveness is the degree to which a person's physical features are considered aesthetically pleasing or beautiful. The term often implies sexual attractiveness or desirability, but can also be distinct from either. There are many factors which influence one person's attraction to another, with physical aspects being one of them. Physical attraction itself includes universal perceptions common to all human cultures such as facial symmetry, sociocultural dependent attributes, and personal preferences unique to a particular individual.

<span class="mw-page-title-main">Psychological adaptation</span>

A psychological adaptation is a functional, cognitive or behavioral trait that benefits an organism in its environment. Psychological adaptations fall under the scope of evolved psychological mechanisms (EPMs), however, EPMs refer to a less restricted set. Psychological adaptations include only the functional traits that increase the fitness of an organism, while EPMs refer to any psychological mechanism that developed through the processes of evolution. These additional EPMs are the by-product traits of a species’ evolutionary development, as well as the vestigial traits that no longer benefit the species’ fitness. It can be difficult to tell whether a trait is vestigial or not, so some literature is more lenient and refers to vestigial traits as adaptations, even though they may no longer have adaptive functionality. For example, xenophobic attitudes and behaviors, some have claimed, appear to have certain EPM influences relating to disease aversion, however, in many environments these behaviors will have a detrimental effect on a person's fitness. The principles of psychological adaptation rely on Darwin's theory of evolution and are important to the fields of evolutionary psychology, biology, and cognitive science.

<span class="mw-page-title-main">Mate choice</span> One of the primary mechanisms under which evolution can occur

Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior. In other words, before an animal engages with a potential mate, they first evaluate various aspects of that mate which are indicative of quality—such as the resources or phenotypes they have—and evaluate whether or not those particular trait(s) are somehow beneficial to them. The evaluation will then incur a response of some sort.

<span class="mw-page-title-main">Estratetraenol</span> Chemical compound

Estratetraenol, also known as estra-1,3,5(10),16-tetraen-3-ol, is an endogenous steroid found in women that has been described as having pheromone-like activities in primates, including humans. Estratetraenol is synthesized from androstadienone by aromatase likely in the ovaries, and is related to the estrogen sex hormones, yet has no known estrogenic effects. It was first identified from the urine of pregnant women.

Claus Wedekind is a Swiss biological researcher notable for his 1995 study that determined a major histocompatibility complex (MHC) dependent mate preference in humans.

<span class="mw-page-title-main">Sexual selection in humans</span> Evolutionary effects of sexual selection on humans

Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection, as it affects humans. Sexual selection is a biological way one sex chooses a mate for the best reproductive success. Most compete with others of the same sex for the best mate to contribute their genome for future generations. This has shaped human evolution for many years, but reasons why humans choose their mates are not fully understood. Sexual selection is quite different in non-human animals than humans as they feel more of the evolutionary pressures to reproduce and can easily reject a mate. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that sexual selection played a part in the evolution of the anatomically modern human brain, i.e. the structures responsible for social intelligence underwent positive selection as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model. Fisher also stated that the development of sexual selection was "more favourable" in humans.

<span class="mw-page-title-main">Odor</span> Volatile chemical compounds perceived by the sense of smell

An odor or odour is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and many animals can perceive via their sense of smell. An odor is also called a "smell" or a "scent", which can refer to either an unpleasant or a pleasant odor.

<span class="mw-page-title-main">Major histocompatibility complex and sexual selection</span> Adaptive immune gene selection

The major histocompatibility complex in sexual selection concerns how major histocompatibility complex (MHC) molecules allow for immune system surveillance of the population of protein molecules in a host's cells. In 1976, Yamazaki et al. demonstrated a sexual selection mate choice by male mice for females of a different MHC.

Mate preferences in humans refers to why one human chooses or chooses not to mate with another human and their reasoning why. Men and women have been observed having different criteria as what makes a good or ideal mate. A potential mate's socioeconomic status has also been seen important, especially in developing areas where social status is more emphasized.

Sexual motivation is influenced by hormones such as testosterone, estrogen, progesterone, oxytocin, and vasopressin. In most mammalian species, sex hormones control the ability and motivation to engage in sexual behaviours.

Female intrasexual competition is competition between women over a potential mate. Such competition might include self-promotion, derogation of other women, and direct and indirect aggression toward other women. Factors that influence female intrasexual competition include the genetic quality of available mates, hormone levels, and interpersonal dynamics.

<span class="mw-page-title-main">Copulation (zoology)</span> Animal sexual reproductive act in which a male introduces sperm into the females body

In zoology, copulation is animal sexual behavior in which a male introduces sperm into the female's body, especially directly into her reproductive tract. This is an aspect of mating. Many animals that live in water use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch. Internal fertilization with many vertebrates occurs via cloacal copulation, known as cloacal kiss, while most mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.

No study has led to the isolation of true human sex pheromones, though various researchers have investigated the possibility of their existence. Sex pheromones are chemical (olfactory) signals, pheromones, released by an organism to attract an individual, encourage it to mate with it, or perform some other function closely related with sexual reproduction. While humans are highly dependent upon visual cues, when in proximity, smells also play a role in sociosexual behaviors. An inherent difficulty in studying human pheromones is the need for cleanliness and odorlessness in human participants. Experiments have focused on three classes of putative human pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the vomeronasal organ.

The ovulatory shift hypothesis holds that women experience evolutionarily adaptive changes in subconscious thoughts and behaviors related to mating during different parts of the ovulatory cycle. It suggests that what women want, in terms of men, changes throughout the menstrual cycle. Two meta-analyses published in 2014 reached opposing conclusions on whether the existing evidence was robust enough to support the prediction that women's mate preferences change across the cycle. A newer 2018 review does not show women changing the type of men they desire at different times in their fertility cycle.

In humans, males and females differ in their strategies to acquire mates and focus on certain qualities. There are two main categories of strategies that both sexes utilize: short-term and long-term. Human mate choice, an aspect of sexual selection in humans, depends on a variety of factors, such as ecology, demography, access to resources, rank/social standing, genes, and parasite stress.

<span class="mw-page-title-main">Olfactic communication</span> Social interaction through smell

Olfactic communication is a channel of nonverbal communication referring to the various ways people and animals communicate and engage in social interaction through their sense of smell. Our human olfactory sense is one of the most phylogenetically primitive and emotionally intimate of the five senses; the sensation of smell is thought to be the most matured and developed human sense.

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