What is Love? Understanding The Process of Human Mate Selection

Introduction

Attraction is a billion-dollar industry. People around the globe frequent online-dating websites, speed-dating clinics, matchmakers, and more, all in the hopes of finding “the one,” their perfect partner. Most humans experience some form of physical attraction, yet most of the time they cannot rationally explain its cause. Attraction has become a mystery attributed to fate or luck. But, what if attraction is not an enigmatic, inexplicable process? What if there are underlying scientific principles that govern who people are attracted to, and what if there is a logical and reasonable explanation for who they consider to be their perfect mate? Is love at first sight real, or is it simply science?

In recent years, researchers have found that olfactory-based biological processes play an important role in the mate selection process not only in animals, but in humans as well. Studies have shown that particular genes, such as the ones that code for the Major Histocompatibility Complex (MHC), which controls what the immune system considers native or foreign, influence attraction patterns through the production of maximally dissimilar scent profiles between partners. Differences in MHC allele locations have also indicated an increase in the potential for attraction between individuals, suggesting that humans tend to mate with MHC dissimilar partners 1, 2, 5. These scent profiles, in conjunction with other biochemical processes, such as the production of chemosignals and pheromones, increase levels of sexual responsiveness between well-matched individuals and maximize the potential for successful mating1, 6, 9, 10. The neuroendocrine system, which controls hormonal releases, works in tandem with these biological mechanisms, using systematic chemical cascades in the brain to further increase feelings of love and attraction 4, 8, 11.

A cohesive view of attraction does not yet exist, limiting the knowledge that is available on a fundamental and mysterious part of human nature. This paper aims to unite the separate, recent conversations on the processes of attraction into one unified overview. My research acts as a means for other scientists to discover a broader context for their work and to showcase the overarching system of olfaction that links what has traditionally been thought of as an individualized processes. In doing so, I hope to aid not only scholars but also to help the public find answers for some of the fundamental questions as to why they feel the way they do. Exposure to a deeper understanding of attraction will allow readers to begin thinking more logically about their emotions and discover the intimate effect that science has on their daily lives.

The current, compartmentalized method for research in attraction stifles the growth of interdisciplinary thought and prevents new discoveries from influencing future scientific inquiry effectively. If a unified conception of present research on the mechanisms of mate selection existed, a broader definition and scope of attraction could be developed. This would promote collaborative investigation and aid in a holistic understanding of how both the human mind and body work. My research will be an initial attempt to solve this problem, analyzing and synthesizing current information on how the human body’s numerous chemical and biological processes work cohesively to manifest feelings of physical attraction under the unifying theme of olfaction.

Attraction is a complex idea with definitions in multiple fields of the social and biological sciences. In social psychology, attraction is defined as “any force that draws people together,” 4 while in sociology it is defined as “a positive attitude one holds for another person, or the positive behaviors an individual displays in response to another person over a prolonged period of time.” 7 As these related, yet distinct, definitions show, one description that completely encompasses the broad nature of attraction does not exist. These varying ideas lead to a historically inconsistent concept of attraction. What society has considered beautiful or attractive in the past is not necessarily what is considered beautiful in the present. Attraction and beauty are not constant ideas, but rather fluctuating concepts based on popular culture and social norms. However, while the psychological, social, and cultural implications of attraction, along with the accepted standard of beauty, are important in noting the effects of attraction on society as a whole, they are not the focus of this research. This project focuses solely on the underlying, biological processes that create a fundamental, scientifically determined basis for attraction in all mammals, specifically humans.

The overarching theme of this research paper is the relationship of olfaction to the main biochemical mechanisms of attraction. In this article, I will first explore current research on the process of olfaction and its role in human processes and society. I will then examine the relationship between olfaction and specific genes, such as MHC, that have been determined to have an effect on attraction through scent, along with their relationship to other chemical impulses, specifically chemosignals and pheromones. The olfactory processing of pheromones and chemosignals will then be used to explore the hormonal and neuroendocrine releases in the brain that increase feelings of attraction and are also linked to scent. This information will be synthesized to connect what were once considered disparate processes and to develop an integrated overview of attraction.

Olfaction and Attraction

In both humans and other mammals, olfaction acts as a foundation for the biochemical mechanisms that generate feelings of attraction. Olfaction, like attraction, is an intricate process that is dictated by the successful interworking of numerous biological structures. Overall, olfaction provides the ability for individuals to detect chemicals and stimuli in the gaseous or volatile state, which can then induce specific neuroendocrine and hormonal releases 1, 6. There are two olfactory systems present in mammals: the main olfactory system and the accessory olfactory system 1, 6, 7. These two systems have separate, but related, roles in the process of scent recognition. The main olfactory system detects both volatile airborne scents and scents that are perceived at a distance from their emitting source 1, 3. The accessory olfactory system discovers scents through contact as detected molecules move into the vomeronasal organ (VNO) 1, 3. This peripheral structure, which is located in the nasal septum or roof of the mouth in most mammals, is a sensory receptor organ that perceives certain chemical stimuli in the environment 1, 3. However, humans do not have fully functional vomeronasal receptors, causing them to rely almost entirely on the main olfactory system for scent recognition 1, 4. Despite this, chemical communication does occur among humans; it simply does so independently of the VNO, which has been found to regress in humans during fetal development 1, 3. In conjunction with this regression, humans also have a slightly diminished capacity of obtaining sensory information through smell as compared to other mammals 3. Due to humans’ limited capability for scent recognition, olfaction has been greatly underestimated in its importance to reproductive behaviors. It was previously assumed that humans relied predominantly on visual cues, rather than scent, when assessing potential mates. However, recent studies have begun to suggest that olfaction plays a significant role in reproductive biology and mate selection 5-8. Researchers have found that humans have the capability to recognize thousands of discrete odors, which often evoke emotions and memories upon perception 4. In this way, smell acts as a primal receptor sense in humans, causing specific odors and scents to trigger instinctive behaviors that are not consciously decided by the perceiving individual 4. This process is what causes salivation when one smells food cooking or the surfacing of a particular memory when a correlating smell is perceived. Over time, human evolution has caused auditory and visual cues to become tools for the refinement of underlying sexual impulses initiated by underlying scent attractants 4. One study found that women considered a man’s smell to be more important than appearance, voice, or money when selecting a lover, and, in surveyed women, the influence of smell also outranked all social factors except pleasantness 7. Most current research leads to the conclusion that scent plays a key role in mediating sexual behaviors and in sex recognition, affecting almost all of the fundamental processes of human attraction and mate selection 1.

The Major Histocompatibility Complex

Certain biological molecules have been found to have a direct correlation with the processes of both olfaction and mate-value determination. Research has shown that the major histocompatibility complex, MHC, a complex gene with many possible allele combinations, is the cause of an intricate and complicated relationship between olfaction and attraction. In humans, the MHC gene is known as Human Leukocyte Antigen (HLA) complex and is found on the shorter chromatid of chromosome 1, 2, 6, 7, 11. The HLA complex is comprised of three interacting gene regions that each code for numerous alleles and proteins, making it almost impossible for two individuals to have the same HLA allele combination 1. This individuality of the HLA complex works in tandem with its function in self and nonself immune system determination, a process that allows the body to determine what is an invader and what is a regulated and acceptable part of internal systems 1, 2, 7. In conjunction with its immune system function, HLA also acts as a measure of genetic relatedness and compatibility through the formation of individualized scent profiles 1. HLA determines which organisms can live healthily in or on the body, including the microscopic bacteria that cover all humans and that have a direct effect on an individual’s scent 7. The extreme level of HLA variance in humans causes the formation of unique and personal scent profiles for every individual 7, 11. In simpler terms, just as no two people have the same HLA combination, no two people smell exactly the same.

This formation of unique scent profiles has shown to be directly correlated with the process of mate selection in humans. Humans generally tend to rate the odors of individuals with only a few HLA alleles similar to their own as the most pleasant, and studies have shown that this indicates a tendency for humans to select mates that display a maximal level of HLA-dissimilarity 2, 5. In conjunction to this, other studies have found that family members, who display similar HLA profiles, have the tendency to smell the most displeasing to each other 8. This unpleasant odor is a biological attempt to assess the degree of relatedness of a prospective mate and prevent mating between related pairs, reducing the potential for inbreeding. This natural tendency to avoid inbreeding works in combination with the trend for humans to select mates with HLA alleles different from their own. In a study conducted using genome-wide data and the HLA genotypes of both African-American and European-American couples, researchers concluded that, in European Americans, spouses were significantly more MHC-dissimilar from each other than random pairs of individuals 2. The data from this study was not due to demographic or sociocultural changes as it was based on information from the entire genome, rather than simply the tested couples. This allowed researchers to separate MHC-specific effects from effects caused by social or cultural changes. Other research studies support these hypotheses, displaying a human proclivity towards selecting mates with maximally dissimilar MHC allele pairs 5. Further, the genetic analysis of HLA in reproductively isolated Hutterite communities showed that they avoided spouses with similar HLA allele combinations to their own. This selection went beyond the prevention of inbreeding, with individuals selecting non-related mates with a maximal level of HLA dissimilarity 2. The analysis of the Hutterite communities also found that a preference for HLA dissimilar mates was based on matches of only paternally inherited HLA alleles, suggesting that mate selection is independent of learned environmental factors and is solely dependent on individual genetic makeup 2. Another study showed that women in relationships with a lower percentage of MHC similarity were more sexually satisfied with their partners and less likely to have intercourse with individuals outside of their relationship 5. This unconscious trend towards the pairing of dissimilar HLA alleles provides an insight into the biological benefit of such matches. Partners with dissimilar MHC alleles not only benefit in higher levels of sexual satisfaction, but also maximize the immune system fitness of potential offspring, ensuring that the healthiest offspring are those that are introduced into the future gene pool 8.

Chemosignals and Pheromones

Chemosignals and pheromones are an integral part of mammalian communication, creating a complex emitter and receiver relationship between well-matched individuals and possessing unique qualities that allow them to trigger base biological reactions. The emitted chemical signals work with scent profiles determined by HLA alleles to produce an overall odor signature for individuals and to release specific pheromones that can cause unconscious increases in sexual responsiveness 4. In mammals, these volatile chemical signals or chemosignals, a subset of which are known as pheromones, are perceived by both the main and accessory olfactory systems 1. However, the importance of each system in recognizing and processing these signals is tied to the nature of the stimuli, not the response from the receiver 1. These signals, specifically pheromones, have a specific and measurable effect on the body 4. Many of the olfactory signals imperative to both mate selection and reproduction are pheromone-based 5. In reproduction and attraction, these pheromones have four main functions: opposite-sex attractants, same-sex repellants, mother-infant bonding, and menstrual cycle modulation 5. Chemosignals strongly drive reproductive development and influence reproductive behaviors. The signals can alter the sexual behavior and reproductive physiology of both sexes, ensuring that individuals become sexually mature and active at optimal mating times, while suppressing sexual drive at inopportune times 5. Males and females rely on both chemosignals and pheromones to find, attract, and evaluate potential mates 4.

In humans, sex attractant pheromones create an emitter-receiver relationship that increases the level of sexual attractiveness in the emitting individual 1. The volatile pheromones released upon the perception of an individual deemed to be a viable partner cause distinct, and even unconscious, changes in the receiver. These changes make the emitter seem more attractive and increase the possibility for physical intimacy or sexual encounters 6. In one study, women showed a direct increase in four sociosexual behaviors with the application of a synthetic sex-attractant pheromone 6, 11. These sociosexual behaviors included sexual intercourse, sleeping next to a romantic partner, formal dates, and affectionate behaviors 6, 11. The data showcased the underlying, and often unnoticed, influence that pheromones play in both sexual attraction and the mate-selection process, as women who acted as emitters for the pheromone experienced increased levels of sexual and physical intimacy with a romantic partner. This result suggests an increased level of attraction in the receiver toward the emitter along with an increase in the perceived level of attractiveness of the emitter. Another study showed that the male pheromone androsterone, found in underarm sweat, is an attractant to females during ovulation, a time when they are at their most fertile and maximally predisposed for reproduction 11. Chemosignals and pheromones further the biological desire for reproduction by increasing perceived levels of attractiveness and maximizing the likelihood that an individual will find a partner with whom they can reproduce. This boost of attraction works in tandem with HLA dissimilarity between well-matched partners to increase the potential for the production of offspring with the greatest level of immune system fitness.

Chemical Pathways of Attraction

The effects of olfaction manifest in the neurochemical pathways forged between pleasant scents and the chemicals they release in the brain. Two sections, the primitive brain and the emotional brain, control the majority of neurochemical processes that influence attraction 12. The primitive brain is composed of the brain stem, or hindbrain, and controls automated, basic activities such as breathing, heartbeat, sleeping, and arousal 11, 12. The primitive brain also includes the spinal cord, which sends messages between the brain and the body, including to and from the sex organs. The emotional brain is located within the midbrain and acts as a gateway between the primitive brain and the cerebrum, or the “thinking brain” 8. The limbic system, located within the emotional brain, has been determined to be the likely origin for sexual attraction and deep bonding between individuals 8, 12. This system regulates survival mechanisms, which include hunger, emotion, sensory input, sex hormones, and pleasure 8. The limbic system works with other parts of the brain to send messages to the hypothalamus, which directs the production and release of chemicals that trigger direct bodily responses 11.

This release process of the numerous chemicals that control attraction works through a complex system of receptors and chemical cascades. Each region of the brain is made up of specialized cells called neurons 8. For the brain to perform at a high functioning level, these neurons must be able to signal each other through the cascade release of chemical messengers. The brain manifests feelings of attraction through a similar process, using chemical cascades to release a variety of neurotransmitters due to the perception of positive scents by the olfactory system 12. This perception excites neurons located in the nasal cavity, beginning the process of release that continues into the brain. This process occurs hundreds of thousands of times as countless neurochemicals and hormones move between neurons, inducing a cascade effect to continue the process of neuron excitation until specific neurotransmitters reach necessary sections of the brain 8. This signals hormone production in the pituitary gland, which then causes the release of hormones such as testosterone and estrogen, which have a direct effect on the level of attraction felt by an individual 12.

Attraction in the brain is controlled by this chemical release and involves a combination of over 100 hormones and neurochemical messengers, including testosterone, norepinephrine, dopamine, and oxytocin 8. Each of these chemicals has a unique yet interconnected role in the process of attraction. Testosterone drives lust and sexual aggression in both men and women and is triggered by dopamine, a neurohormone 11. Dopamine is vital for attentiveness, motivation, and pleasure, and it is a key component in both ecstasy and love; it is “the fire behind the fireworks.” 8 The main chemical associated with attraction is oxytocin, which has been labeled the “love hormone.” Oxytocin is secreted by the pituitary gland and plays a key role in social bonding and the reinforcement of attachment and trust 4. In both men and women, oxytocin increases during sexual intercourse and other pivotal moments, influencing pair bonding and attraction 4, 8. Norepinephrine, also known as adrenaline, is produced by the adrenal glands during periods of excitation, and is what provides the “rush” that individuals feel towards a potential mate 4, 8. These chemicals work to increase feelings of attraction in various stages of attraction and arousal, including, but not limited to, initial encounters of a mate deemed physically attractive, sexual intercourse, and moments of emotional intensity. These chemicals are present throughout the process of attraction and mate selection, changing in concentration composition throughout the progression of a relationship. The change in concentrations of certain neurotransmitters has been thought to aid in the transition from initial sexual attraction to deeper feelings of love and commitment throughout the course of a relationship. Overall, these chemicals are a vital part of the process of mate selection in humans, and work with chemosignals and pheromones to increase feelings of attraction and the potential for offspring.

Conclusion

The overarching theme of this research paper and the link that connects three seemingly unrelated causes of attraction, MHC, chemical signals, and the neuroendocrine system, is olfaction and its influence on human mate selection. Olfaction is the bridge that connects the varied components of this mysterious process, components that, in the past, have been incorrectly deemed as separate and isolated. The mechanisms that control attraction are linked at a fundamental level by the sense of smell. Specific gene complexes, such as HLA in humans, have been determined to have an effect on attraction through scent, creating a unique odor profile for every individual—much like a fingerprint. The role of HLA in the regulation of the immune system, paired with its role in creating individualized scent profiles, allows humans to assess the viability of a potential mate and pair with partners that will produce offspring with the strongest immune system based on their smell. An individual who is well-matched will smell better to a prospective partner, causing increased potential for attraction and a higher chance of healthy offspring. This pleasant smell works in tandem with the volatile chemicals, such as chemosignals and pheromones, to increase the sexual responsiveness of one partner to the other. These emitted pheromones cause specific base, biological reactions to occur within the body of receiving individuals that result in an increase in the sexual attractiveness of emitters. These chemosignals are scent-based, and, when paired with the pleasant smell of a maximally HLA-dissimilar and well-matched individual, excite the vomeronasal neurons located in the nasal cavity and accessory olfactory system. These pleasant smells are the first step in the chemical cascade process that eventually floods the brain and body with neurotransmitters and hormones, such as dopamine, oxytocin, and testosterone, which stimulate and solidify feelings of attraction between individuals. These three processes, each of which occurs at a different level of biological processing—in in the brain, in an individual’s genetic code, and externally—are influenced and interrelated through the process of olfaction. Overall, these processes all increase feelings of attraction between partners that are well-matched to produce healthy and fit offspring.

Attraction is more than just a random collection of biological processes; it is a complex and intricate system that requires further study to begin to understand completely. In the past, research on this subject has been inefficient and sectioned off, limiting the knowledge that can be gained from the investigation of this fundamental part of human life. The topic of attraction has the potential to spawn important medical applications through the study of the relationship of olfaction and pheromones to brain chemistry. Specific pheromones involved in attraction are also involved in mother-infant bonding. Understanding the impact of pheromones on the release of certain neurotransmitters could prove useful in the creation of antidepressants for mothers suffering from postpartum depression and other diseases. A holistic understanding of attraction is vital to a greater understanding of the human body and the processes that drive base human nature. Attraction, mate-selection, and reproduction are fundamental parts of human life, and yet most of the population do not understand what causes these feelings of love or desire. If nothing else, understanding attraction will help people across the globe understand their own emotions and allow them to simply live happier lives, something that will benefit everyone.