The biology of romantic love has been explored by such biological sciences as evolutionary psychology, evolutionary biology, anthropology, and neuroscience. Neurochemicals and hormones, such as dopamine and oxytocin, are studied along with a variety of interrelated brain systems (including the reward system) which produce the psychological experience and behaviors of romantic love. The study of romantic love is still in its infancy. As of 2021, there were a total of 42 biological studies on romantic love.
Definition of romantic love
The meaning of the term "romantic love" has changed considerably throughout history, making it difficult to define simply. It was initially coined to refer to certain attitudes and behaviors described in a body of literature now referred to as courtly love. However, academic psychology and especially biology also consider romantic love in a different sense, which refers to a brain system (or systems) related to pair bonding or mating with associated psychological properties. The evolutionary anthropologists Bode and Kushnick undertook a comprehensive review of romantic love from a biological perspective in 2021. They considered the psychology of romantic love, its mechanisms, development across the lifespan, functions, and evolutionary history. Based on the content of that review, they proposed a biological definition of romantic love:
Romantic love is a motivational state typically associated with a desire for long-term mating with a particular individual. It occurs across the lifespan and is associated with distinctive cognitive, emotional, behavioral, social, genetic, neural, and endocrine activity in both sexes. Throughout much of the life course, it serves mate choice, courtship, sex, and pair-bonding functions. It is a suite of adaptations and by-products that arose sometime during the recent evolutionary history of humans. The early stage of romantic love (which has obsessive and addictive features) might also be referred to as "being in love", passionate love, infatuation, limerence, or obsessive love. Research has never settled on a unified terminology or set of methods, although "romantic love" being used to refer to this state is the most common in biopsychology. Distinctions are drawn between this early stage of romantic love and the "attachment system" theorized by the attachment theorists like John Bowlby. Romantic love is also distinct from sexual attraction, although they most often occur together. Typically, intense romantic love is limited to a duration of 12–18 months or as long as 3 years, depending on the estimate; however, in the rare phenomenon of long-term intense romantic love, some people experience intense attraction inside a relationship, even for 10 years or more. This is similar to early-stage intense romantic love, but at this later stage they exhibit less of the obsessional features. Romantic love is not necessarily "dyadic", "social", or "interpersonal", despite being related to pair bonding. Romantic love can be experienced outside the context of a relationship, for example in the case of unrequited love where the feelings are not reciprocated.
Independent emotion systems
Helen Fisher and her colleagues proposed that the brain systems involved with mammalian reproduction can be separated into at least three parts:
Neuroscientists currently believe that the basic emotions arise from distinct circuits (or systems) of neural activity; that humans share several of these primary emotion-motivation circuits with other mammals; and that these brain systems evolved to direct behavior [...]. It is hypothesized that among these primary neural systems are at least three discrete, interrelated emotion-motivation systems in the mammalian brain for mating, reproduction, and parenting: lust, attraction, and attachment [...]. Lust is the sex drive, or libido. Attraction (or early-stage romantic love, also called passionate love or infatuation) is associated with feelings of exhilaration, obsessive (or "intrusive") thoughts, and the craving for emotional union. Attachment (the attachment system from attachment theory, and also called companionate love) is associated with feelings of calm, security, and comfort, but separation anxiety when apart. In Fisher's theory, the systems tend to act in unison, but may become disassociated and act independently. For example, a person in a long-term partnership may feel deep attachment for their spouse, while experiencing intense romantic love (attraction) for some other individual, while being sexually attracted (lust) to still others, all at the same time. Lisa Diamond has also used independent emotions theory to explain why people can 'fall in love' sometimes without sexual desire, as in the case of "platonic" infatuation for a friend. Fisher associates each system with different neurotransmitters and/or hormones (lust: estrogen & androgens; attraction: dopamine, norepinephrine & serotonin; attachment: oxytocin & vasopressin), but modern research shows these associations are not as clearly defined as Fisher's theory proposes. Additionally, romantic love has been associated with endogenous opioids, cortisol, and nerve growth factor, which are not included in Fisher's earlier model. With respect to the idea that the systems are independent, a more modern theory holds that the attachment system is active in early-stage romantic love, in addition to the infatuation component. Fisher's model is considered outdated, although the idea of interrelated systems is useful.
Evolution of systems
Evolutionary psychology
Evolutionary psychology is seen as an organizing framework which offers explanations behind psychological functions (rather than merely describing them), as well as specifying theoretical constraints, like requiring that a given trait is adaptive in the form of providing reproductive benefit to an individual. Evolutionary psychology has proposed a variety of explanations for romantic love.
Romantic love is a powerful commitment device. Romantic love suppresses the search for alternative mates (even irrationally so, when a more desirable one comes along), and signals this to the partner. Romantic love may also signal to alternative mates, disincentivizing them from pursuing oneself. The emergence of longer pair bonds in the evolution of humans coincided with the emergence of concealed ovulation, where it cannot (in general) be determined when a woman is ovulating, requiring partners to stay together while having sex during the entire menstrual cycle. Commitment is seen as adaptive to facilitate this, and to facilitate child care. Love feelings might be the psychological reward produced by the brain when the problem of commitment is being solved. The intensity of romantic love feelings and why people become "fools for love" can be explained in terms of the handicap principle, which states that a contention arises between honest and fake signaling. When real emotions evolved, a niche would have been created for sham emotions (e.g. fake facial expressions) which are less risky to express. One explanation for why honest signals can evolve without becoming worthless (because of competing fakers) is that the honest signal can evolve if it is too expensive to fake. One example in nature is the peacock's tail, an example of conspicuous consumption, a cumbersome display which consumes nutrients. Only a healthy peacock can afford it, so in that case it may have evolved because it was a handicap, and used by females of the species as an indicator of health. Romantic love may have evolved to be as bewitching and besotting as it is, "like handcuffing oneself to railroad tracks", as a handicap meant to prove that one's commitment is truly real. Romantic love may have evolved to override rationality, so that one reproduces regardless of the considerable costs of raising a child, and regardless of any rational will to be single or child-free. Romantic love signals parental investment. Paternal investment in the form of pair bonds has been linked to better outcomes for children, both as infants and as they grow older. Children raised in pair bonds are more socially competitive and more likely to survive to reproductive age. Being in love makes people more creative, so romantic love may have evolved as a courtship display. It has been suggested that art, music, and literature serve a function like a peacock's tail, but as a display of mental prowess, designed to impress and make a potential partner swoon. Creativity is believed by some authors to be especially a part of the male courtship display. Romantic love may conserve time and metabolic energy by focusing courtship efforts on a specific individual over others. Successful pair bonding predicts better health and survival. Happy, well-functioning romantic relationships contribute to mental and physical health, especially when stress is encountered. The end of a pair bond (e.g. divorce) is associated with vulnerability, such as to disease, depression, substance abuse, or negative outcomes for children. Victims of a heart attack, for example, are more likely to have another when they live alone. Romantic love promotes exclusivity via mate guarding. Romantic jealousy is one of the most common correlates of being in love, which evolved as a protection from the threat of losing one's love to a romantic rival. Jealousy may be adaptive up to a point (when it motivates one to maintain their relationship), but can also take the form of pathological jealousy where a sufferer has a delusional or paranoid belief in their partner's infidelity regardless of actual evidence. Monogamous pair bonding helps prevent sexually transmitted diseases (STDs) which compromise fertility, especially for women. Certain STDs (e.g. syphilis) increase the risk of miscarriage, and otherwise harm or can be passed to an unborn child. The strongest predictor of contracting an STD is the number of sexual partners, so limiting this is the best way to limit the risk of contracting a disease which would harm one's reproductive health.
Time of evolution
Although the exact moment during human evolution is unknown, modern romantic love is usually believed to have evolved either during or after the time of bipedalism. The earliest hominid found with extensive evidence of bipedalism (and some evidence of pair bonding) is Ardipithecus ramidus, from about 4.4 million years ago, although it may also be the case that bipedalism is older than this. It has been proposed that monogamous pair bonding (which is rare among mammals) evolved during this time, because walking bipedally requires mothers to carry infants in their arms or on their hip, instead of on their backs. With their hands occupied, mothers would be more vulnerable, requiring additional help for food and protection from males of the species (hence, husbands or fathers). A different selection pressure which has been proposed is the evolution of infant altriciality (immaturity and helplessness) and large brain size at birth, which occurred around 2 million years ago. At this time, brain size became so large that a fully-developed infant's head could not fit through the mother's pelvic birth canal (known as the obstetricial dilemma), requiring the infant to be born early and underdeveloped in comparison to other species. This would have also placed a greater burden on mothers, and made paternal support more valuable. Due to the general scarcity of evidence, it is still also possible that romantic love (or a precursor to it) predated bipedalism and altriciality, possibly originating in a common ancestor of humans and chimpanzees, 5–8 million years ago. While chimpanzees primarily mate opportunistically, some of their rarer reproductive strategies have features reminiscent of romantic love (involving mate guarding, and a more-than-fleeting association). One assumption behind hypotheses based on fossil evidence is that less sexual dimorphism in body mass (i.e., similarity) is indicative of monogamy, but the comparative similarity between the sexes in human body mass occurred as recently as 500,000 years ago. This suggests that there may have been multiple steps in the evolution of human pair bonding, and romantic love may have evolved during any of these periods.
Courtship attraction
Helen Fisher's theory is that romantic love (which she considers distinct from attachment) is a motivation system for choosing and focusing energy on a preferred mating partner. According to Fisher, this brain system evolved for mammalian mate choice, also called "courtship attraction". In this phenomenon, a preferred mating partner is chosen based on a display of physical traits (such as a peacock's tail feathers) or other behaviors. For humans, Fisher also includes the attraction to personality traits and other characteristics in her mate choice theory. In most species, courtship attraction is as brief as lasting only minutes, hours, days, or weeks, but intense romantic love can last much longer in humans. Fisher believes that during the timeline of human evolution, mammalian courtship attraction may have become prolonged and intensified as pair bonding evolved, eventually becoming the phenomenon of romantic love today. A critique of Fisher's theory published by Adam Bode holds that courtship attraction only encompasses love at first sight attraction or a crush, and the core components of romantic love (including the intense attraction and obsessive thoughts, in addition to attachment) evolved as a co-option of mother-infant bonding. A study on love at first sight found that, even though people reporting the experience retrospectively will recall features resembling passionate love ("constant thoughts about the person and the desire to be with [them]"), people reporting love at first sight currently after just meeting the potential partner only report neutral scores (neither agreeing nor disagreeing) on a romantic love measure that includes a passion component. Some authors have speculated that the remembered account of falling in love at first sight (with high passion) is often actually a memory confabulation. Furthermore, the study found that the experience of love at first sight was related to the physical attractiveness of the potential partner. This led the researchers to conclude that love at first sight is actually a strong initial attraction, rather than resembling the state of being in love. Bode argues this more closely resembles the concept of courtship attraction, and can be considered a separate system from core romantic love components. Courtship attraction shares similar behaviors with romantic love in humans, and both involve activation of dopaminergic reward circuits. Courtship attraction may be characterized by dopamine, oxytocin and opioid activity, but little is known about it because existing studies were not designed to target it.
Co-option of mother-infant bonding Co-option is an evolutionary process whereby a given trait is repurposed to take on a new function. One example is how a number of species of fish (e.g. catfish) have co-opted their gas bladder to produce sound. Co-opted traits can be morphological, but also behavioral. Co-option has been used as an explanation of how a species can develop an evolutionary adaptation very quickly sometimes, seemingly faster than Darwinism could explain. With this process, a seemingly "new" trait can develop quickly because its structure predated the time of adaptation, only needing to be modified to function in a new way. In some cases, co-option involves one gene whose function is altered, while in other cases the co-opted gene is a duplicate and the function of the original gene is retained. The terms "co-option" and "exaptation" are closely related, but have different connotations, as exaptation refers to structural continuity when a trait takes on a new function. Adam Bode has proposed that romantic love is "a suite of adaptations and by-products" consisting of a number of interrelated systems, several of which evolved by co-opting mother-infant bonding (attraction for bonding, obsessive thinking, and attachment). The co-option theory says that the genes that regulate mother-infant bonding were recreated and took on a new function. Courtship attraction and sexual desire are "causally linked adjuncts" which were not co-opted, but were combined and modified in romantic love. The theory is based on the available human evidence, but also a literature arising from research on prairie voles that pair bonding uses the same mechanisms that mother-infant bonding uses. Academic literature has drawn a parallel between romantic love and the mother–infant dyad since the 1980s, with attachment theorists like Cindy Hazan and Phillip Shaver believing they share a common biological process. In 1981, Glenn Wilson suggested a close analogy between adult lovers and the kind of infant attachment studied by John Bowlby. In 1999, James Leckman and Linda Mayes compared features of romantic love and early parental love, finding substantial similarities. Both are altered mental states featuring preoccupations, exclusivity of focus, longing for reciprocity, and idealization of the other. The trajectories of both also share similarities, with preoccupation increasing during courtship (for romantic love) and around the time of birth (for parental love), then diminishing after a relationship is established (for romantic love) or shortly after the postpartum period (for parental love). (The use of "baby talk" by romantic lovers is another "uncanny" similarity.) In 2004, Andreas Bartels and Semir Zeki were the first to compare romantic love and maternal love with fMRI. This comparison looked at areas known to contain high densities of receptors for the attachment hormones oxytocin and vasopressin. Bartels and Zeki found precise overlap in some specific areas including the striatum (putamen, globus pallidus, and caudate nucleus) and some overlap in the ventral tegmental area, areas with dopamine, and oxytocin receptors. Each type of love was also associated with other unique activations. Notably, maternal love involved the periaqueductal gray matter, an area associated with endogenous pain suppression during intense emotional experiences such as childbirth. Two meta-analyses of fMRI experiments have also found similarities between maternal love and romantic love. A 2022 meta-analysis by Shih et al. found that both types of love were associated with the left ventral tegmental area (more associated with the pleasurable aspect of reward, or "liking"), while in addition romantic love also involved the right ventral tegmental area (more associated with reward "wanting"). In 2003, Lisa Diamond suggested that adult pair bonding is an exaptation of the affectional bond between infants and caregivers, using this to explain phenomena such as romantic friendships and "platonic" infatuations, or i.e. "romantic" passion without sexual desire. Some instances of this are reported by Dorothy Tennov in her study of "limerence" (i.e. love madness, commonly for an unreachable person), in which a younger woman who otherwise considered herself heterosexual would have this type of reaction towards an older woman. Among other examples are schoolgirls falling "violently in love with each other, and suffering all the pangs of unrequited attachment, desperate jealousy etc." (historically called a "smash"), and Native American men who seemed to fall in love with each other and form intense, but non-sexual bonds. Helen Fisher's theory that sexual desire is a separate system from romantic love and attachment is also given as theoretical evidence. Diamond argues that romantic love without sexual desire can even happen in contradiction to one's sexual orientation: because it would not have been adaptive for a parent to only be able to bond with an opposite sex child, so the systems must have evolved independently from sexual orientation. People most often fall in love because of sexual desire, but Diamond suggests time spent together and physical touch can serve as a substitute. Diamond believes the connection between romantic love and sexual desire is "bidirectional" in that either one can cause the other to occur because of shared oxytocin pathways in the brain.
New model Based on contentions over evolutionary theories and Fisher's outdated neurochemical model, Bode has suggested Fisher's model, while useful and the predominant one for a time, is oversimplified and proposes five systems:
Sexual desire is associated with a drive to initiate and be receptive to sexual activity. Testosterone, dopamine, serotonin, norepinephrine, acetylcholine, histamine, and opioids have been implicated in sexual behavior. Courtship attraction is for choosing and focusing energy on a preferred mating partner and promotes courtship behaviors. It can take the form of love at first sight attraction or a crush and also be intertwined with other forms of attraction, but might not precede a relationship in all cases. Courtship attraction may be associated with dopamine, oxytocin, and opioids. Bonding attraction is the type of attraction for pair bond formation, characterized by a strong desire for proximity, separation anxiety when apart, exclusivity of focus, and heightened awareness of the loved one. Bonding attraction is associated with dopamine and oxytocin activity, especially in the ventral tegmental area. According to Bode's arguments, this is the type of romantic attraction shown in fMRI experiments of early-stage romantic love. Obsessive thinking involves preoccupation or intrusive thinking about the loved one. Some authors have drawn a comparison between this feature and obsessive-compulsive disorder, suggesting they share similar neurobiology, but the evidence for that is limited and ambiguous. Attachment is for pair bond maintenance, or maintaining very close personal relationships, with psychological features like a heightened sense of responsibility, longing for reciprocity, and a powerful sense of empathy. Attachment is associated with oxytocin, dopamine, and opioid activity, but there is also some evidence for the involvement of vasopressin. Bode suggests that the systems of bonding attraction, obsessive thinking and attachment (the three systems which were co-opted from mother-infant bonding) together form the core of romantic love (the necessary components). However, all five systems are merged into one single phenomenon of romantic love, with a variety of different outcomes depending on the circumstances.
Mechanics
Reward, motivation and addiction
The early stage of romantic love is being compared to a behavioral addiction (i.e., addiction to a non-substance) but the "substance" involved is the loved person. Addiction involves a phenomenon known as incentive salience, also called "wanting" (in quotes). This is the property by which cues in the environment stand out to a person and become attention-grabbing and attractive, like a "motivational magnet" which pulls a person towards a particular reward. Incentive salience differs from craving in that craving is a conscious experience while incentive salience may or may not be. While incentive salience can give feelings of strong urgency to cravings, it can also motivate behavior unconsciously, as in an experiment where cocaine users were unaware of their own decisions to choose a low dose of cocaine (which they believed was placebo) more often than an actual placebo. In the incentive-sensitization theory of addiction, repeated drug use renders the brain hypersensitive to drugs and drug cues, resulting in pathological levels of "wanting" to use drugs. People in love are thought to experience incentive salience in response to their beloved. Lovers share other similarities with addicts as well, like tolerance, dependence, withdrawal, relapse, craving, and mood modification. Incentive salience is mediated by dopamine projections in the mesocorticolimbic pathway of the brain, an area generally involved with reward, motivation, and reinforcement learning. Dopamine signaling for incentive salience originates in the ventral tegmental area (VTA) and projects to areas such as the nucleus accumbens (NAc) in the ventral striatum. The VTA is one of two main areas of the brain with neurons which produce dopamine (the other being the substantia nigra pars compacta). Projections from the VTA innervate the NAc, where dopamine activity attaches motivational significance to stimuli associated with rewards. Brain scans of people in love using fMRI (commonly while looking at a photograph of their beloved) show activations in these areas like the VTA and NAc. Another dopamine-rich area of the reward system shown to be active in romantic love is the caudate nucleus, containing 80% of the brain's dopamine receptor sites, located in the dorsal striatum. The dorsal striatum is implicated in reinforcement learning, and the caudate nucleus has shown activity in response to a monetary reward and cocaine. This activity in reward and motivation areas suggests that early-stage intense romantic love is a motivation system or goal-oriented state (rather than a specific emotion), consistent with the description of romantic love as a desire or longing for union with another person. These activations are also consistent with the similarity between romantic love and addiction. In addiction research, a distinction is drawn between "wanting" a reward (i.e., incentive salience, tied to mesocorticolimbic dopamine) and "liking" a reward (i.e., pleasure, tied to hedonic hotspots), aspects which are dissociable. People can be addicted to drugs and compulsively seek them out, even when taking the drug no longer results in a high or the addiction is detrimental to one's life. They can also irrationally "want" (i.e. feel compelled towards, in the sense of incentive salience) something which they do not cognitively wish for. In a similar way, people who are in love may "want" a loved person even when interactions with them are not pleasurable. For example, they may want to contact an ex-partner after a rejection, even when the experience will only be painful. It is also possible for a person to be "in love" with somebody they do not like, or who treats them poorly. Academics have proposed a number of theories for how addictions begin and perpetuate. One prominent theory developed by Wolfram Schultz involves a dopamine signal which encodes a reward prediction error (RPE): the difference between the predicted value of a reward, which would be received by performing a particular action, and the actual value upon receiving it: whether the reward was better, equal to or worse than expected. In this theory, dopamine is also part of a mechanism for reinforcement learning which associates rewards with the cues which predicted them. Drugs of abuse like cocaine hijack this mechanism by artificially overstimulating dopamine neurons, mimicking an RPE signal which is much stronger than could be produced naturally. An alternative model developed by Kent Berridge and Terry Robinson states that dopamine signaling causes the motivational output (incentive salience) which is proportional to RPE, but that the dopamine signal itself may be an effect of learning rather than causing it directly. There is, however, said to be overwhelming evidence that dopamine guides learning in addition to motivation. The computation of dopamine signaling is complicated, with inputs from a variety of areas in the brain, although its output (primarily from the VTA) is a relatively homogeneous signal encoding the level of RPE. One study has investigated whether people in long-term romantic relationships experienced RPE in response to having expectations about their partners' appraisal of them validated or violated, indicating they do. This study used fMRI to find that reward areas like the VTA and striatum responded in a way consistent with other research on RPE. Most fMRI studies of romantic love have had participants look at a photograph, and the resulting reward system activity has been interpreted in terms of salience. Research has not investigated whether romantic love shares all of the neurobiological aspects of addiction. Despite similarities, there are also differences between romantic love and addiction. One of the major differences is that the trajectories diverge, with the addictive aspects tending to disappear over time during a relationship in romantic love. By comparison, in a drug addiction, the detrimental aspects magnify over time with repeated drug use, turning into compulsions, a loss of control and a negative emotional state. It has been speculated that the difference between these could be related to oxytocin activity present in romantic love, but not in addiction. Oxytocin seems to ameliorate the effects of drug withdrawal, and it might inhibit the more long-term, excessive effects of addiction. Academics do not universally agree on whether or not love is always an addiction or when it needs to be treated. The term "love addiction" has had an amorphous definition over the years and does not yet denote a psychiatric condition, but recently one definition has been developed that "Individuals addicted to love tend to experience negative moods and affects when away from their partners and have the strong urge and craving to see their partner as a way of coping with stressful situations." Other authors include rejected lovers as love addicts, or specify that love is an addiction when it involves abnormal processes which carry negative consequences. A broader view is that all love is addiction, or simply an appetite, similar to how humans are dependent on food. Research on behavioral addictions is more limited than research on drugs of abuse; however, there is a growing body of evidence that some people are susceptible to showing brain patterns in response to natural rewards (food, sex, etc.) similar to drug addicts, particularly in the case of gambling addiction. Romantic love may be a "natural" addiction, which differs from the nature of drug addiction in that love may be prosocial and has been evolved for the purpose of pair bonding. Helen Fisher, Arthur Aron and colleagues have proposed that romantic love is a "positive addiction" (i.e. not harmful) when requited and a "negative addiction" when unrequited or inappropriate.
Oxytocin, bonding, and attraction
Oxytocin is sometimes called the "love hormone", because of its involvement in the mechanisms of maternal behavior and adult pair bonding. Oxytocin is synthesized primarily in an area of the hypothalamus and released into the blood via the pituitary gland, where it has been found circulating in people in the early stages of romantic love. Additionally, the hypothalamus projects oxytocin to other areas of the brain, like the ventral tegmental area (VTA), nucleus accumbens (NAc), amygdala, and hippocampus. The projections to reward areas (VTA and NAc) are thought to modulate social salience, or i.e. the level of dopamine activity in response to socially-relevant stimuli. This oxytocin signaling in reward pathways may also be the source of salience in response to a loved one. A placebo-controlled study found that administering intranasal oxytocin enhanced facial attractiveness of a romantic partner while viewing a photograph, as compared to an unfamiliar face. The effect was also measured using fMRI, which found enhanced brain activity in reward areas like the VTA and NAc. Another fMRI study found dopamine-rich genetic expression of an oxytocin receptor gene in the left VTA, and the left VTA has also been found active in response to facial attractiveness. In humans, circulating oxytocin levels have been associated with higher levels of interaction between partners, and also predicted which couples would still be together 6 months later. Anna Machin calls the combination of oxytocin and dopamine the "glue" which makes the early stages of a relationship possible. The role of oxytocin in human behavior is varied and complex. Oxytocin lowers inhibitions to forming new relationships by deactivating the amygdala, involved with processing fear and anxiety. Oxytocin can be released with physical touch, hence it's also sometimes called the "cuddle hormone". Oxytocin also plays a role in sexual behavior, being released during sexual arousal and orgasm. Aside from romantic and parental bonding, oxytocin activity has a role in the interactions with peers or strangers, for example facilitating facial recognition and eye contact. Oxytocin is believed to facilitate trust and altruistic behaviors towards in-groups (e.g. partners or children), but also aggression towards out-groups (e.g. strangers or conspecifics). Much of the research on oxytocin comes from experiments on monogamous prairie voles (notably by Larry Young), but this research is also used for making inferences about humans. In prairie voles, both oxytocin and dopamine signaling have been shown to influence pair bond development. For example, the number of oxytocin receptors in the NAc is positively related to how fast a partner preference is formed. A partner preference can also be prevented by injecting either a dopamine or oxytocin receptor antagonist (a drug which blocks transmission) into the NAc of a prairie vole directly. In a contemporary model of the brain systems involved with romantic love, this type of salience (or 'bonding attraction') is present throughout the entire time a person is experiencing romantic love, including during the early stages. This contrasts with some previous theories (e.g. proposed by Helen Fisher in 1998) which stated that oxytocin activity and dopamine activity were distinct (and independent) systems, and that oxytocin activity only became prominent at some later stage of a relationship. Levels of oxytocin would still vary from situation to situation because of differing types of stimuli, for example because of less regular interaction and physical touch in cases of unrequited love. This could be used to explain some of the maladaptive symptoms of infatuation (e.g. sleep difficulties, social anxiety, clammy hands, etc.), when dopaminergic activity is high without the calming effect of oxytocin from the attachment system.
Brain opioid theory of social attachment
Modern research is increasingly showing the importance of endogenous opioids in love and social attachment, particularly the β-endorphin (the most potent endogenous opioid) and the μ-opioid receptor system. While opioids have their origin being the body's natural painkiller, they're also implicated in a variety of other systems, essentially like neurotransmitters. Opioid receptors are located throughout the brain, including in the limbic system (affecting basic emotions) and neocortex (affecting more conscious decision-making). Opioids are linked to the consummatory part of reward, or i.e. "liking" or pleasure, and released in areas of the brain called hedonic hotspots (or pleasure centers). Hedonic hotspots are located in the nucleus accumbens, the ventral pallidum and other areas. This function includes social reward, or the pleasurable aspect of social interactions. The brain opioid theory of social attachment (BOTSA) is a long-running theory summarizing this connection, originally formulated in the 1980s and 1990s, based on a proposal by the psychiatrist Michael Liebowitz and research by the neuroscientist Jaak Panksepp. Starting in the 1990s, opioids were overshadowed by the interest in oxytocin and largely overlooked until more recently, possibly because of the difficulty studying them (requiring e.g. a PET scan, which is expensive). Opioids have been connected to a variety of social experiences, including the early stage of romantic love and attachment styles. While the addictive aspects of love have been compared to cocaine or amphetamine addiction, other aspects may also resemble an opioid addiction. BOTSA (as it was originally conceived of) predicts that in the absence of social relationships, individuals will have comparatively lower levels of endogenous opioids, motivating them to initiate contact with other people. Social contact then leads to feelings of euphoria and contentment, but individuals also need to continue contact to avoid withdrawal symptoms. Liebowitz originally argued that romantic relationships resemble narcotic addiction and that individual neurochemical differences could also explain why some people are unable to commit, or stay in abusive relationships. Earlier experiments on BOTSA were animal studies, but in the 2000s this has been expanded to include human experiments. Among the animal studies which have been done, there is some evidence that separation distress is akin to opioid withdrawal. Studies on chicks, puppies, Guinea pigs, rats, sheep, and monkeys have shown that administrating morphine reduces distress vocalizations when separated from the mother, and administrating naloxone (an opioid antagonist) increases them, even in the presence of other members of the same species. In another study, mutant mouse pups with a μ-opioid receptor knockout (lacking the μ-receptor gene) vocalized less frequently in response to isolation than normal mice. Administration of morphine had no effect on distress vocalization frequency in the knockout mice, despite reducing it in normal mice. Furthermore, these knockout mice had a reduced preference for their mother's odor, which is normally the result of conditioning mediated by the endogenous opioid system. In nonhuman primates, studies have suggested that endogenous opioids provide the euphoria behind dyadic social grooming behaviors. Other animal studies have shown that endogenous opioids play a role in the desire for rough-and-tumble play (a physical, but also social behavior). In humans, physical activity with a social element (rowing, dancing, laughing) increased pain tolerance more when the activities were synchronized with other people. An fMRI experiment in 2010 investigated whether viewing a picture of a romantic partner could reduce pain sensitivity, and which areas of the brain became active. Participants were exposed to high temperatures (resulting in moderate or high pain levels) while viewing a picture of a romantic partner (whom they were intensely in love with), or a friend, or performing a word association task which has also been shown to reduce pain via distraction. Participants were then asked to rate how much pain they felt on a pain scale, and both viewing a romantic partner and performing the distraction task (but not viewing a friend) were found to reduce pain levels. The fMRI scans revealed that viewing a romantic partner activated reward circuits in the brain, while the distraction task did not. Brain areas were also correlated with pain relief to reveal that reward analgesia and distraction analgesia involved distinct areas. Some areas associated with sensory processing of pain also had decreased activity while viewing a romantic partner. An earlier experiment showed that viewing photos of a romantic partner reduced experimental pain, but did not pair it with a brain scan. A PET scan experiment in 2016 investigated whether non-sexual social touching between romantic partners was mediated by endogenous opioid activity. This study found that social touch did have an effect, but unexpectedly found that social touching decreased opioid activity in the brain rather than increasing it (despite being rated as pleasurable by participants). This is in contrast with prior PET research that pleasant affect is related to increased opioid activity. One possible explanation is that touching decreases stress, so this might also decrease the ongoing opioid activity in response to distress and pain. As this is also at odds (to