Biology of romantic love

The biology of romantic love has been explored by such biological sciences as evolutionary psychology, evolutionary biology, anthropology and neuroscience. Specific chemical substances such as oxytocin and dopamine are studied in the context of their roles in producing human experiences, emotions and behaviors that are associated with romantic love.

Definition of romantic love
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."

Contemporary literature also draws distinctions between romantic love and the attachment system theorized by attachment theorists such as John Bowlby. Romantic love is related to or involves attachment over time, but is thought to also involve additional brain systems.

Role of courtship attraction
Courtship attraction is a phenomenon exhibited by many species which serves the role of mate choice, or intersexual selection. This refers to physical and behavioral characteristics, as well as brain systems which motivate an individual to pursue a preferred mating partner. Courtship attraction shares similar behaviors with romantic love in humans, and both involve activation of dopaminergic reward circuits. In most species, courtship attraction lasts only minutes, hours, days or weeks, but romantic love in humans can last much longer, 12–18 months or more.

It has been proposed that courtship attraction and romantic love are related phenomena, either that romantic love evolved as a form of courtship attraction, or that courtship attraction is one of several interrelated components.

Independent emotions theory
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.
 * Attraction (also called romantic love or passionate love) is associated with feelings of exhilaration, intrusive thinking and the craving for emotional union.
 * Attachment (also called companionate love) is associated with feelings of calm, security and comfort, but separation anxiety when apart.

The team suggests that these systems tend to act in unison, but may become disassociated. For example, a person in a long-term partnership may feel attachment for their spouse, but become attracted to somebody else. Lisa Diamond has argued based on independent emotions theory and other evidence that people can 'fall in love' without sexual desire, even in contradiction to their sexual orientation.

Adam Bode has suggested Fisher's model, while useful and the predominant one for a time, is oversimplified and proposes five systems:


 * Sexual desire, similar to what Fisher called lust.
 * Courtship attraction, for selecting a preferred mating partner.
 * Bonding attraction, characterized by a strong desire for proximity.
 * Obsessive thinking, involving preoccupation or intrusive thinking about the loved one.
 * Attachment, for maintaining very close personal relationships.

Bode suggests that the systems of bonding attraction, obsessive thinking and attachment together comprise the core of romantic love. He suggests these three systems evolved by co-opting brain systems originally for mother-infant bonding, and courtship attraction and sexual desire are causally linked adjuncts.

Co-option theory
Academic literature has drawn a parallel between romantic love and the mother-infant dyad since the 1980s. In 1998, Leckman & Mayes compared features of romantic love and early parental love, and suggested they share common neurobiologic systems. Leckman & Mayes speculate that there is an evolutionary reason behind the similarity, asking "If some part of the behavioral repertoire works in one context to help create and sustain a relationship, why not borrow it to form another?" In 2004, Bartels and Zeki compared romantic love and maternal love with fMRI.

Lisa Diamond has suggested that adult pair bonding is an exaptation. Adam Bode writes that "exaptation is when a trait retains its original form but takes on a new function; co-option is the process whereby any trait takes on a new function, regardless of whether the original form is retained or not."

Bode has proposed that romantic love evolved by co-opting mother-infant bonding. The co-option theory says that the genes that regulate mother-infant bonding were recreated and took on a new function. The theory is based on a literature arising from research on prairie voles that pair bonding uses the same mechanisms that mother-infant bonding use as well as the available human evidence. The theory was used to critique a previously asserted evolutionary theory of romantic love proposed by Helen Fisher, that romantic love is a form of courtship attraction. Bode's theory explains not only one process in the emergence and subsequent evolution of romantic love, but also proposed a new model of the mechanisms of romantic love.

Evolutionary psychology
Evolutionary psychology has proposed several explanations for love. Monkey infants and children are for a very long time dependent on parental help. Love has therefore been seen as a mechanism to promote mutual parental support of children for an extended time period. Another is that sexually transmitted diseases may cause, among other effects, permanently reduced fertility, injury to the fetus, and increase risks during childbirth. This would favor exclusive long-term relationships reducing the risk of contracting an STD.

From the perspective of evolutionary psychology the experiences and behaviors associated with love can be investigated in terms of how they have been shaped by human evolution. For example, it has been suggested that human language has been selected during evolution as a type of "mating signal" that allows potential mates to judge reproductive fitness. Since Darwin's time there have been similar speculations about the evolution of human interest in music also as a potential signaling system for attracting and judging the fitness of potential mates. It has been suggested that the human capacity to experience love has been evolved as a signal to potential mates that the partner will be a good parent and be likely to help pass genes to future generations. Biologist Jeremy Griffith defines love as 'unconditional selflessness', suggesting utterly cooperative instincts developed in modern humans' ancestor, Australopithecus. Studies of bonobos (a great ape previously referred to as a pygmy chimpanzee) are frequently cited in support of a cooperative past in humans.

Neurochemistry
There have been fewer than 45 biological studies of romantic love. One view in biology is that there are three major drives in love – libido, attachment, and partner preference. The primary neurochemicals (neurotransmitters, sex hormones, and neuropeptides) that govern these drives are testosterone, estrogen, dopamine, oxytocin, and vasopressin.

Central dopamine pathways mediate partner preference behavior, while vasopressin in the ventral pallidum and oxytocin in the nucleus accumbens and paraventricular hypothalamic nucleus mediate partner preference and attachment behaviors. Sex drive is modulated primarily by activity in the mesolimbic dopamine pathway (ventral tegmental area and nucleus accumbens). Trace amines (e.g., phenethylamine and tyramine) play a critical role in regulating neuronal activity in the dopaminergic pathways of the central nervous system.

Testosterone and estrogen contribute to these drives by modulating activity within dopamine pathways. Adequate brain levels of testosterone seem important for both human male and female sexual behavior. Norepinephrine and serotonin have a less significant, contributing role through their neuromodulatory effects upon dopamine and oxytocin release in certain pathways.

The chemicals triggered that are responsible for passionate love and long-term attachment love seem to be more particular to the activities in which both persons participate rather than to the nature of the specific people involved. There is mixed evidence about the role of cortisol in romantic love.

A less speculative account of the neurochemistry of romantic love was provided by Bode & Kushnick in 2021. While they recognize that other factors play a role, they summarize the available human endocrinological evidence that implicates testosterone, dopamine, oxytocin, serotonin, cortisol, and nerve growth factor in romantic love. There is no evidence that estrogen and norepinephrine play a major role in romantic love (although they might) and only neuroimaging and genetic evidence of vasopressin's involvement.

Role of the limbic system
The role of the limbic system in emotion was first explained by James Papez in 1937 within his paper titled "A proposed mechanism of emotion". The model described is known as the Papez circuit. The Papez circuit highlighted the presence of neuronal pathways between the vestibular and the limbic system. The vestibular apparatus is in the inner ear and coordinates the body balance and movement. This requires extensive neuronal networking. Vestibular stimulation, which comes from the apparatus, can cause changes in mood and emotion. It can also impact emotions either independently or as part of the general limbic system networks by influencing the hypothalamus. These emotions can include extreme passivity, loss of drive/motivation, excessive eating and drinking, and rage and violent behavior. Studies show Romantic Love uses reward and motivation systems to focus on a specific individual. The limbic cortical regions process individual emotion factors. In A General Theory of Love, three professors of psychiatry from UCSF provide an overview of the scientific theories and findings relating to the role of the limbic system in love, attachment and social bonding. They advance the hypothesis that our nervous systems are not self-contained, but rather demonstrably attuned to those around us and those with whom we are most close. This empathy, which they call limbic resonance, is a capacity which we share, along with the anatomical characteristics of the limbic areas of the brain, with all other mammals. Their work builds on previous studies of the importance of physical contact and affection in social and cognitive development, such as the experiments conducted by Harry Harlow on rhesus monkeys, which first established the biological consequences of isolation.

Brain imaging
Brain scanning techniques such as functional magnetic resonance imaging have been used to investigate brain regions that seem to be involved in producing the human experience of love.

In 2000, a study led by Semir Zeki and Andreas Bartels of University College London concluded that at least two areas of the brain become more active when in love. These were foci in the media insula, which the brain associates with instinct, and part of the anterior cingulate cortex, which is associated with feelings of euphoria.

Ortigue et al. found that an unconscious prime of the name of a romantic partner activated similar brain regions as when subjects were consciously aware of seeing partners' faces. Subliminal priming with either a beloved's name or a favorite hobby activated emotion and motivational brain regions: caudate nucleus, insula, bilateral fusiform regions, parahippocampal gyrus, right angular gyrus, occipital cortex, and cerebellum. However, the love prime evoked more activation in bilateral angular gyri and bilateral fusiform regions than the hobby prime. These regions are associated with integrating abstract representations, and the angular gyrus in particular is involved with abstract representations of the self. The authors also found a correlation (r=0.496, p=0.002) between activation of a region of the angular gyrus with a passionate-love scale measuring subjective feelings of love.

Love and motivation
Conscious thoughts about a romantic partner activate brain regions related to reward and motivation. Ortigue et al. investigated whether unconscious priming by a partner's name could also affect motivation. They found that priming by either a beloved or a favorite hobby improved reaction times in identifying whether a string of letters was a word or not compared against priming by a neutral friend. The authors suggest this effect happens because a beloved's name "may call for a goal-directed state" and produce "dopaminergic-driven facilitation effects." Similarly, the love one feels for their friends may also be biologically motivated. Isern-Mas and Gomila argue that while the love we feel for our friends is not romantic, it is still motivated through feelings of moral obligations as well as changes in the brain resulting from prosocial experiences. The common motivation whether it be love romantically or through a non-intimate companion can be connected to positive feelings and rewards that in turn, form social bonds. As seen in other animals as well, the immediate connections between the love of a mother and their infant impacts their personality as they age. Harlow described love as a secondary drive for all animals, but it is essential for proper development. The animals that were left abandoned, had trouble socializing with others and often had personality issues as well. The Behavioral Activation System (BAS), which plays a role in directing behavior, is believed to play a role in romantic love.