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. According to the researcher Enzo Emanuele, in light of the comparative lack of neurochemical studies on the early stage of romance, "it would not be surprising that a diversity of biological mechanisms could be involved", due to the complexity of love.
Definition of romantic love
The meaning of the term "romantic love" has changed considerably throughout history, making it difficult to define simply.: "The cultural history of 'romance' and various meanings of the word 'romantic' make it extremely difficult to define 'romantic love'. Academic psychology—usually quite pedantic about its terminology—has been unable to establish a consensus. Some psychologists use the term in accordance with its courtly origins, whereas others use it interchangeably with 'passionate love'. As a culture, we seem to have settled on the latter usage, viewing 'romantic love' and 'falling in love' as much the same thing." Initially it referred to the attitudes and behaviors found in certain literary works, 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:
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 phenomenon of long-term intense romantic love, people can experience intense attraction inside a relationship, even for 10 years or more. This is similar to early-stage intense romantic love, but with less 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, as 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:
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.