Human bonding is the process by which close, enduring interpersonal relationships develop between two or more people. These selective and long-lasting attachmentsâÂÂwhich include parent-child bonds, romantic pair bonds, and friendshipsâÂÂare fundamental to human survival, development, and well-being. Modern neurobiology views bonding not just as a psychological experience, but also as a set of motivational and emotional states rooted in ancient, conserved mammalian brain systems.
It is often said that man is a social animal. Human bonding encompasses a wide range of affiliative behaviors, including parentâÂÂchild bonding, romantic attachment, friendship, and kinship ties. With technical progress, scientists are now able to examine the neurobiology of the various forms of bonding that enter into play in human societies. These neurobiological mechanisms of connection have played an essential role in survival and social cohesion in the context of human evolution. Such mechanisms are also partially conserved in other mammalian species, though humans exhibit unique complexities due to language, culture, and cognition.
The neurobiological machinery for human attachment is thought to have evolved from the primal mother-infant bond, which is essential for the survival of altricial (helpless at birth) mammalian young. Under some theories, the human system of maternal love was later repurposed through evolution to facilitate pair bonding (monogamy) and possibly other types of attachment. A meta-analysis of neuro-imaging studies states that evidence âÂÂsupports the notion that maternal and passionate love share a common evolutionary origin and neurobiological basis in the human brain.âÂÂ
For example, humanity's broad tendency to develop monogamous male-female pair bonds may trace its origins to the mechanisms for parent-child bonding (though there are wide variations across individuals and cultures regarding monogamy in practice). One neurobiological study using fMRI compares maternal and romantic love. It finds that both types of love "activate [brain] regions that are specific to each, as well as overlapping regions in the brain's reward system that coincide with areas rich in oxytocin and vasopressin receptors. Both [types of love] deactivated a common set of regions associated with negative emotions, social judgement and 'mentalizing', that is, the assessment of other people's intentions and emotions. [The authors] conclude that human attachment ... overcomes social distance by deactivating networks used for critical social assessment and negative emotions, while it bonds individuals through the involvement of the reward circuitry... ." These neurological mechanisms are also thought to promote broader social affiliations, thereby enhancing human ability to function in groups.
Human bonding is governed by an integrated "cocktail" of neuropeptides, neurotransmitters, and hormones:
Oxytocin is often dubbed the "love hormone,". It is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. It is involved in social-affiliation processes, with effects modulated by context and receptor distribution. It is thought to be critical for promoting prosocial behaviors, trust, and feelings of closeness. More specifically, it plays a role in:
In the area of pair bonding, oxytocin works by dampening the activity of stress-related circuits (like the amygdala) and increases activity in reward-related areas, promoting a state of "immobility without fear" that is conducive to social engagement.
Vasopressin, another neuropeptide related to oxytocin, is thought to play a key role, particularly in males, in regulating social and pair-bonding behavior. In addition to its other functions, vasopressin is released directly into the brain from the hypothalamus, and is thought to play an important role in social behavior, sexual motivation and pair bonding, and maternal responses to stress. In close interaction with oxytocin, vasopressin seems to have the following effects:
Dopamine is the primary neurotransmitter of the brain's reward and motivation system, the mesolimbic pathway. Although dopamine is often portrayed as the main chemical of pleasure, scientists now believe that dopamine instead confers motivational salience; in other words, dopamine signals the perceived motivational prominence (i.e., the desirability or aversiveness) of an outcome, which in turn propels the organism's behavior toward or away from achieving that outcome.
For that reason, it is heavily implicated in the phase of intense attraction and early love. When an individual is "in love," viewing or thinking about the partner activates major dopamine-rich areas, including the Ventral Tegmental Area and the Nucleus Accumbens. This activation creates feelings of euphoria, intense motivation, and focused attention, similar to the neurochemical response seen in addiction. The interaction of dopamine with oxytocin is proposed to link the rewarding sensation (dopamine) with the specific social stimuli of the partner (oxytocin), enabling the individual to focus their motivational drive exclusively on the chosen partner.
Serotonin is a monoamine neurotransmitter with a wide range of functions in both the central nervous system and also peripheral tissues. It is involved in mood, cognition, reward, learning, memory, and several physiological processes. It often shows fluctuating levels during the early, intense stage of romantic attraction. Studies suggest that serotonin levels in people newly in love can resemble those found in individuals with obsessive-compulsive disorder. This may account for the intrusive, all-consuming thoughts and idealization associated with early infatuation. As relationships transition to long-term attachment, serotonin levels typically normalize.
Functional neuroimaging studies (fMRI and PET) in humans have identified several brain regions that show consistent changes in activity when subjects are exposed to cues of their attachment figures (e.g., viewing a photo of a romantic partner or child). The formation and maintenance of social bonds are mediated by networks in the limbic system and reward pathways, including:
Functional neuroimaging studies have shown that these regions seem to be activated during both the anticipation and experience of social connection, suggesting that bonding engages mechanisms similar to those involved in other rewarding experiences.
Biological anthropologist Helen Fisher published a "groundbreaking study that include the first functional MRI (fMRI) images of individuals in the throes of romantic love." The study posits 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 or behaviors. Fisher also includes the attraction to personality traits and other characteristics in her mate choice theory for humans.
In her book, Why We Love': The Nature and Chemistry of Romantic Love, Fisher theorized that romantic love in humans can be divided into three distinct, yet interrelated, neurochemical systems:
Understanding the neurobiology of bonding has implications for clinical psychology and psychiatry. It can elucidate both the biological mechanisms of certain disorders, including:
Exogenous administration of oxytocin is being explored in clinical settings with a view to enhancing social functioning and empathy and reducing anxiety in various patient populations. Nasally administered oxytocin has been reported to reduce fear, possibly by inhibiting the amygdala (which is thought to be responsible for fear responses).