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Written by Brett Weiss

October 2019

            Throughout human history, love has inspired the work of artists, musicians, and writers.  In fact, biologists consider the arts, when associated with biological phenomena such as love and reproduction, part of an adaptational process that ensures survival (Esch & Stefano, 2005).  The emotional rush and feelings of well-being which come from love constitute life experiences which most people never forget and which they yearn for.  The following will review some of the current research on the neurobiology of love.

            Research suggests that romantic love in humans evolved from courtship behavior of lower mammals (de Boer et al., 2012; Fisher, 1998).  Courtship behavior in lower mammals shares characteristics with romantic love in humans: increased energy, obsessive following, focused attention, affiliative gestures, mate guarding, and motivation to win the preferred mate (de Boer et al., 2012).  Courtship behavior and romantic love are both evolutionary mechanisms whereby partner choice gives the offspring the best chances.

            The adult attachment system in humans includes romantic love.  The adult attachment system evolved as a mechanism to keep partners together long enough for child rearing (de Boer et al., 2012; Fisher, 1998).  In many species, these bonds last only one mating season.  Whether life-long, monogamous bonds form in humans remains a sensitive and hotly contested debate with divorce rates approaching 50% in Western societies (de Boer et al., 2012).  Interestingly, though, Fisher (1992) studied divorce rates across many cultures and found that divorce rates increase substantially in the fourth year of marriage.  These data led her to develop a “four-year itch” theory, which states that human pair-bonds form for approximately four years, the time when offspring are most vulnerable.  After these four years, the romantic attachment can dissolve, allowing parents to form attachments with other individuals.  Hence, Fisher (1992) surmised that the human mating system constitutes ‘serial monogamy,’ not life-long attachment.  Fisher also found that the pair-bond window can extend to seven years if the couple has more than one child, which would demand that parents cooperate longer to care for the second child.

            Romantic love of the adult attachment system most likely evolved from an evolutionarily much older system, the mother-infant attachment (de Boer et al., 2012).  The mother-infant attachment forms around birth and lasts for as long as the infant cannot function independently.  Romantic love and mother-infant love share something in common in that they both evolved to keep two people together for a certain period of their lives (de Boer et al., 2012).  These two forms of love also share neural and hormonal substrates.  In fact, the main distinction between maternal love and romantic love comes from activation of the hypothalamus and subsequent sexual arousal in romantic love, which does not occur in maternal love.

            The loaded term ‘love’ represents attachment, commitment, passion, intimacy, jealousy, and grief upon separation (Esch & Stefano, 2005).  In the early phases of a loving bond between mating partners, stress and cortisol release ensues with symptoms of sweating, increased heart rate, and even diarrhea (Esch & Stefano, 2005).  Research points to stressors triggering search for pleasure, closeness, and proximity.  Hence, some degree of strong stress may be required for strong bond formation between partners.  The first phase in a relationship where stress ensues lasts about six months and entails “being in love.”  Other characteristics of this phase include high passion, intimacy, and increased commitment (de Boer et al., 2012).  During this phase, testosterone levels rise in females and decrease in males, indicative of a form of biobehavioral synchronization of the pair (de Boer et al., 2012).  Also, during the “being in love” phase, low serotonin levels circulate throughout the body, which may play a role in obsession with the other partner.  The reduced levels of serotonin that go along with “being in love” show similarities to lowered blood serotonin levels in obsessive-compulsive disorder.  After the first several months, a new stage of “passional love” ensues which includes feelings of safety, calm, and balance.  Hormone and neurotransmitter imbalances (neuroendocrine imbalances) normalize during this phase as passion remains high with intimacy and commitment increasing steadily.  Stress decreases during “passional love,” giving way to several health benefits.  Oxytocin and vasopressin, two hormones, play major roles in pair-bond formation during this period.  Passional love lasts several years before the third phase, “companionate love” comes into play.  “Companionate love” entails reduced passion with intimacy and commitment remaining high.  The loving relationship of this phase is similar to friendships with oxytocin and vasopressin as dominant hormones reinstating and maintaining pair-bonds (de Boer et al., 2012).  The “four-year itch” referred to previously coincides with the end of the second “passional love” phase, indicating that the transition from “passional love” to “companionate love” can be a fragile time in a relationship (de Boer et al., 2012).  Some couples claim to have highly passionate love after 20 years of marriage, which indicates that not all relationships evolve into “companionate love (de Boer et al., 2012).”

            The neurotransmitters and hormones playing key roles in romantic bond formations include oxytocin, vasopressin, dopamine, and serotonin (de Boer et al, 2012; Esch & Stefano, 2005).  Research has utilized the life-long monogamous mammal, the prairie vole, to study these hormone and neurotransmitter systems in love with hopes of applying findings to human research (de Boer et al., 2012).  In prairie voles, oxytocin plays a dominant role in attachment in females, while vasopressin plays a more important role in males (de Boer et al., 2012).  Receptor distribution for oxytocin and vasopressin in the brain differs between sexes in these animals, indicating different mechanisms of romantic attachment between sexes.  With that said, oxytocin increases social contact induction, partner formation, and social bonding while decreasing aggression and anxiety (Esch & Stefano, 2005).  Vasopressin increases positive social behaviors, social attachment, territorial behavior, and sexual behavior (Esch & Stefano, 2005).  Dopamine, a neurotransmitter involved in pleasure and motor movement, interacts with oxytocin and vasopressin signaling (de Boer et al., 2012).  Dopamine, the main signaling neurotransmitter in reward systems of the brain, releases to play a central role in the formation of monogamous pair-bonds (de Boer et al., 2012).  Dopamine systems play crucial roles in addiction; hence, some researchers consider properties of neural activation in love similar to processes of addiction (de Boer et al., 2012).  As previously referred to, early stages of romantic love show similarities to obsessive-compulsive disorder with depletion of serotonin levels in the brain (de Boer et al., 2012).

            Another interesting facet to romantic love entails biobehavioral synchrony, which occurs between partners in romantic bonds.  Biobehavioral synchrony includes four types of synchrony: behavioral, autonomic, hormonal, and brain.  Examples of biobehavioral synchrony between lovers include synchronized nonverbal patterns of behavior, coordinated self-disclosure and empathy, heart rate coordination during and following interaction, coordination of oxytocin and cortisol levels among parents and lovers, coordination of brain response in networks of the brain involved in mentalizing, and coordination of gamma oscillations in the temporal cortex of lovers (Feldman, 2017).  Findings in relation to biobehavioral synchrony give interesting details regarding ways where lovers in a pair-bond do not only mesh with thought patterns and emotion but also with physiological patterns, including behavior, heart rate, hormones, and brain patterns.

            Neural correlates of love found through brain imaging have discovered that brain areas involved in critical assessment during socialization become less activated when lovers interact.  An area of the brain called the amygdala, which plays a prominent role in emotional processing of fear and critical social thinking, has less activation during interactions between lovers.  Another area involved in the network of brain areas of critical social analysis, the frontal lobe, shows less activity as well (Zeki, 2007).  Therefore, these findings lend credibility to the statement, “Love is blind,” as people in love often cannot act as accurate judges of a partner’s character.

            From maternal love to romantic love, human attachment seems to have evolved to enable survival and proliferation of the species.  The hormones and neurotransmitters oxytocin, vasopressin, dopamine, and serotonin, among others, play crucial roles in forming these bonds.  Brain networks which display reduced activity when one ‘falls in love’ make it hard to discern the character of a love partner.  In its initial stages, love can seem like an emotional rollercoaster with increased levels of stress; however, once one has found a stable, loving relationship with a partner, he/she often finds positive health benefits in reduced stress and stabilization of hormone levels.  With love as one of the most important ‘altered states of consciousness’ that people experience, insights into this state of mind can only help to advance knowledge of the brain and human biology.

References

De Boer A, Van Buel EM, & Ter Hortst GJ (2012).  “Love is more than just a kiss: a neurobiological perspective on love and affection.”  Neuroscience.  201: 114-124.

Esch T & Stefano GB (2005).  “The Neurobiology of Love.”  Neuro Endocrinol Lett.  26(3): 175-192.

Feldman R (2017).  “The Neurobiology of Human Attachments.”  Trends Cogn Sci.  21(2): 80-99.

Fisher H (1998).  “Lust, attraction, and attachment in mammalian reproduction.”  Hum Nat.  9:23-52.

Fisher HE (1992).  “Anatomy and love: the natural history of monogamy, adultery and divorce.”  p. 432.  New York: WW Norton & Co.

Zeki S (2007).  “The neurobiology of love.”  FEBS Lett.  581(14):  2575-2579.

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