The Neurobiology of Love and Pair Bonding from Human and Animal Perspectives

Abstract

Love is a powerful emotional experience that is rooted in ancient neurobiological processes shared with other species that pair bond. Considerable insights have been gained into the neural mechanisms driving the evolutionary antecedents of love by studies in animal models of pair bonding, particularly in monogamous species such as prairie voles (Microtus ochrogaster). Here, we provide an overview of the roles of oxytocin, dopamine, and vasopressin in regulating neural circuits responsible for generating bonds in animals and humans alike. We begin with the evolutionary origins of bonding in mother-infant relationships and then examine the neurobiological underpinnings of each stage of bonding. Oxytocin and dopamine interact to link the neural representation of partner stimuli with the social reward of courtship and mating to create a nurturing bond between individuals. Vasopressin facilitates mate-guarding behaviors, potentially related to the human experience of jealousy. We further discuss the psychological and physiological stress following partner separation and their adaptive function, as well as evidence of the positive health outcomes associated with being pair-bonded based on both animal and human studies.

Keywords: dopamine; oxytocin; pair bond; prairie vole; romantic love; vasopressin.

Figure 1

Techniques available for dissecting the neural underpinnings of behavior and emotion. (a) Rodent research is enhanced by the utilization of invasive techniques and behavioral assays that are not available for human use. Site-specific manipulations or recordings, such as chemogenetics, optogenetics, electrophysiology, pharmacological infusion, or viral knockdown, aid in determining the function of specific cellular populations. Post-mortem histological analysis allows for the visualization, segmentation, and quantification of neural substrates, including receptors and fibers, along with activity-induced protein expression. A wealth of assays have been developed to test for a range of behaviors and processes under experimentally controlled conditions, for example, the partner preference test (PPT) used to test the strength of pair bonds in prairie voles. Together, these techniques used in rodents allow for greater understanding of the neural mechanisms underlying processes such as pair bonding that can be translated to human work. (b) Research in humans involves less invasive techniques to dissect the neural networks underlying bonding. Neuroimaging through functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG) allows for visualization of neural activity in response to experimentally-presented stimuli. fMRI and EEG can be paired with self-report, physiological, and psychological measures from subjects, providing further context for changes in neural activity captured through neuroimaging. Figure created with BioRender.com (accessed 4 June 2023).

Figure 2

Comparative processes of bonding. (a) Rodent and human bonding are driven by shared processes across species. Mother–infant bonds are the evolutionary antecedent for pair bonding, relying on selective recognition of infant stimuli and persistent motivational drive to care for offspring. (b) Recognition of and attraction to potential mates in rodents uses primarily auditory and olfactory signals, whereas, in humans, potential partners are initially evaluated primarily based on visual information. (c) For rodents and humans alike, mating facilitates pair bond formation; however, human sexual behavior is complex, with individuals engaging in sexual activity for a variety of reasons, including for pleasure and to strengthen connections. (d) Once a bond has formed, monogamous prairie voles will prefer to mate and spend time with their partner, as well as engage in biparental care. Pair bonding in humans, referred to as “being in love” is accompanied by similar affiliative behaviors. (e) Long-term bonds take work to maintain, especially in the face of other potential mates. Prairie voles engage in mate guarding and have decreased receptiveness to extra-pair bond formations, allowing for bonds to remain stable over time. Humans experience feelings of jealousy and devalue the attractiveness of other individuals, both contributing to long-term bond maintenance. (f) The loss of a partner results in physiological and psychological risk factors across species. Rodents experience a reduction in oxytocin (OT) release accompanied by an increase in corticotropin-releasing factor (CRF), as well as elevated heart rate (HR). Loss of a loved one in humans results in elevated cardiovascular (CV) risk, increased experiences of anxiety or depression, and overall feelings of grief. Figure created with BioRender.com (accessed 8 June 2023).

Figure 3

Comparison of brain regions involved in pair bonding and love across species. Similar brain regions are involved in pair bonding and romantic love across species. In rodents, the involvement of neural circuits and neuromodulators can be studied, whereas, in human fMRI, nodes of activation and deactivation are examined. (a) In prairie voles, oxytocin (OT) produced in the paraventricular nucleus of the hypothalamus (PVN) is released to medial prefrontal cortex (mPFC) and nucleus accumbens (NAc). Neural entrainment of the mPFC and NAc predicts pair bond formation. Representations of the partner are likely encoded by cells in the hippocampus (HPC) and the extended amygdala (AMY), regions that project to the NAc and may contribute to the rewarding properties of partner-related stimuli. The ventral tegmental area (VTA) releases dopamine (DA) to the NAc. Vasopressin (AVP) is released from the AMY to the ventral pallidum (VP) to facilitate pair bonding. (b) In humans, the NAc and VTA are activated upon viewing images of one’s partner, both regions in the mesolimbic DA system, and this activation is enhanced by IN-OT. Dotted lines represented inferred circuits based on neuroimaging studies in conjunction with IN-OT administration. Other regions activated include the insular cortex (IC), anterior cingulate cortex (ACC), and substantia nigra (SN). Deactivation is seen in the AMY and PFC. Figure created with BioRender.com (accessed 4 June 2023).