Falling in love is one of the best feelings in the world—and humans might not be the only creatures who do it. Pair bonding, the two-by-two partnering of creatures, has been seen across the animal kingdom. Whether or not a nesting pair of robins can be said to truly love each other, we're still awfully interested in why animals might pair off. The tools available to biologists have advanced immensely in the last few decades, and some are using that technology to decipher physiology involved in both pair bonding and love.
To unravel the mysteries behind pair bonding, researchers studied not robins, but voles. Prairie voles and montane voles are closely related rodents with a stark difference in mating behavior: prairie voles form lifelong pair bonds after mating, while montane voles are promiscuous. The work of Thomas Insel, director of the National Institute of Mental Health (NIMH), and colleagues showed that the different mating behaviors can be linked to the hormones oxytocin and vasopressin. Oxytocin stimulates childbirth and lactation, while vasopressin regulates the kidneys and constricts blood vessels. More recent research has implicated the two hormones in many different social behaviors, from working cooperatively to selecting mates to inferring the emotions of others. Oxytocin in particular has calming effects and seems to help build trust between people.
The crucial physiological difference between these two species of vole is in the distribution of oxytocin and vasopressin receptors in the voles' brains. Prairie voles have a higher density of both types of receptors in the amygdala, the area of the brain involved in emotion-related memory formation, and in various parts of the dopamine reward system. Both hormones are released when prairie voles mate, prompting pair bonds to form. When the hormone release is blocked, prairie voles become promiscuous. Perhaps more tellingly, when montane voles are genetically modified to have prairie vole-like distributions of vasopressin receptors, they become monogamous.
Dopamine is an important part of the picture as well. The release of dopamine gives a pleasurable feeling, and is used in the brain to reward behaviors such as procreating or eating a hearty meal. Voles that are made more sensitive to dopamine can develop partner preferences without mating—a friendly encounter will do. Voles whose dopamine receptors are blocked do not form partner preferences at all. Many of the dopamine receptors and pathways responsible for pair bonding in voles are also involved in cocaine addiction in rodents. This suggests a possible explanation for the addictive feeling of love.
This is your brain on Love
In humans, Donatella Marazziti, Professor of Psychiatry and Director of the laboratory of Psychopharmacology at the University of Pisa, has found that early stages of romance are linked with diminished levels of the neurotransmitter serotonin and of a serotonin receptor. These two molecules are also depleted in obsessive-compulsive disorder. Since both conditions (to different extents) also give rise to feelings of anxiety and obtrusive thinking, it is tempting to think of early love as a mild, temporary form of obsessive behavior. Early romance is also characterized by higher levels of several different molecules related to stress response. Twelve to 18 months into a relationship, both serotonin and the stress molecules are restored to normal levels.
Researchers like Helen Fisher of the Department of Anthropology at Rutgers University and Semir Zeki of UCL in London have done functional MRI studies to identify the regions of the brain that are activated or deactivated by romantic love. They've found that people in romantic love show increased activity in a number of different regions of the brain that are involved in the dopamine reward system. Parental love activates most of the same regions but not the hypothalamus, suggesting that the hypothalamus may be responsible for the sexual component of romantic love.
Areas that show reduced activity include the amygdala and the frontal and prefrontal cortecies. The amygdala is associated with fear and aversive learning, or learning from one's mistakes. The frontal and prefrontal cortecies are associated with the executive functions of analysis and judgment, delayed gratification, and predicting the outcomes of events. We can speculate that diminished activity in these regions explains why lovestruck persons do not seem to have full grasp of these particular functions. Fisher summarizes some of her findings magnificently in her 2008 TED talk.
The physiology of love is not totally understood, and research is ongoing. Researchers have begun to explain the pair bonding of animals such as voles, and have identified patterns of hormones and brain activity that show up among people in romantic relationships. What physiology tells us about love is what we knew all along—that it is a stressful matter bordering on an addiction or an obsessive disorder, that it muddles one's judgment and leads one to act rashly, and that as it grows, these disturbances fade and give way to calm and joy.