Romeo and Juliet died for it. Plato contemplated it. And, it has inspired musicians and artists throughout history. But what happens in the brain when we fall in love, and what keeps some couples together while others fall apart?
To understand the neural basis of adult partnerships, researchers study prairie voles, a rare example of a species that mates for life.
Scientists study the brains of monogamous animals and people who report being “in love” in hopes of better understanding the neurobiology underlying partnerships.
Basis for long-term attachment
Much of what scientists know about the neural basis for long-term attachment stems from decades of work studying the bonds that form between animals. For clues about the partnerships between adults, many scientists study the prairie vole, a small North American rodent that forms monogamous lifelong bonds — a rare occurrence in the animal world.
In the lab, scientists test the fidelity of voles after they've mated by measuring the amount of time one partner chooses to spend with the other when a new vole of the opposite sex is present. Prairie voles spend more time with their partners than with the strangers during the test. However, species of voles that fail to form monogamous bonds, such as the meadow vole, prefer the novel animal.
Studies examining the brains of prairie and meadow voles show that, in both types of voles, sex activates the ventral tegmental area, a group of nerve cells located at the base of the brain. In response to something that makes you feel good (e.g., food, sex, or drugs), these cells release a chemical called dopamine, triggering the brain’s reward system.
Brain chemicals promote fidelity
In female prairie voles (the monogamous ones), sex also leads to the release of the hormone oxytocin in the reward pathway. Oxytocin, which stimulates birth and milk production in mothers, has long been associated with the bond that forms between a mother and an infant. According to Larry Young, a neuroscientist at Emory University who studies the mechanisms underlying social attachment in voles, the interaction between oxytocin and dopamine in the brain’s reward region in female voles likely increases the desire to spend more time with her partner.
In male prairie voles, the hormone arginine vasopressin, which is involved in aggression and territorial behavior, also appears to play an important role in pair-bonding.
Multiple studies show scientists can increase or decrease the monogamous and promiscuous voles’ interest in partners by manipulating the amount of oxytocin and vasopressin in the brain. While scientists have yet to come up with the perfect chemical cocktail for fidelity in people, neuroscientists say the knowledge that oxytocin, vasopressin, and dopamine play a role in pair-bonding in animals may indicate what is happening in people.
Recent genetic studies show men who have a specific form of the gene that codes for vasopressin receptors are less likely to be married and report more relationship problems than other men. Although scientists caution such data cannot be used to predict how a person will behave in a relationship, the findings suggest the mechanisms involved in bond formation between voles may be relevant to people.
“I believe the mechanisms involved in the formation of long-term bonds are evolutionarily antecedent to love,” Young says. “Human love is more complex. But, I think, at the foundation, the same mechanism is involved in human love.”
Romantic love triggers reward system
Scientists have started to explore the brain activity of people who report being “in love” using a technique called functional magnetic resonance imaging (fMRI). By comparing the pattern of brain activity in the participants as they stare at a picture of their beloveds to the activity of the brain when staring at a picture of a close friend, a brain pathway begins to emerge.
While many brain regions are active when the participants think about their partners, the area that stands out as being most active during the fMRI is the ventral tegmental area, consistent with what researchers see in monogamous prairie voles. “This brain region is responsible for wanting, for craving, for obsession,” says Helen Fisher, a biological anthropologist at Rutgers University, who together with Lucy Brown, a neuroscientist at Einstein College of Medicine, and others, has scanned the brains of an estimated 100 people in love.
The data from the team’s fMRI studies suggest that when you are in love, the brain’s reward system drives you to want to be with your partner, Brown said. Now, scientists are starting to examine how the brains of people in long-term partnerships compare with people in early-stage romantic love.
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