No matter what you've heard, you don't love anything with all of your heart. You love from the depths of your ventral tegmental area, your hypothalamus, your nucleus accumbens, and other vital areas of the brain. In the last two decades, scientists have joined the throngs of poets, philosophers, artists and others striving to comprehend the ways of love. Scientific techniques for exploring how the brain experiences love ranges from animal experiments to traditional surveys to advanced radiological techniques, such as functional magnetic resonance imaging (fMRI) and positron emissive tomography (PET).
According to Dr. Helen Fisher, one of the preeminent researchers in the field of human affections, love can be divided into three major systems of the brain: sex, romance, and attachment. Each system involves a different network within the brain, involving different constituents, hormones, and neurotransmitters at different stages in the relationship.
The Sex Drive
Lust stems predominantly from the hypothalamus, a region of the brain that also controls such basic desires as hunger and thirst. The hypothalamus is closely tied to the autonomic nervous system that controls our heart rate and how fast we breathe. Specific receptors on the hypothalamus for hormones such as testosterone -- which exists in you too, ladies -- fire off connections to all kinds of physical reactions. The result is a strong, familiar drive for reproduction.
The Romance System
This is the culprit behind many an all-night poetry fit. This is the reason lovers fight armies, swim oceans, or walk hundreds of miles to be together. In a word, they're high. Imaging studies confirm new lovers have high amounts of activity in the ventral tegmental area and nucleus accumbens, the same reward systems that fire off in response to inhaling a line of cocaine. These regions are flooded with the neurotransmitter dopamine, a chemical that drives us toward a perceived reward. Other chemicals related to stress and excitement are elevated as well, such as cortisol, phenylephrine (found in chocolate), and norepinephrine. A neurotransmitter called serotonin is low in early romantic love. Serotonin can also be low in obsessive compulsive disorder, depression, and anxiety. The result is obsessive pursuit of the desired, a relentless optimism, and even a kind of addiction.
The Affection System
This is why some people stick together when the dopaminergic thrill is gone. In animals, the responsible chemicals are oxytocin and vasopressin. Interestingly, these calming chemicals are secreted by the same hypothalamus that fuels our lust.
Some may see the above systems as a kind of progression in a relationship. First lust ("hey, he or she is cute"), then romance ("I'll write a love song"), then marriage (calmer and cozier). While it's true that these aspects of our brains and our relationships change over time, it's important to remember that they never dwindle to nothing, and often interact in important ways. For example, oxytocin and vasopressin are connected with the dopamine reward system as well. Perhaps that's why it's a good idea to refresh the romance now and then so affection can bloom.
Heartache or Headache?
Relationships change. Sometimes they evolve into something that lasts forever, and usually they don't. Most of us date prior to marriage, going through a string of relationships prior to meeting "the one." And sadly, it's not uncommon that "the one" becomes an ex-spouse.
Researchers who have taken pictures of the brain in people who have just gone through a break-up show changes in the ventral tegmental area, ventral pallidum, and putamen, all of which are involved when a reward is uncertain. While this might be reading too much into the study, uncertainty is certainly common after a break-up. Areas in the orbitofrontal cortex involved with obsessive-compulsive behaviors and in anger control also light up initially, though this extra activity may fade over time. In 2011, researchers published functional MRI findings suggesting that the brain does not distinguish between the pain of social rejection and the pain of physical injury, though these results and methods have been called into question. Not surprisingly, changes in other neural networks involved with major depression have also seen after a break-up.
How and if evolution has helped to shape human mating habits is a topic that frequently leads to lively debate. For example because men produce millions more sperm than women produce eggs, there is a theory that the mating strategy of women will be more focused on protecting and nurturing the relatively few reproductive opportunities she has, whereas men are "pre-programmed" to spread their seed far and wide.
However, this theory is probably simplistic, as it fails to account for a number of other factors. For example, in species where nurturing a newborn requires parental cooperation, monogamy becomes more common. Dr. Helen Fisher has proposed a "four-year" theory, which attributes a spike in divorce rates in the fourth year of marriage to the notion that this is when a child has passed through the most vulnerable phase of their youth and can be cared for by one parent. The "four-year" theory is somewhat flexible. For example, if the couple has another child, the time period may be extended to the infamous "seven-year itch."
None of this, however, explains those enviable couples who walk hand in hand together through their entire lives into the twilight of their years. It's also important to remember just how complicated the topic of human affection is. Our culture, our upbringing, and the rest of lives help to change those chemicals and networks. Love's complexity means that questions about the nature of love will continue to fascinate poets, philosophers, and scientists for many years to come.
A. de Boer,E.M. van Buel,G.J. Ter Horst, Love is more than just a kiss: a neurobiological perspective on love and affection, Neuroscience Volume 201, 10 January 2012, Pages 114-124
Kross E, Berman MG, Mischel W, Smith EE, Wager TD (2011) Social rejection shares somatosensory representations with physical pain. Proc Natl Acad Sci USA 108:6270-6275. Abstract/FREE Full Text
Helen E Fisher, A Aron, D Mashek, H Li, LL Brown. Defining the brain systems of lust, romantic attraction and attachment. Archieves of Sexual Behavior, October 31, 2002. (5): 314-9