Love on the Brain

Scientists peeking inside our brains and psyches have more clues than ever about the biology of love -- why we're attracted, why we fall so hard, and what makes us stay.

Medically Reviewed by Louise Chang, MD on February 04, 2009
8 min read

He's analytical, driven, not very verbal, and not always compassionate.

She's gregarious, intuitive, whimsical, warm, and compassionate.

Before you say "not a chance," hear another view.

It's probably a good match, says Helen Fisher, PhD, a cultural anthropologist from Rutgers University and a leading researcher on love, attraction, and romance. One of her findings: Biology matters, and these two people's biology -- their chemical "profiles" -- may complement each other nicely.

In recent years, Fisher and a host of other researchers have been looking deep into our psyche and brains -- helped by high-tech imaging and genetic analysis. They've come up with some intriguing information about what makes us become attracted to someone, what underlies the crazy-in-love feeling, what's up with the transition from butterflies to a more comfortable relationship, and what keeps us attracted.

"It's all much less of a mystery than it was five years ago and certainly 30 years ago," says Arthur Aron, PhD, a professor of psychology at the State University of New York at Stony Brook and another top researcher in the field. The science of the biology of love is relatively new. Research picked up steam in the 1980s, Aron says, and since then experts have made multiple discoveries. Here's a sampling of their findings:

When it comes to whom you are attracted to, "your biology plays a role,'' says Fisher, who wrote Why We Love and several other books. It's not only a similar socioeconomic status, level of education and family backgrounds that make people attractive to you, she says, but also hormones -- ones that differ from your own.

We're attracted, Fisher says, to those with a chemical "profile" for estrogen, testosterone, dopamine, and serotonin that's different from our own, yet complements it. For instance, she says, "If you tend to be high estrogen, you will gravitate to the high testosterone type.''

That explains why Mr. Analytical and Driven and Ms. Gregarious and Warm are a match. He's probably a ''high testosterone'' type, Fisher says, and she is probably a "high estrogen" type. "For good Darwinian reasons, they are very complementary," says Fisher. She can likely see many ways of doing things and become indecisive. To the rescue, the analytical man. Likewise, she might inspire more compassion in him. Fisher is working with, an offshoot of, to develop this chemical profile match strategy.

Love involves three basic brain circuits, according to Fisher. There's the sex drive, which motivates us to seek out partners; romantic love, the in-the-clouds feeling when you first fall in love; and the attachment phase, the comfortable-but-fewer-fireworks stage.

"The sex drive is a very simple drive," Fisher says. "It's simply the craving for sexual gratification, driven largely by testosterone in both men and women."

The three brain systems, however, don't always come into play in any kind of order. They can kick in separately of be intertwined. Or they can trigger each other. For instance: you can have sex with someone but not fall in love, of course; you can be in love with someone with whom you've never had sex.

"Of these three systems, in many respects I think the most powerful one is intense romantic love," Fisher says. With Aron and others, Fisher has used functional magnetic resonance imaging (fMRIs) to examine the brains of people in love and get clues about romantic love.

In one study, 17 people who were newly in love and asked to look at a photo of their beloved showed intense activity in two brain regions associated with reward and motivation -- called the ventral tegmental area and the right caudate nucleus. The findings led Fisher's team to suggest that the crazy-in-love feeling is more a motivation system than an emotion. The report was published in 2005 in The Journal of Comparative Neurology.

"Both the VTA and the caudate nucleus are part of the brain reward system," Fisher says. And the VTA, she says, is a "mother lode" for cells that make dopamine, a brain chemical important for controlling emotional response and the ability to feel pleasure and pain. As dopamine levels in the madly-in-love increase, she says, it accounts for focused attention on the new partner, motivation to get the reward -- and the lover's high.

In this romantic love phase, Fisher says, lovers are motivated to win each other over. Obsessive thinking is part and parcel.

"What we are seeing is activation in the same area as when you expect to receive a large reward," Aron says. It's the same area that "lights up" in cocaine users, he says, as they anticipate using the drug.

"What we think is what's going on when one falls in love is, one perceives incredible opportunities for one's life to be enriched," Aron says. "Perhaps the most important reward for most people is falling in love."

Aside from biology and brain activity, body odor is important and help may dictate who we are attracted to and our romantic behavior. "It may be one of the first things that inspires us to say yes or no," says Charles Wysocki, PhD, a researcher at Monell Chemical Senses Center in Philadelphia.

Preference for human body odors is influenced by both gender and sexual orientation, Wysocki and his colleagues found in their research, published in 2005 in Psychological Science. When his study participants of different orientations and genders were asked to choose between distinct odors -- straight men, gay men, straight women, lesbian women -- each picked the odor of a partner of the preferred gender and orientation.

''A person's body odor is determined by a number of factors," Wysocki says," and among them is a set of genes that regulate the immune system." This cluster of genes is called the major histocompatibility complex or MHC. "This MHC confers on an individual an odor print," says Wysocki, citing others' research. And experts have found that a person will seek out a partner with an MHC different than his or her own. "MHC is so variable, no two are alike," Wysocki says.

Once you're initially attracted to someone -- helped along by hormones, odor, or other unconscious factors -- what the other person does or doesn't do counts, too. "You become more attracted to people who are attracted to you," Fisher says.

For instance, one research participant told Aron: "I sort of liked this woman and she came over and sat by me." Things developed.

A woman told Aron she was talking to a friend about her piano instructor and the friend said, "You know he likes you." At that moment, the woman told Aron, she realized she had feelings for him, too.

"When people fall in love, that is the most common scenario," Aron says. "We are looking for the opportunity to love and be loved back."

After people have been in love a while, the activity in the brain reward areas wanes, Fisher has found in further research. "As the relationship matures, it links in new brain areas associated with emotion," she says. "We aren't exactly sure what is going on, but everyone knows romantic love changes over time."

Still, she says, "chemistry" can persist. "We have started a new study, of those in long-term marriages," she says. Only five people have undergone the fMRI imaging so far, she tells WebMD, but it looks promising for those who yearn for long-term chemistry. "They still show activity in some brain regions associated with romantic love and also with some associated with attachment," Fisher says.

Two other hormones -- oxytocin and vasopressin -- may come into play once you are settling into a more comfortable relationship. At least it's true in small rodents called prairie voles, according to Sue Carter, PhD, a professor of psychiatry at the University of Illinois at Chicago, who has studied the monogamous animals for decades. Both hormones seem important in the animals' attachments to one other vole, she says.

Oxytocin, sometimes called the hormone of love, is plentiful in women in labor and in lactating women and is released by men and women during orgasm. Some human studies have suggested it plays a role in maintaining interpersonal relationships. Vasopressin is released by the pituitary gland.

In voles, at least, Carter says, the hormones seem to play a role in social bonding, and perhaps in reducing fear, making them feel less anxious. So that may play a role in the voles' decision to mate with just one other vole.

Avoiding boredom is crucial for the health of a relationship, Aron tells WebMD. In a study, he randomly assigned couples to participate in activities both considered highly exciting but moderately pleasant or highly pleasant but moderately exciting.

"The group who did highly exciting but only moderately pleasant activities had a much bigger increase in marital satisfaction," he says. The study was published in the Journal of Personality and Social Psychology.

Meanwhile, another expert is tracking the long-term effect of picking a partner with a different major histocompatibility complex. Martie Haselton, PhD, a psychologist and researcher at the University of California Los Angeles, is working with the web site to track newlyweds, noting the effect of different MHC patterns between partners.

"There is some evidence that fertility is higher in those with dissimilar MHC genes," says Haselton. And children who inherit different MHC genes from each parent are thought to have broader immunity, she says. She also wants to determine if picking someone with different MHC genes than your own bodes well for the relationship long term.

Women in a relationship with a man with very different MHC genes are more sexually responsive to that partner and less likely to be attracted to other men than are women who pair up with a guy with not-so-different MHC genes, says Haselton, citing a study by other researchers published in 2006 in Psychological Science. How that plays out long term will be Haselton's focus as she follows couples for five years or so.

So how much of a role does all this chemistry going on in our brains play in all this? "Chemistry isn't quantifiable," Fisher says. In the making of a relationship, she says, several variables come into play -- such as personality, which includes your character and your temperament. "Your character is formed by everything you grew up with," she says. "And your temperament is built by your biology. Together they create who you are."

So it's difficult to put a percent or a number on the role of chemistry in a relationship. And like some of us, it can be fickle. "One moment chemistry rules and the next moment your upbringing will rule," Fisher says. As in: "I'm madly in love with this guy." to "What I am thinking? He's a different religion."

One thing's for sure. There's much more to discover about the biology of love, guaranteeing that relationship scientists will have jobs for years to come.