On a neurochemical level, people in long-term relationships are just like those who are falling in love.
The feeling of intense love does not have to fade as time wears on.
Dopamine seems to play a part in both new and long-term love by motivating people to see their partners.
Nothing compares to the rush of being newly in love. But even after a decade of marriage or more, some people say their love remains intense.
And when these people see pictures of their beloved, found a new study, their brains respond in an area associated with deeply rewarding and motivating feelings -- one of the same areas that lights up in people who are still entangled in the passionate throws of a new relationship.
By finding physical parallels between people who are newly in love and those who have been intensely in love for a long time, the study offers biological evidence that romantic love is real and can be long-lasting.
The study, which focused on a group that held particularly strong feelings for their long-term partners, also suggests that couples who simply strive to be happy together may not be aiming high enough, said Arthur Aron, a social neuroscientist at Stony Brook University in New York.
"This should be inspiring to people who are considering getting married," Aron said. "For long-term couples, it suggests there's a higher possibility than people were imagining."
A handful of studies in recent years have scanned the brains of people who are in the early stages of love -- all with the same results.
When people looked at pictures of a new partner, results show, their brains fired in a region that processes the reward-inducing brain chemical dopamine.
It's the same region that responds to food, alcohol and cocaine, and motivates people to want more of something. When study participants looked at similarly attractive faces of people of the same age and gender, on the other hand, that brain region stayed quiet.
As time wears on, relationships often change. And people commonly debate whether intense romantic love can last. Some believe that passion inevitably fades over the course of a partnership. Others think that desire evolves into deep friendship. And even though there have always been partners who claim they are still madly in love many years after their first date, even some psychologists speculate that these people are fooling themselves.
To begin to test these theories, Aron and colleagues conducted the first brain-scan study of people in long-term love. The study included 17 adults who had been married for between 10 and 29 years. All rated the intensity of love for their partner as at least five on a seven-point scale.
In the lab, participants looked at pictures of their partner's faces. Then they looked at a variety of comparison faces, including people they were close to but not passionately in love with. All the while, an fMRI machine recorded activity throughout their brains.
Compared to the brain scans of people who had recently fallen in love, there were some differences, the researchers reported in the journal Social Cognitive and Affective Neuroscience. For example, only the new lovers had activity in the parts of the brain related to tension and obsession. And only long-term lovers showed extra activity in areas related to attachment and pair bonding.
But both groups showed comparable activity in a dopamine-processing region of the brain called the ventral tegmental area. Married partners who rated themselves highest on the scale of intense love showed more activity in that area than did long-term partners who reported slightly lower levels of intensity.
"The question is if these claims of intense love in long-term relationships are real," Aron said. "This adds to our confidence that the answer is yes. This is confirmation that there's something real here. It's not just people kidding themselves."
Findings like these bring us a step closer to understanding the biology behind long-lasting love, said Stephanie Ortigue, a neuroscientist of love at Syracuse University in New York.
But, she added, the work has limitations and the findings need to be taken with a grain of salt. It would be helpful, for example, to scan the brains of people in long-term relationships who are not intensely in love. That would show whether simply being part of a pair affects the brain -- passion or no passion.
As evidence from ongoing studies comes in, we may start to better understand a feeling that is both eternally desired and frustratingly elusive.
"We all wonder if love really exists and if it's really worth it because everybody looks for love," Ortigue said. "And when love doesn't go well, everybody gets depressed."
"By studying the brain networks of love and showing there is something scientific behind that... and that even people who are in long-term relationships can still have the same brain networks as younger people," she added, "it tells us that love is not only an emotion, but something more complicated and sophisticated. It can be an intellectual process, and it can last for a long time."