So this, in the end, is what love is.

Former Justice Sandra Day O’Connor’s husband, suffering from Alzheimer’s disease, has a romance with another woman, and the former justice is thrilled — even visits with the new couple while they hold hands on the porch swing — because it is a relief to see her husband of 55 years so content.

What culture tells us about love is generally young love. Songs and movies and literature show us the rapture and the betrayal, the breathlessness and the tears. The O’Connors’ story, reported by the couple’s son in an interview with a television station in Arizona, where Mr. O’Connor lives in an assisted-living center, opened a window onto what might be called, for comparison’s sake, old love.

Of course, it illuminated the relationships that often develop among Alzheimer’s patients — new attachments, some call them — and how the desire for intimacy persists even when dementia steals so much else. But in the description of Justice O’Connor’s reaction, the story revealed a poignancy and a richness to love in the later years, providing a rare model at a time when people are living longer, and loving longer.

“This is right up there in terms of the cutting-edge ethical and cultural issues of late life love,” said Thomas R. Cole, director of the McGovern Center for Health, Humanities and the Human Spirit at the University of Texas, and author of a cultural history of aging. “We need moral exemplars, not to slavishly imitate, but to help us identify ways of being in love when you’re older.”

Historically, love in older age has not been given much of a place in culture, Dr. Cole said. It once conjured images that were distasteful or even scary: the dirty old man, the erotic old witch. 

That is beginning to change, Dr. Cole said, as life expectancy increases, and a generation more sexually liberated begins to age. Nursing homes are being forced to confront an increase in sexual activity.

And despite the stereotypes, researchers who study emotions across the life span say old love is in many ways more satisfying than young love — even as it is also more complex, as the O’Connors’ example shows.

“There’s a difference between love as it is presented in movies and music as this jazzy sexy thing that involves bikini underwear and what love actually turns out to be,” said the psychologist Mary Pipher, whose book “Another Country” looked at the emotional life of the elderly. “The really interesting script isn’t that people like to have sex. The really interesting script is what people are willing to put up with.”

“Young love is about wanting to be happy,” she said. “Old love is about wanting someone else to be happy.”

That’s one way to look at it, at any rate. And it’s not just that relationships are seasoned by time and shared memories — although that’s part of it, as is the inertia the researchers call the familiarity effect, which keeps people from leaving a longtime relationship even though he nags and she won’t ask for directions.

It’s also that brain researchers say older people may simply be better able to deal with the emotional vicissitudes of love. As it ages, the brain becomes more programmed to be happy in relationships.

Researchers trying to understand aging and emotion performed brain scans on people across a range of ages, gauging their reactions to positive and negative scenes. Young people tended to respond to the negative scenes. Those in middle age took in a better balance of the positive. And older people responded only to the positive scenes.

“As people get older, they seem to naturally look at the world through positivity and be willing to accept things that when we’re young we would find disturbing and vexing,” said John Gabrieli, a professor of cognitive neuroscience at the Massachusetts Institute of Technology and one of the researchers. 

It is not rationalization: the reaction is instantaneous. “Instead of what would be most disturbing for somebody, feeling betrayed or discomfort, the other thoughts — about how from his perspective it’s not betrayal — can be accommodated much more easily,” Dr. Gabrieli said. “It paves the way for you to be sympathetic to the situation from his perspective, to be less disturbed from her perspective.”

Young brains tend to go to extremes — the swooning or sobbing so characteristic of young love. Old love puts things in soft focus. 

“As you get older you begin to recognize that this isn’t going to last forever, for better or for worse,” said Laura L. Carstensen, director of the Stanford Center on Longevity and a research counterpart of Dr. Gabrieli’s in the brain imaging research. “You understand that the bad times pass, and you understand that the good times pass,” Dr. Carstensen said. “As you experience them, they’re more precious, they’re richer.”

Of course, not everyone would show the emotionally generous response that Justice O’Connor did. As Dr. Cole said, “I have many examples in my mind of people who are just as jealous, just as infantile, just as filled with irrationality when they fall in love in their 70s and 80s as she is self-transcendent.”

And it still is possible to have a broken heart in old age. But in general, Dr. Carstensen said: “A broken heart looks different in somebody old. You don’t yell and scream and cry all day long like you might if you were 20.”

In one of the few cultural examples exploring old love — the film “Away From Her,” based on an Alice Munro short story and released in the spring — the starting point is similar to the O’Connors’ story. A man who cannot imagine life without his sparkling wife of some decades watches her slip into Alzheimer’s and then a romance with another patient in a nursing home. In the fictional example, the spousal devotion is such that he arranges for her new boyfriend to return to the nursing home after seeing how crushed she is when the man moves away.

But the story is more complex. The husband had a series of affairs years earlier, so what seems like devotion is also a desire to pay her back and to ease his own remorse.

For Olympia Dukakis, whose mother had Alzheimer’s and who played the wife of the other man in the film, that wrinkle explains the resonance of Ms. Munro’s story.

“She was very aware that contradictory things live together,” Ms. Dukakis said. “You can’t look at it and say he did it purely for love. It’s a complicated issue, because there’s a lot of life that has been lived. It’s not going to be simple.”

Still, for all those kinds of complications, those who study aging can only smile at young lovers who say they never want to become like an old married couple. Despite the popular preference for young love, the O’Connors’ example suggests that we should all aspire to old love, for better and for worse.
“Young love is very privileged, and as a culture that may be a mistake,” Dr. Pipher said. “If you want a communal culture where people make sacrifices for each other and work for the common good, you would have a culture that privileges the stories of older people.” 

Those stories would not be without their troubles. But nor would they be without rewards. “If you stay married,” Dr. Pipher said, “there’s riches in store that nobody 25 years old can imagine.”