LOVES FROM THE PAST SHAKE UP THE PRESENT

She was not looking for an affair. She loved her husband. Her life was fine.

But then the 41-year-old Ohio woman reconnected with her first boyfriend. “Your heart is kind of open to this person already,” she said of the meeting three years ago. “It’s like with an old friend, the way you pick up where you left off, but it’s not as innocent as an old friend. And then you start thinking about it and going crazy.”

The woman, who asked to remain anonymous because she is still married, had fallen victim to what appears to be a growing phenomenon: long-lost sweethearts who look each other up years or decades later and find that their old love returns with a passion – at the expense of their current marriages.

It’s a little discussed downside to technology that has made finding old flames remarkably easy, thanks to such enormously popular websites as classmates.com that help people locate long lost friends and relatives.

Though no one seems to keep national numbers on reunions of former lovers, psychologist Nancy Kalish of California State University in Sacramento may be the closest thing to a “lost love” expert, having studied 2,000 such relationships in the last 10 years.

Kalish believes the romances are increasingly common; earlier this month, 82-year-old actress Carol Channing married her junior high sweetheart. Her Internet website, which solicits lost-love stories from visitors and offers paid consultation for relationship issues, is increasingly visited by married people who are seeking past boyfriends or girlfriends.

Rekindled love, she warned, “has a life of its own. One person said that, for her, it was an emotional steamroller, and it just rolls over everything in sight.”

Not every reunion sparks a romance, but when it does, her research shows, the couples tend to be unusually happy compared with others. Among those she has studied, Kalish said three-quarters of first loves who reunite years later decide to stay together, even when the reunion begins as an adulterous affair. Normally, most marriages that begin as affairs fail.

But the reunited lovers’ happiness comes at a clear cost, from the destruction of marriages to the bitterness of children when parents leave to be with a past lover. About half of those in Kalish’s sample who divorced to get back together reported that before renewing contact with their earlier love, their current marriages had been good.

Kalish’s work focuses increasingly on how the reunion of past lovers affects married people, who account for 82 percent of those who contact her through her website, lostlovers.com, compared with about 30 percent when she began.

Almost all of them find each other using the Internet, she said, and that is part of the problem: It has gotten easier and easier to look somebody up, and what begins as idle curiosity can so easily morph into big trouble.

Some people use search engines to reconnect, plugging in names in hopes of a hit. But many use websites such as classmates.com, the Web’s 15th busiest site, which adds tens of thousands of members each day to its current 35 million-plus. By filling out a short registration form, visitors gain access to classmates.com’s database, which can be broken down to individual graduating classes all over the country. It also offers access to a site called kiss.com that specializes in finding high school sweethearts.

“It’s one of the most amazing catalysts for humanity that you can imagine,” said its spokesman, John Uppendahl.

Reunion.com, a similar site, boasts 10 million members and similar growth, and includes a dating directory. And Kalish’s own site is also growing fast, she said.

They are on fertile ground. A national survey commissioned by classmates.com found that 36 percent of respondents had used the Internet to look up or contact an old boyfriend or girlfriend.

Thinking about an old flame is fine, said a 43-year-old Colorado woman who, like the other unfaithful spouses quoted here, responded to a Globe query posted on Kalish’s website and spoke on condition of anonymity. But, she said, beware of contacting them because it can escalate into an affair with amazing speed and force.

“It’s like you’re falling in love all over again,” thrown back to those dewy teenage days, she said.

Her first boyfriend found her on classmates.com, she said, and before she knew it, she was obsessed, and then lying to her husband, and then sexually unfaithful, and then caught by her husband – who, to her continuing gratitude, stuck with her instead of divorcing her.

Therapists tend to underestimate the powerful nature of such old loves, especially first loves, Kalish argues. As a result, they tend to tell such patients that their feelings for their re-found loves are based on fantasy and that they can find the same feelings in their own marriages if they only try. But that fails to take into account that reunited lovers really do know and love each other, and a first love, in particular, remains unique.

“This is not about sex, it is not about the spouse or the marriage, it is not a midlife crisis,” she said. “The reunion is a continuation of a love that was interrupted.”

Shirley Glass, a leading authority on infidelity, offered similar warnings. Her latest book, “Not Just Friends,” explores how friendships can turn into affairs. “If friendships and collegial relationships can become overheated, former lovers are positively flammable,” she said.

Some research indicates that a teenager may attach specifically to a first lover in much the same way as a baby attaches to a mother, said Linda Waud, a psychologist who wrote her dissertation on three reunited couples.

“There is an actual neurological attachment that happens between these individuals,” she said, “and that’s why it’s enduring and it never leaves your mind. It’s there forever and ever.”

Waud, 61, was reunited with her current husband after more than 35 years apart; they met at a high school reunion after each of their marriages to other people had ended.

“The first time I got to be alone with him, I felt such power and such extreme happiness, that I can’t even describe it,” she said.

Her experience prompted her dissertation and her in-depth interviews of the three couples found, among other things, that they had unusually intense sexual connections, which made her posit that sexual attachment may work with the same kind of specificity as baby-mother attachment

That may help explain why a 54-year-old Illinois man who is now having an affair with his first girlfriend could never get her out of his mind, through 30 years of marriage to another woman.

“There was a kind of crystallization that went on in my heart of `This is the one,’ ” he said.

Two years ago, he left her a message on classmates.com, and they e-mailed for months before starting to arrange secret meetings.

“I care for my wife,” he said, “but it’s like my first love and I were together first, and it seems like my wife kind of came between us.”