In the 1960s, young Americans questioned social traditions one after another, ringing in the feminist movement and celebrating sexual freedom. Today they have become the divorced generation.

While the overall divorce rate in the U.S. has declined over the past 20 years, it has doubled for Americans aged 50 and over. Today, more than one in three in this category has ended a marriage.

No doubt, there have been a variety of causes, including empty nests, midlife crises, better health in late middle age (allowing the energy to pursue extramarital affairs) and even Generation Me’s vaunted desire to remain forever young. But one force that is helping to fracture those marriages has received less attention: the people who are now approaching retirement age were unprepared for an altered technological landscape that allows them to reconnect with long-lost love.

More than 80 percent of divorce attorneys recently surveyed by the American Academy of Matrimonial Lawyers said that in the past few years they have witnessed “an increase in the number of cases using social networking evidence.” Although it is difficult to definitively establish cause and effect here, it seems likely that the divorce rate among baby boomers has been elevated by the Internet.

Nancy Kalish, a professor of psychology at California State University, Sacramento, suspects that online connections may lead to growing numbers of what she terms “accidental affairs,” meaning they involve people who don’t set out to have a physical or emotional relationship outside their marriage. Kalish studies couples who reunite after years apart.

Before there was an Internet, when someone wanted to track down a past love, he or she had to go through the effort of locating a friend or relative to make contact. “Unless they were single, divorced or widowed, they just didn’t typically do that,” Kalish told me.

Getting Reacquainted

But now the ghosts of romance past are alive and well online, popping up on chat services and sending greetings on Facebook. In the 21st century, old friends are virtually at our fingertips, and a seemingly harmless email sent to someone with the innocent intention of “catching up” can quickly go further. Many of those who engage in accidental affairs tell Kalish that they had happy marriages before they strayed. “They still bear responsibility for the affairs, of course; no one made them write, call or meet in a hotel room,” Kalish said. “But these are probably people who would not have cheated years ago, even with a lost love.”

What makes the possibility of reconnection so alluring?

Our brains often romanticize the past, in ways not entirely within our conscious control. Recollecting people, places and experiences can affect our neurochemistry.

Years or decades after ending a relationship, the name or photograph of a former partner has the potential to stir up some of the same feelings that sparked the love affair in the first place. It may cause, for example, an increase in uptake of the brain chemical dopamine, which can in turn promote feelings of desire, or even craving, for that person. The memory may also lead to a surge of serotonin, another neurotransmitter, and this can trigger obsessive thoughts.

Finally, there is oxytocin, a hormone that operates in the brain. Recent research at Mount Sinai School of Medicine, in New York City, suggests that oxytocin fosters our sense of attachment to others and in many people also enhances positive memories of close relationships.

Love Lasts

In other words, powerful feelings don’t necessarily disappear after a breakup, even after many years. Stimulating the memory can release the associated chemicals -- which can then resume their effects on our emotions and behavior.

If those feelings drive the two people to arrange a meeting, still greater temptations may arise. A former lover’s voice, or even scent, can unleash many vivid memories stored in the hippocampus and frontal cortex of the brain. Because of the way humans are wired, a reunion, even if meant to be platonic and lighthearted, may suddenly feel very intimate.

That isn’t to say that people can’t control their own actions when strong emotions are involved. It’s just that Internet reunions can be more challenging than people anticipate. A click of the mouse can end up rekindling an old flame that is difficult to put out.

(Sheril Kirshenbaum is a research scientist at the University of Texas’s Center for International Energy and Environmental Policy and the author of “The Science of Kissing.” The opinions expressed are her own.)

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To contact the author of this column: Sheril Kirshenbaum at srkirshenbaum@yahoo.com

To contact the editor responsible for this column: Mary Duenwald mduenwald@bloomberg.net