(CNN) Each week, Sheri McGregor gets hundreds of emails from parents shut out of their children's lives. Every story is different, she said. What the parents have in common is a profound sense of isolation.
"They say, 'I thought I was the only one,'" said McGregor, founder of a website for estranged parents who lives in the foothills of California's Sierra Nevada mountains. "A lot of these people have been suffering alone for years. ... You feel like you're the only one, so you don't tell other people."
After being cut off by her own adult son, McGregor had felt the same. In the years since, she has written extensively about the healing process, and heard from countless families coping with similar losses.
McGregor, and the people who write to her, are not alone in their rifts with family members. It's something they have in common with millions of people.
In one recent high-profile case, multiple family members of Republican Rep. Adam Kinzinger of Illinois released a letter shunning the politician for his criticism of then-President Donald Trump. "What a disappointment you are to us and to God!" they wrote. "It is now most embarrassing to us that we are related to you."
Over a quarter of adults responding to a US survey by the Cornell Family Reconciliation Project reported being estranged from a family member. The representational survey, which is the first of its kind, suggested by extension that tens of millions of Americans may be estranged from at least one relative.
That number is probably low, said Karl Pillemer, professor of human development at Cornell University, who led the study and explored his findings in the recent book "Fault Lines: Fractured Families and How to Mend Them."
"People find this to be an embarrassing problem," he said, noting that even in a confidential survey, some topics can simply feel too shameful to share.
The groundbreaking survey sheds light on a topic Pillemer said is poorly understood by scientists, given how widespread and painful estrangement is. No more than 20 reliable scientific articles about estrangement exist, he said, "and those are all based on small and non-representative samples."
Today, however, researchers and mental health professionals are tuning into the problem. Here's what experts say about why estrangements happen, why they may be rising and how families can begin to heal.
Every family is different, but there are six main "paths" to estrangement between family members, Pillemer said.
Divorce can have long-term impacts on families. Conflict over money and inheritance can play a major role in blowups.
Relationships with in-laws can cause tension, sometimes to the point of estrangement. A family member might also have unmet expectations, seeing their relatives as failing them in some crucial way.
Differences in values and lifestyles can come between families, too, in conflicts over sexual identity, religion and other deeply personal issues. Even politics can come into play, or strains related to interracial dating and marriage.
For example, tennis champion Naomi Osaka's Japanese mother, Tamaki Osaka, was estranged from family members for over a decade because they disapproved of her relationship with Naomi's Haitian father, Leonard Francois.
The most prominent "path," though, may be a painful history that proves just too hard to move on from, Pillemer said.
"Problems in childhood, problems in the family of origin" were a main cause in many estrangements, he said. In some ways, that reflects how what he calls "positive shared history" can provide a buffer against the stress of normal conflict, Pillemer explained.
Imagine a pair of siblings facing a conflict about money, for instance. Or a parent-child relationship strained by a difference in values, like the family situation faced by Tamaki Osaka.
Remembering a lifetime of positive, loving interactions could see the family through a rocky patch.
"If there's been this long and solid basis of childhood attachment and affection, you're more likely to reconcile. It's more likely to be a temporary thing," Pillemer said.
A family rift is intensely personal, yet each story plays out against a broader cultural backdrop of values and behavioral norms. While no historical data exist to demonstrate a clear rise, Pillemer said he suspects estrangements have gone up over time.
"Someone feeling comfortable saying 'I never want to speak to my family members again,' is probably increasing," he said. Divorce, which correlates to likelihood of family estrangements, has risen dramatically over past decades.
There is also a change in perspective, Pillemer said. "The sense that 'I will stick with my relatives no matter what' ... I think that's still there to some extent. But I think there's a lower-threshold breaking point, for younger people in particular."
Many Americans now place a greater emphasis on individual well-being, said psychologist Joshua Coleman, author of the new book "Rules of Estrangement: Why Adult Children Cut Ties and How to Heal the Conflict."
"Staying in contact is much more tied to identity, to personal growth, to the pursuit of happiness," he said. In the past, Coleman explained, such bonds were more likely to be grounded in a sense of duty or obligation.
That's different now, said Coleman, whose focus is mainly on estrangements between parents and adult children. Such ruptures are particularly painful, and the Cornell University survey found they're the most common of all.
Today, "parents are held to a much higher standard," Coleman said. "The only thing that keeps an adult child tied to a parent is whether the adult child wants the relationship."
That's not necessarily a bad thing, he said. The cultural shift makes it easier for adult children to separate from parents who have been abusive, or who reject their sexuality, gender identity and basic values.
"Prior generations of parents had too much power in terms of relationships with their children, while today it's much more equal," he said. In some cases, though, Coleman thinks US culture has swung too far away from family cohesion to support overall social well-being.
The equilibrium between cohesion and individual happiness varies between cultures and families. In the Cornell University study, for example, participants from families who immigrated to the US from the Caribbean, Africa and Latin America reported feeling strong social pressure to repair any rifts with estranged relatives.
Overall in the US, though, "we're wedded to this more individualistic narrative of personal happiness," Coleman said, "that if a relationship doesn't make you feel good, or makes you feel bad in any way, then you should consider this person toxic and cut them out of your life."
That, in turn, might not actually make us very happy, Coleman said. "There's enormous loneliness in our culture," he said.
Older adults tend to be especially isolated, a situation that has been aggravated by the pandemic. "Being part of a group — caring about what other people think, feel and need — is important."
Some estranged families make their way to Coleman's Oakland, California, therapy practice, where the psychologist works with parents hoping to reconcile with their children. It's a predicament he can relate to, because he, too, has experienced such loss firsthand.
"I had an estrangement with my daughter, which has made this kind of a mission," said Coleman, who has since reconciled with his child. "The parents I work with are heartbroken, they're miserable."
Reconciliation is possible for many families, Coleman said, but it's not easy. In most cases of successful reconciliations between parent and child, he said parents initiate the process.
Parents must "show empathy for the adult's child's perspective, they have to take responsibility." Coleman often invites parents to write their children a letter that does just that, acknowledging why the child felt they needed to cut off the relationship.
It's not all about making amends, he said. It's also important to signal that you're ready for a relationship that respects your family member's ideas of what a healthy connection looks like, even if that differs from your own expectations.
"Not all parents, frankly, are capable of doing that," Coleman said. "And sometimes parents may do all of these things and the child is still not willing to reconcile."
Coleman underscores empathy when he's talking to adult children, too. If they were open to reconciliation, "I would pursue with them a different way of looking at their parent, one that's borne more of compassion and empathy," he said.
Such a shift in perspective can be difficult for people on each side of a rift. For many families, though, he said the benefits of reconciliation means it's worth the hard emotional work.
"People sometimes say, 'How successful are you?'" Coleman said. "We're very successful when both people are willing to come to the bargaining table and are open to change. But both people have to be willing."