CHELSEA, Mich. » Kristi and Michael Burns have a lot in common. They love crossword puzzles, football, going to museums and reading five or six books at a time. They describe themselves as mild-mannered introverts who suffer from an array of chronic medical problems. The two share similar marital résumés, too. On their wedding day in 2011, the groom was 43 and the bride 39, yet it was marriage No. 3 for both.

Today, their blended family is a sprawling, sometimes uneasy ensemble of two sharp-eyed sons from her two previous husbands, a daughter and son from his second marriage, ex-spouses of varying degrees of involvement, the partners of ex-spouses, the bemused in-laws and a kitten named Agnes that likes to sleep on computer keyboards.

If the Burnses seem atypical as an American nuclear family, how about the Schulte-Waysers, a merry band of two dads, six kids and two dogs? Or the Indrakrishnans, a successful immigrant couple in Atlanta whose teenage daughter divides her time between prosaic homework and the precision footwork of ancient Hindu dance; the Glusacs of Los Angeles, with their two nearly grown children and their litany of middle-class challenges that seem like minor sagas; Ana Perez and Julian Hill of New York, unmarried and just getting by, but with Warren Buffett-size dreams for their three young children; and the alarming number families with incarcerated parents, a sorry byproduct of the United States’ status as the world’s leading jailer.

The typical American family, if it ever lived anywhere but on Norman Rockwell’s Thanksgiving canvas, has become as multilayered and full of surprises as a holiday turducken — the all-American seasonal portmanteau of deboned turkey, duck and chicken.

Researchers who study the structure and evolution of the American family express unsullied astonishment at how rapidly the family has changed in recent years, the transformations often exceeding or capsizing those same experts’ predictions of just a few journal articles ago.

"This churning, this turnover in our intimate partnerships is creating complex families on a scale we’ve not seen before," said Andrew J. Cherlin, a professor of public policy at Johns Hopkins University. "It’s a mistake to think this is the endpoint of enormous change. We are still very much in the midst of it."

Yet for all the restless shape-shifting of the American family, researchers who comb through census, survey and historical data and conduct field studies of ordinary home life have identified a number of key emerging themes.

Families, they say, are becoming more socially egalitarian overall, even as economic disparities widen. Families are more ethnically, racially, religiously and stylistically diverse than half a generation ago — or even half a year ago.

In increasing numbers, blacks marry whites, atheists marry Baptists, men marry men and women women, Democrats marry Republicans (and start talk shows). Good friends join forces as part of the "voluntary kin" movement, sharing medical directives, wills, even adopting one another legally.

Single people live alone and proudly consider themselves families of one — more generous and civic-minded than so-called "greedy marrieds."

"There are really good studies showing that single people are more likely than married couples to be in touch with friends, neighbors, siblings and parents," said Bella DePaulo, author of "Singled Out" and a visiting professor of psychology at the University of California, Santa Barbara.

But that doesn’t mean they will be single forever.

"There are not just more types of families and living arrangements than there used to be," said Stephanie Coontz, author of the coming book "Intimate Revolutions," and a social historian at Evergreen State College in Olympia, Wash. "Most people will move through several different types over the course of their lives."

At the same time, the old-fashioned family plan of stably married parents residing with their children remains a source of considerable power in the U.S. — but one that is increasingly seen as out of reach to all but the educated elite.

"We’re seeing a class divide not only between the haves and the have-nots, but between the I do’s and the I do nots," Coontz said. Those who are enjoying the perks of a good marriage "wouldn’t stand for any other kind," she said, while those who would benefit most from marital stability "are the ones least likely to have the resources to sustain it."

Yet across the divide runs a white picket fence, our unshakable star-spangled belief in the value of marriage and family. We marry, divorce and remarry at rates not seen anywhere else in the developed world. We lavish $70 billion a year on weddings, more than we spend on pets, coffee, toothpaste and toilet paper combined.

We are sappy family romantics. When an informal sample of 52 Americans of different ages, professions and hometowns were asked the first thought that came to mind on hearing the word "family," the answers varied hardly at all: Love! Kids! Mom! Dinner!

"It’s the backbone of how we live," said David Anderson, 52, an insurance claims adjuster from Chicago.

"It means everything," said Linda McAdam, 28, who is in human resources on Long Island.

Yes, everything, and sometimes too many things.

"It’s almost like a weight," said Rob Fee, 26, a financial analyst in San Francisco, "a heavy weight."

Or as the comedian George Burns said, "Happiness is having a large, loving, caring, close-knit family in another city."

THE BABY ANTI-BOOM

In charting the differences between today’s families and those of the past, demographers start with the kids — or rather the lack of them.

The nation’s birthrate today is half what it was in 1960, and last year hit its lowest point ever. At the end of the baby boom, in 1964, 36 percent of all Americans were younger than 18; last year, children accounted for just 23.5 percent of the population, and the proportion is dropping, to a projected 21 percent by 2050.

Fewer women are becoming mothers — about 80 percent of those of childbearing age today versus 90 percent in the 1970s — and those who reproduce do so more sparingly, averaging two children apiece now, compared with three in the 1970s.

One big reason is the soaring cost of ushering offspring to functional independence. According to the Department of Agriculture, the average middle-class couple will spend $241,080 to raise a child to age 18. Factor in four years of college and maybe graduate school, or a parentally subsidized internship with the local theater company, and say hello to your million-dollar bundle of joy.

As steep as the fertility decline has been, the marriage rate has fallen more sharply, particularly among young women, who do most of the nation’s childbearing. As a result, 41 percent of babies are now born out of wedlock, a fourfold increase since 1970.

The trend is not demographically uniform, instead tracking the nation’s widening gap in income and opportunity. Among women with a bachelor’s degrees or higher, 90 percent adhere to the old playground song and put marriage before a baby carriage. For everybody else, maternity is often decoupled from matrimony: 40 percent of women with some college but no degree, and 57 percent of women with high school diplomas or less, are unmarried when they give birth to their first child.

More than one-quarter of these unwed mothers are living with a partner who may or may not be their child’s biological father. The rise of the cohabiting couple is another striking feature of the evolving American family: From 1996 to 2012, the number jumped almost 170 percent, to 7.8 million from 2.9 million.

Nor are unmarried mothers typically in their teens; contrary to all the talk of an epidemic of teenage motherhood, the birthrate among adolescent girls has dropped by nearly half since 1991 and last year hit an all-time low, a public health triumph that experts attribute to better sex education and birth-control methods. Most unmarried mothers today, demographers say, are in their 20s and early 30s.

SHIFTING BREADWINNERS

Also dimodi is the old debate over whether mothers of dependent children should work outside the home. The facts have voted, the issue is settled, and Paycheck Mommy is now a central organizing principle of the modern American family.

The share of mothers employed full or part time has quadrupled since the 1950s and today accounts for nearly three-quarters of women with children at home. The number of women who are their families’ sole or primary breadwinner also has soared, to 40 percent today from 11 percent in 1960.

"Yes, I wear the pants in the family," said Ana Perez, 35, a mother of three and a vice president at a financial services company in New York, who was, indeed, wearing pants. "I can say it brings me joy to know I can take care of my family."

Cultural attitudes are adapting accordingly. Sixty-two percent of the public, and 72 percent of adults younger than 30, view the ideal marriage as one in which husband and wife both work and share child care and household duties; back when Jimmy Carter was president, less than half of the population approved of the dual-income family, and less than half of 1 percent of husbands knew how to operate a sponge mop.

Mothers are bringing home more of the bacon, and the mortarboards, too. While most couples are an even match scholastically, 28 percent of married women are better educated than their mates; that is true of just 19 percent of married men. Forty years ago, the asymmetry went the other way.

Some experts argue that the growing legion of mothers with advanced degrees has helped sharpen the already brutal competition for admission to the nation’s elite universities, which stress the importance of extracurricular activities. Nothing predicts the breadth and busyness of a child’s after-school schedule better, it turns out, than the mother’s level of education.

One change that caught many family researchers by surprise was the recent dip in the divorce rate. After many decades of upward march, followed by a long, stubborn stay at the familiar 50 percent mark that made every nuptial feel like a coin flip, the rate began falling in 1996 and is now just above 40 percent for first-time marriages.

The decline has been even more striking among middle- and upper-middle-income couples with college degrees. For them, fewer than 1 in 3 marriages is expected to end in divorce, a degree of stability that allows elite couples to merge their resources with confidence, maximally invest in their children and otherwise widen the gap between themselves and the struggling masses.

There are exceptions, of course. Among baby boomers, the rate of marriage failure has surged 50 percent in the past 20 years — perhaps out of an irritable nostalgia, researchers said, for the days of free love, better love, anything but this love. Nor do divorce rates appear to have fallen among those who take the old Samuel Johnson quip as a prescription, allowing hope to triumph over experience, and marrying again and again.

A NEW START, ONLINE

For both Mike and Kristi Burns, the first marriage came young and left early, and the second stuck around for more than a dozen years.

Kristi was 19, living in South Carolina, and her Marine boyfriend was about to be shipped to Japan. "I wasn’t attached to him, really," she said, "but for some reason I felt this might be my only chance at marriage."

In Japan, Kristi gave birth to her son Brandon, realized she was lonely and miserable, and left the marriage seven weeks after their first anniversary. Back in the U.S., Kristi studied to be a travel agent, moved to Michigan and married her second husband at age 23.

He was an electrician. He adopted Brandon, and the couple had a son, Griffin. The marriage lasted 13 years.

"We were really great friends, but we weren’t a great husband and wife," Kristi said. "Our parenting styles were too different."

Besides, she went on, "he didn’t verbalize a lot, but he was mad a lot, and I was tired of walking around on eggshells."

After the divorce, friends persuaded her to try the online dating service match.com, and just as her free trial week was about to expire, she noticed a new profile in the mix.

"Kristi was one of the first people to ping me," said Mike Burns, an engineer for an e-commerce company. "This was at 3 in the morning."

They started chatting. Mike told Kristi how he’d married his first wife while he was still in college — "definitely too young," he said — and divorced her two years later. He met his second wife through mutual friends, they had a big church wedding, started a software publishing company together, sold it and had two children, Brianna and Alec.

When the marriage started going downhill, Mike ignored signs of trouble, like the comments from neighbors who noticed his wife was never around on weekends.

"I was delusional, I was depressed," he said. "I still had the attitude that divorce wasn’t something you did."

After 15 years of marriage, his wife did it for him, and kicked him out of the house. His divorce papers hadn’t yet been finalized, he told Kristi that first chat night. "I’ll help you get through it," she replied.

Mike and Kristi Burns admit their own 3-year-old marriage is not perfect. The kids are still adjusting to one another. Sometimes Kristi, a homemaker, feels jealous of how much attention her husband showers on his daughter Brianna, 13. Sometimes Mike retreats into his computer. Yet they are determined to stay together.

"I know everyone thinks this marriage is a joke and people expect it to fail," said Kristi. "But that just makes me work harder at it."

"I’d say our chances of success are better than average," her husband added.

In the U.S., family is at once about home and the next great frontier.

© 2013 The New York Times Company