In the United States, summer is the most popular time of year to get married. Thousands of couples will wed between now and September. But while that wedding tradition lives on, American matrimony is changing dramatically in other ways, says social historian Stephanie Coontz. She's the director of Research and Public Education at the Council on Contemporary Families and the author of Marriage, A History: From Obedience to Intimacy or How Love Conquered Marriage (Viking).

When Stephanie Coontz hears Americans worry that the traditional marriage is in crisis, she can respond with a host of arguments. There have been more step-families and out-of-wedlock births at other times in history than today, says The Evergreen State College professor. Same-sex marriages have also taken place in societies around the world. And while divorce rates rose for much of the last century in the United States, Ms. Coontz says they have fallen by 26% since 1981.

What is different is that for thousands of years, marriage was not really about love and romance. "It's not that societies of the past didn't have love and romance," she explains, "but it was considered a very poor thing to get married for that reason. Marriage was about acquiring a good set of in-laws, making economic and political alliances. And then about 200 years ago, we began to develop in Western Europe and America this ideal of the 'love match,' that you should marry for love."

Marriage changed even more radically, Ms. Coontz believes, with the women's equality movement of the past 30 years. "Even after marriage began to be about love, it remained about the subordination or at least the dependence of women," she says. "And now, because women have got economic independence, they are increasingly being educated, and they are able to approach marriage as a relationship, not, so to speak, a social safety net."

That means that single women like Melanie Cashdan, 29, a social worker living in New York, can voice ideas about marriage that would have seemed revolutionary a few hundred years ago. She believes she has options that make her more selective about whom she marries, and she cannot imagine NOT marrying for love. "I have a very simple idea of what I want -- someone who makes me laugh, someone I'm attracted to, someone who's stable," she says. "There were opportunities for me to get married with people who were perfectly nice people, but I just didn't feel that connection."

And while she believes young people still feel the pressure to find a mate as they enter their 30s, Melanie Cashdan says she can also imagine having a happy life without being married. "I was in an airport," she recalls, "and the guy looking at my passport said, 'Are you single or married?' And I said single and proud of it!"

Stephanie Coontz believes a lot of what is happening in American marriages today is good news. Many couples are finding greater fulfillment in marriage, and they are dividing household and parenting tasks more equally. The problem is that what makes for a good relationship does not necessarily make for a strong institution. "For thousands of years marriage was a strong institution because no one expected it to be a very satisfying relationship," the author explains. "Today, the things that make it a flexible relationship that can be negotiated also give people the options not to marry or to leave a marriage, and so marriage as an institution has been weakened."

Stephanie Coontz says changing ideas about marriage are having an impact around the world, although not always the same impact. "In countries where woman have fewer options and where the men have changed least in marriage, those women are responding by putting off marriage in much greater numbers," she notes. "So in Japan and Italy and Spain the divorce rate and out-of-wedlock rate is much lower than in the United States. However, the marriage rate is also much lower. There are very high proportions of unmarried women over 25."

What Stephanie Coontz did find almost everywhere in the world is the impulse to have some form of marriage. The most widespread kind of wedlock historically has been polygamy, she says, where a man has several wives. In Tibet, and parts of India, Kashmir and Nepal, a woman may marry 2 or more brothers. Ms. Coontz found only 1 culture that has no marriage at all -- the Na people of southwestern China. "Within that group siblings live together," she says. "They're the basic unit that raises children, that cooperates economically. Sexual relationships are much more casual. And even when a couple continues a relationship for a long time, they do not normally consider the man to have a responsibility to his kids. It's the woman's brother who has a responsibility to the kids."

Even in countries where arranged marriages have long been the norm, Stephanie Coontz says young people are being given a growing voice in that process. But she also found little difference in the happiness levels reported by couples in love matches and arranged marriages. She believes Americans tend to see marriage as the focal point of family happiness, sometimes at the expense of other personal ties. "Within marriage you owe everything. Outside of marriage you owe nothing," she notes. "We are less inclined than many other nations to emphasize your duty to neighbor, to extended family. One of the biggest challenges is that Americans, more than other countries who maybe respect marriage a little less, are more likely when they do leave a marriage to walk away entirely from their sense of obligation."

In Marriage, A History, Stephanie Coontz aims to show that the American style of marriage is not so much an old tradition that is fading away, as a relatively recent one that is still evolving -- posing unprecedented challenges as well as new rewards.