Researchers haven't figured out the formula for the perfect marriage, but they do know more about what makes unions today different from those in the past: love.
Common sense suggests that love might always have been part of the equation. But the idea first surfaced in the 18th century and of late is changing the nature of the institution.
Today, marrying for love is making marriages both more fulfilling and more fragile than ever, reports Stephanie Coontz, author of the recently published book, "Marriage, a History: from Obedience to Intimacy or How Love Conquered Marriage."
Modern marriages are distinguished by the ability to negotiate and mold the relationship so that it meets the needs of both parties. "That makes marriage, when it works, more fulfilling and loving and passionate than any of the marriages I've studied in the past," says the author. "But the same things that make it possible to negotiate and change, make it possible not to enter marriage at all, or to leave a marriage that doesn't meet those expectations."
That freedom is a big change from the institution's early vestiges. For thousands of years, notes Ms. Coontz, joining two people together was more about gaining in-laws - and satisfying economic and political needs - than establishing a relationship between a husband and wife. "I was surprised to find how unanimous people were until the late 18th century that love was not a good reason for marriage," she says.
Back then, traditionalists predicted that if couples were allowed to freely pick their own partners, chaos would ensue. Increased divorce, women gaining more rights - and potentially more control over men - were but a few of their concerns.
It took another 150 years for those predictions to play out, says Coontz, because women remained economically dependent on men. "What I've come to think of as the perfect storm occurred between 1960 and 1980, as all of those barriers to truly making love your first priority disappeared," she explains in a phone interview.
During that time, birth control became acceptable and the stigma on illegitimate children was lifted. Men were increasingly judged less for their personal reputation - including marital status - and more for their credentials. Perhaps most significant of all was the entry of women into the workforce, making it easier for them to support themselves without marriage or after leaving a marriage.
Coontz, who teaches history and family studies at the Evergreen State College in Olympia, Wash., doesn't like using the word crisis to describe what's happening to marriage because she says that implies that the changes are all bad. Marriage as an institution is definitely different, and has less of the monopoly it once did, as people are staying single longer and choosing to live together, for example. "You will never shove everybody back into universal, lifelong, male-breadwinner marriages," she notes.
But Coontz says that once people accept that, they might see that society is also better able to deal with the problems associated with modern marriage. The possibility is there of helping men and women negotiate their roles, she says, and advising them on how to minimize the damage to themselves and their families when they divorce. "The most exciting paradox of this book for me," she says, "was to get past the notion that all these changes have been good or all these changes have been bad, and to understand that the good ones are kind of inextricably wrapped up with what we often see as the bad ones."