Christie Brinkley and Peter Cook; Mr. and Mrs. A-Rod — apparently, divorce is in. Between 1998 and 2000, there were nearly a million divorces annually, launching two million adults back into the single life each year, compared to about half as many between 1950 and 1970. How did "till death do us part" become so negotiable?

If you had told the Founding Fathers that "irreconcilable differences" would one day be a valid reason for ending a marriage, they would have laughed in your face. Back then, marriage was about economics and dynastic politics, not personal happiness. Henry VIII ended the medieval church's ban on divorce only because because he needed a male heir. In a world where marriage was an economic institution, the divorce rate was relatively low.

This doesn't mean divorce never happened, however. The "wife sales" made famous in Thomas Hardy's The Mayor of Casterbridge were, in reality, English rural-folk divorces in which the participants knew one another, and the wife being "sold" was often already sleeping with her "buyer." The "better sort," on the other hand, didn't get divorced — they just downed another bottle of claret or went off to the bordello.

The situation in America, where a man could lose himself on the vast frontier, had a different twist. Divorce was a woman's weapon, argues Norma Basch in Framing American Divorce.

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Men could make like the Pet Shop Boys and go west, but women tended to stay rooted in their communities, and needed the social sanction of a divorce to remarry once their husbands were on the road. Getting out of a marriage remained an onerous process, however. The litigant had to prove the other party had failed in the marriage contract through domestic violence, infidelity, or simple abandonment. Even then, many judges were loathe to dissolve marriages, so simply shacking up with someone new without notifying the state was more common. Both men and women were more concerned with real relationships than legal ones: "In part, the need for marriage was a story about labor, about the impossibility of doing what was needed without at least two sets of adult hands," Hendrik Hartog wrote in Man and Wife in America. Though marriage has long been portrayed as an institution under attack, divorce is nothing new — it just didn't always take place in a way statisticians could measure.

That said, there's no doubt that the divorce rate, which began creeping upward during the Civil War, spiked dramatically in the 1970s. There were several reasons for this.

Many judges were loathe to dissolve marriages, so simply shacking up with someone new without notifying the state was more common.

First, while divorce laws had traditionally been more lenient in western states — due to an overpopulation of single men who had left wives and families back East — California's 1969 Family Law Act changed everything by creating "no-fault" divorce. Instead of treating marriage as an institution that should be preserved at all costs, no-fault divorce viewed it as a voluntary union between two equals that could be dissolved at will. A spouse who wanted a divorce no longer had to prove wrongdoing in an adversarial setting; the separation could be amicable, and mutual incompatibility — "irreconcilable differences" — was enough to begin proceedings. Other states quickly followed California's lead; today, some form of no-fault divorce is available in all fifty states.

A second reason was social. By the early '70s, older adults, many of whom had married young in the domesticity-oriented postwar era, were slowly beginning to accept the tenets of the sexual revolution. While some began swinging or hanging out at places like Plato's Retreat, for the vast majority of couples, the solution to an unhappy situation was rejecting the "until death do us part" clause.