No group is more supportive of living together—despite evidence that four out of five couples who begin cohabiting will not build a lasting marriage—than the young. While 90 percent of teenagers believe in marriage, they view cohabitation as a stepping-stone in the relationship—a good way to get know their partner and avoid a future divorce. Sixty-nine percent say they “approve” of couples “living together before they get married.” They say, “If things don’t work out, we can chalk it up to experience and move on. At least we will have learned something about ourselves and marriage.”

Pamela Smock, Ph.D., a sociologist at the University of Michigan, writes: “Common sense suggests that premarital cohabitation should provide an opportunity for couples to learn about each other, strengthen their bonds, and increase their chances for a successful marriage. . . . The evidence, however, suggests just the opposite. Premarital cohabitation tends to be associated with lower marital quality and to increase the risk of divorce, even after taking into account of variables known to be associated with divorce. . . . The degree of consensus about this central finding is impressive.”

What starts as lower levels of commitment among those who choose to cohabit eventually translates into lower levels of relationship happiness both before and after the wedding, if there is a wedding. This outcome will come as a surprise to men who insist on living with a woman before considering marriage. The truth is, instead of improving their odds of success, they unwittingly improve their odds of failure.

Why is the divorce rate so much higher for couples who marry after cohabiting? Two theories have credence.

1. The “Selection Effect”

The first theory, put forth by Dr. Bumpass, is the “selection effect.” Those who live together before marriage differ substantially from those who do not, and those differences increase the likelihood of marital instability for former cohabitors. Cohabitors tend to be less educated. For example, cohabiting women are twice as likely to be high-school dropouts than college graduates. Cohabitors also tend to have nontraditional values and attitudes.

They are less likely to be religious and more likely to dismiss advice to remain chaste before marriage. They are more likely to be children of divorce. They are less committed to marriage and, when troubles arise, more open to divorce.

The problem with this theory is that most high-school dropouts in 1960 didn’t cohabit before they married, nor did the less religious nor the more liberal. It simply was not done. Additionally, few of their parents had divorced.

What has changed the culture so dramatically? The Sexual Revolution. When the birth control pill was introduced, the perceived dangers of premarital sex were lessened and the era of “free love” was ushered in. Premarital sexual activity brought less of a stigma; it actually became a badge of honor and a sign of modernity. Eventually sex without the bonds of marriage became accepted as the norm. The “Playboy Philosophy,” popularized by Hugh Hefner, promoted consensual sex anywhere, anytime, with anyone. Widespread cohabitation is the logical outgrowth of such a societal frame of reference.

2. The Cohabitation Effect

The other, more probable, theory to explain why living together dooms so many marriages is that the experience of cohabiting changes young adults in ways that increase their chances of divorce. The very act of cohabiting, with its casual, impermanent bonding, diminishes respect for commitment, especially the till-death-do-us-part vows of marriage.

As Pamela Smock notes: “Through cohabitation people learn about and come to accept the temporary nature of relationships and in particular that there are alternatives to marriage.” She cites a study showing that living together between the ages of eighteen and twenty-three “significantly alters young men’s and women’s attitudes toward marriage and divorce.” It changes “people’s attitudes in ways that make them more prone to divorce.”