In the summer of 1965, one of the most explosive government reports ever written disclosed that one-quarter of all black children were born out of wedlock.

The report, by then-Assistant Labor Secretary Daniel Patrick Moynihan, touched off waves of alarm, then defensiveness, then recrimination. The subject proved so sensitive that public policy-makers became convinced there was nothing they could do to address it. Eventually, a protective silence descended.

Twenty-five years later, there is a new statistic: More than one-quarter of all children born in America are born out of wedlock.

The black out-of-wedlock rate has more than doubled to 63.5 percent (in 1988, the last year for which figures are available). But the white rate has risen faster, more than quadrupling from just over 4 percent in 1965 to 17.8 percent in 1988. Of all the babies born out of wedlock in 1988, 539,696 were white and 426,665 were black.

"Illegitimacy levels that were viewed as an aberration of a particular subculture 25 years ago have become the norm for the entire culture," said Moynihan, now a Democratic senator from New York.

Few changes in social mores have been more sweeping or less understood. For example, the popular wisdom that the rise in nonmarital births is mainly the result of a rise in teenage sexual activity doesn't stand scrutiny. Teenagers are more sexually active than ever, but teenage birthrates have declined since 1965 (though not nearly as fast as adult birthrates). Moreover, teenagers today account for about one-third of all out-of-wedlock childbirths, down from about one-half in 1965. The drop is due to the shrinking pool of teenagers in the population.

"There is no question this is one of the primary puzzles of social science," said Mark Testa, a University of Chicago sociologist. "In the 1960s, there was a global movement away from marriage as the locus of sexual relations. Since then we've seen an increasing detachment of childbearing from wedlock, and we may be entering a third stage in which marriage is no longer the locus of child-rearing. And we're not really sure why all this has happened."

There has been no shortage of theories.

Moynihan's 1965 report, which focused on black out-of-wedlock birthrates, traced the phenomenon to weaknesses in the black family structure stemming from slavery. However, research has shown that two-parent child-rearing was more common among slaves, and remained more common among blacks for a century following their emancipation, than it has been during the past quarter-century. The black out-of-wedlock birthrate was just 18 percent in 1950.

Another theory holds that the transition to a post-industrial economy has made marriage a less viable economic package for disadvantaged young men because they cannot fulfill the traditional male breadwinner role.

In the mid-1980s, University of Chicago sociologist William Julius Wilson developed what he called the Male Marriageable Index (MMI) Pool, which showed that the ratio of employed black men to black women under age 35 had fallen sharply during the 1970s and early 1980s, in a pattern that closely tracked the decline in black marriages.

However, more recent studies show that Wilson's MMI increased from 1980 to 1988, while the rate of black out-of-wedlock births continued to rise and marriage rates continued to decline. Moreover, the marriage rate of employed black males has fallen more since 1970 than has the rate for unemployed black males, according to a study published last fall by David T. Ellwood, a professor of public policy at Harvard.

Marriage is no longer the norm for young blacks. According to Ellwood, just 30 percent of all black men and women age 15 to 44 were married in 1988, down from 50 percent in 1960. Among whites of the same age group, the figure was 51 percent in 1988, down from 65 percent in 1960.

Another theory is that Aid to Families with Dependent Children (AFDC), the principal welfare program for the poor, has created perverse incentives for out-of-wedlock childbearing because it targets benefits mainly to single mothers. This proposition has long been the subject of heated ideological sparring. In a book on single mothers that reviews a multiplicity of studies, authors Irwin Garfinkel and Sara S. McLanahan note that for the past 15 years, out-of-wedlock birthrates have been rising while AFDC payments have been falling (in constant, or inflation-adjusted, dollars). They estimate that the availability of AFDC accounts for no more than a small fraction -- one-tenth to one-seventh -- of the growth in single parenting over the past generation.

"I wouldn't argue that young women have babies in order to collect welfare, but I do argue that the availability of welfare buffers the decision not to get married once they become pregnant," said Kate O'Beirne, vice president for government relations at the Heritage Foundation, a conservative think tank. Few liberals disagree with that formulation.

With no single cause standing out, scholars are increasingly pointing to a combination of social, cultural, demographic and economic forces to explain the growth in out-of-wedlock births.

"My own view, though I can't prove it, is that there is an interactive process going on, which has several dimensions," said Robert Lerman, a sociologist at American University. "Sexual activity at an earlier age has become more widespread. At the same time, the age at which people get mature jobs and think about mature activities has grown. So it's not uncommon to have a period of five or 10 years between the time when young people become sexually active and the time they are ready to think about getting married. Inevitably, this results in more out-of-wedlock births."

Meantime, "shotgun weddings" have become less common, as fewer young expectant parents feel the tug of social conventions pulling them toward matrimony.

"I suspect that some of what's going on here is trickle-down morality," said Douglas Besharov of the American Enterprise Institute, noting the publicity that surrounds movie stars and other celebrities who bear children out of wedlock.

A small portion of the increase in out-of-wedlock child-rearing comes from middle-class and professional women who want children and who have taken to heart Cher's dictum: "Men are a luxury." But the practice remains heavily concentrated among the poor. A high school dropout is 12 times more likely than a college graduate to bear a child out of wedlock.

There is a similar phenomenon in Western Europe. Between 1970 and 1986, out-of-wedlock births rose from 7 percent to 20 percent in France, from 8 to 21 percent in Britain, from 13 to 23 percent in Austria, and from 6 to 9 percent in West Germany. In Sweden and Denmark, where the lines between adult cohabitation and marriage virtually have disappeared in the last two decades, out-of-wedlock births have soared to 48 and 43 percent, respectively.

All of these post-industrial economies have undergone similar demographic changes: the rapid movement of women into the work force, accompanied by an increase in two-worker households, accompanied by sharp declines in fertility rates. "A big reason that out-of-wedlock percentages have risen so fast is that married women are having fewer babies," said Mary Jo Bane, a professor of social policy at Harvard. "A lot of the action has been in the denominator, not the numerator." Fertility rates among married women have declined by more than 40 percent in the last 25 years.

In the United States, even though teenagers account for only one-third of out-of-wedlock childbirths, roughly 70 percent of all such births are to women who had their first child when they were teenagers.

AFDC notwithstanding, the decision of these women to have children does not make economic sense. Studies show that bearing a child out of wedlock is a ticket to long-term poverty.

Why does it happen? For years, sociologists have theorized that for many young women in the underclass, childbearing is an expression of hope against the bleakness of their lives. The desire is magnified among poor young black women, who are likely to have been raised in a matriarchal subculture. They "invite pregnancy because having children is their only way of becoming adults and of making sure that they will have a family in which they can play the dominant role for which they have been trained by their culture," sociologist Herbert Gans wrote 25 years ago in response to the Moynihan report.

Poor young men also may find psychic rewards in nonmarital fatherhood. "As they come to sexual maturity, often having grown up in fatherless homes, they are under the influence of a peer group that emphasizes sexual prowess as evidence of manhood," said Elijah Anderson, a sociology professor at the University of Pennsylvania and author of "Streetwise: Race, Class and Change in an Urban Community."

"They lure young girls into sexual relationships with vague but convincing promises of marriage. A lot of the young girls do want to get married, but the young men don't want anything to do with marriage unless they can do it in high style. Unfortunately, they have a pretty unrealistic concept of what marriage should involve -- basically, a wife who waits on you hand and foot."

Once children are born out of wedlock, there is a distinct racial pattern to the structure of the families they grow up in. Black mothers are much more likely to raise their children in an extended female kinship network that includes their own mothers. White mothers are more likely to get married, though these marriages are unstable and about two-thirds wind up in divorce. White women are more likely than blacks to receive child support payments from the biological father, but less than one-third of white women get such payments.

Besharov has estimated that between 70 and 90 percent of all children born out of wedlock wind up on welfare at some point in their lives, and Ellwood has found that such children are much more likely to stay on welfare for longer periods than children of divorce.

Among policy prescriptions offered for out-of-wedlock child-rearing are time limits on AFDC payments, toughening enforcement of child-support laws, making birth control more widely available, targeting more benefits to two-parent families, and offering more apprenticeship and training programs that would give men greater access to the job market at a younger age.

However, many experts are skeptical about how much public policy can accomplish. "What has happened in all post-industrial societies is that middle-class people have dramatically reduced their childbearing, and poor people have reduced theirs as well, but not as fast," said Besharov. "Nor do they see the need to get married.

"One hopes that people will change their behavior, but when you get all of these economic and cultural forces acting together," he said, "it is very hard to legislate changes."