WHY, after centuries of relative stability, should marriage suddenly be under such a concentrated 30-year siege? There is no single cause behind the changes. Instead, social historians point to a number of factors. One of the most frequently cited is the change in women's attitudes (to be discussed in tomorrow's article). Others include the following:
The postwar baby boom. After World War II, the United States and many European countries experienced an atypical surge of births. From 1945 to 1962, age at first marriage was low and births were frequent and came early. In America, suburban housing tracts and public schools sprouted up to accommodate the burgeoning population. ``This is an unusual time demographically,'' says Arthur J. Norton of the United States Census Bureau with considerable understatement, describing a period which de mographers think has few if any historical parallels.
It was also, however, a period of relative prosperity in the United States -- which, as many researchers note, helps promote marital stability. Chicago economist Gary S. Becker notes that for all but the very wealthy couples, ``the higher the income, the lower the divorce rate.'' But he also notes that ``divorces fall during bad times and rise during good times'' -- when, he says, women can more readily support themselves apart from the marriage.
One theory of marital instability, built on these twin facts of a baby boom and relative prosperity, holds that marriages encounter strain when the income fails to match aspirations. For baby-boom parents, who grew up during the 1930s depression, aspirations were modest. In fact, however, they found themselves very well off in the 1950s -- a period which, in retrospect, appears to many to have been something of a golden age of marriage.
Unlike their parents, however, baby-boom children encountered a reverse pattern when they reached maturity. Born into 1950s prosperity, they had high aspirations. But because their numbers were so many, they found themselves in tough competition with one another for slots in higher education and, eventually, in the work force. Result: greater turmoil within marriages.
According to this theory, the relatively fewer children of the ``baby bust'' generation now following the earlier ``baby boom'' should once again be well-off. Brought up in a period of declining expectations, they face less competition from one another and will have relatively higher incomes. That should herald greater marital stability.
Increasing age at first marriage. ``The only trend that seems to be continuing into the 1980s,'' says Mr. Norton, ``is the increase in the age of marrying.'' Census Bureau figures for 1984 show that the estimated median age of first marriage was 23 years of age for brides and 25.4 for grooms -- the highest age for women since 1890.
Why? Some observers point to a growing acceptance of cohabitation as a prelude to marriage -- that young people still come together in consensual unions, but feel less pressure to marry than did their counterparts in prior generations.
Other observers point to what demographers call ``the marriage squeeze'' -- a delay in marriage occasioned when the habit of younger women marrying slightly older men encounters a period of population growth.
According to this argument, for every 100 women reaching (for example) age 22, there are fewer than 100 ``eligible'' men aged 24 or 25 -- since (because of expanding birthrates) there were fewer of them born to begin with. The result: a postponement of marriage until the women find someone nearer their own age. That phenomenon occurs around age 30 -- when, according to a recent NCHS report, ``the age difference becomes negative and the husband is younger than the bride, on the average.''
The increasing age of first marriage appears to depress the overall marriage rate -- since some of those who don't marry early never do marry. It also depresses the birthrate, since women of childbearing age spend less time in marriage.
But it may bring down the divorce rate. The strongest predictor that a couple will eventually divorce is generally thought to be the age at marriage: The younger the couple, the greater the probability of divorce.
Prevalence of contraception. Although modern contraceptive devices intended for birth control have been in use throughout the 20th century, it was only in the mid-1960s that reliable female-controlled methods came into wide use.
In 1955 Gregory Pincus at the Worcester Foundation for Experimental Biology in Massachusetts found a group of hormonelike substances that inhibit ovulation in women -- commonly known as ``the pill.'' A few years later, experiments with various mechanical intrauterine devices proved successful. So strong was the trend toward the use of these methods that by 1962 the American Consumers' Union began evaluating contraceptives.
By the 1970s, the increasing availability of contraceptives had effectively removed one of the age-old barriers against extramarital intercourse: the fear of unwanted pregnancy. Not surprisingly, extramarital sexual activity has substantially increased. Among women marrying from 1960 to 1964, 48 percent had chosen to delay sexual intercourse until marriage (NCHS figures). For those marrying between 1975 and 1979, the figure had fallen to 21 percent. With the link clearly severed between marriage a nd sexual gratification, both the delay in marriage and the number of cohabiting couples have increased rapidly.
Growing individualism. Many observers link the instability of marriage to the increased emphasis on individual happiness and fulfillment. They also point to the corresponding decline in commitment to institutions beyond oneself (including religious institutions) that became so prominent during the ``Me Generation'' discussions of the 1970s.
``We undoubtedly have become more individualistic in our way of thinking about family relationships,'' says Samuel H. Preston, a demographer at the University of Pennsylvania and former president of the Population Association of America. These days, he says, we evaluate things ``much more from the point of view of what it means to us as individuals in terms of our own personal agendas, and much less [in terms of] the value of fulfilling esteemed social roles.''
Sociology professor Andrew H. Cherlin of Johns Hopkins University, author of ``Marriage, Divorce, Remarriage,'' says that today ``there's a much greater emphasis on personal satisfaction'' than in the past. Spouses are ``much more concerned about their own personal happiness and well-being than were their parents and grandparents. Our grandparents were much too busy wondering about how to make ends meet.''
Professor Cherlin traces the idea that the individual is supremely important back to the Enlightenment and the French and American Revolutions. But he notes that ``we are the first generation in which most people have the luxury of cultivating their own gardens, so to speak -- of being concerned with our personal welfare, because we don't have to worry about our economic welfare.''
The resulting emphasis on individual happiness, say most observers, has encouraged many people to kick over the traces of miserable marriages -- marriages that, in the past, might simply have been tolerated. One result, according to many experts, is a much greater honesty within the marriages that remain -- and much less tolerance for marriages built on pretense.
But the new individualism has also heightened the stakes for all marriages. ``We expect so much of marriage,'' says Stanford University sociologist Lenore J. Weitzman, author of ``The Divorce Revolution,'' a recently published study of no-fault divorce. As a marriage partner, she says, ``you not only have to be a breadwinner and make a good living and have an interesting career, but you also have to be a great tennis partner and a stimulating conversationalist -- or whatever else it is that you value. It's a very heavy burden.''
Increasing longevity. A century ago, say social historians, there were plenty of single-parent families -- caused not by divorce but by death. But with life expectancy increasing rapidly, and with the mortality of mothers at childbirth all but eradicated, marriages that last 50 years or more are increasingly common -- and make it all the more important, say some observers, to found marriages on enduring common values.
Such longevity also shifts the ground rules for the children of these elderly married couples. Brought up to expect that their own mature years would be relatively free of constraints, they may find themselves spending many years caring for aging parents -- while perhaps putting their own children through school at the same time.
Not surprisingly, these changes sometimes show up on the sensitive barometer of a marriage -- since, as Professor Cherlin notes, ``the family is where the effects of social change are most closely and personally felt, because it's the central arena for our own feelings of personal well-being.''
Romantic love. According to social historians, the idea of love as a prerequisite for marriage is a new and astonishing notion.
``It's the first time in history that marriage has become the place of love,'' says French sociologist Louis Roussel. Never until now was marriage ``a refuge from society,'' he says -- an escape into a self-contained world of affection and romance. In primitive societies, he notes, marriage was sometimes prohibited between those who were infatuated -- precisely because such a couple would have inward-looking goals and aspirations that could run counter to the needs of the society as a whole.
In 20th-century Western cultures, such concerns have all but disappeared.
``There was a time when people were expected to stay married even when they weren't in love,'' says Frank F. Furstenberg Jr., a sociologist at the University of Pennsylvania who writes widely on marrige and the family. ``Now,'' he says, ``they're virtually expected to divorce if they're not in love.''
Professor Furstenberg sees a direct link between romantic love and increases in divorce. Today's marriage, he says, is ``built around companionship, affection, love.'' As a result, society permits young people a wide ``degree of discretion in selecting mates'' -- at the same time it emphasizes ``gratifying marriage, satisfying marriage.'' When such expectations are not met, the marriage breaks up.
Given all the emphasis on romantic love, Furstenberg contends that divorce is ``not an anomaly, but a structural feature of the American marriage system.''
Yet many see the emphasis on love as evidence that marriage is evolving toward a sounder footing -- away from economic convenience and toward a more satisfying (and, for some, a more Christian) basis. In a recent survey of 300 happily married couples, sociologists Jeanette and Robert Lauer asked each partner to rank a list of reasons for why the marriage had endured. The couples gave top priority to variations on the theme of love: ``My spouse is my best friend'' ranked first, followed by ``I like my s pouse as a person.