Paul Vallely ponders the surprising resilience of institutionalised monogamy

How very modern we are. There is a temptation to think that with fewer people getting married, more divorces, more cohabitation and now civil partnerships for gays we have, in recent decades, overturned a traditional view of marriage that goes back thousands of years. But history tells another tale.

It is true that throughout the late 19th and early 20th centuries, as the population of Great Britain grew, the number of marriages rose steadily, reaching a peak around 1970 as the bulge of babies born after World War II reached child-rearing age. Since then the overall number of people marrying has been declining. Moreover, close to two in five marriages in the UK now ends in divorce one of the highest rates in Europe. Yet what we regard as the traditional institution of marriage may merely be a Victorian middle-class invention and all we're doing now is reverting to an even more traditional pattern of behaviour.

Men and women have always paired off. After the Enlightenment there was a school of anthropology suggesting that humankind's natural state was one of tribal promiscuity, but this was always an ideological standpoint rather than one rooted in empirical evidence. There were cultures in which polygamy (many marriages) or, more correctly, polygyny (many wives) was common. In some societies, particularly after war had wiped out large numbers of men, this practice was at times commonplace but often it was restricted to the kings, chiefs and strong men of the community. Even more rare was polyandry, the union of several husbands with one wife.

History, boringly, shows that monogamy has been the norm; indeed, the numerical balance of the sexes, the overpowering force of human jealousy, and the welfare of children would seem to suggest that monogamy is not just normal, but dictated by evolution. "Between husband and wife friendship seems to exist by nature," as Aristotle put it in his Nicomachean Ethics, "for man is naturally disposed to pairing."

The institution through which human society has regulated sexual activity, and minimised the social conflicts that can arise from it, is marriage. That is what turns sex from a carnal indulgence into a form of social cement that brings legal, social, and economic stability to the pleasures of procreation.

Throughout the ages, there have been people who sought to express this in mystical terms. Marriage was considered to be woven deeply into the human spirit. The complementarity of sexual difference goes beyond legal contract or social institution to become, through the business of love, a binding covenant of mutual faithfulness. In Christian metaphor this is expressed as "one flesh" the notion that the couple no longer own their own body; that their body belongs to the other spouse, and to them both jointly. Eastern Orthodoxy even speaks of marriage as a martyrdom in which husband and wife learn to die to themselves for the sake of the other.

And yet for all that theological extravagance it has not been religion contrary to what many might suppose that has been the chief regulator of marriage. It is the state that has taken the leading role.

Four thousand years ago in Babylon, the king enacted a law decreeing that adulterers should be bound together and drowned. In more civilised Ancient Greece, despite Plato's perverse philosophical recommendation that the family should be abolished, a hierarchy of sexual regulation was in place that Demosthenes summarised with the epigram: "We have mistresses for pleasure, concubines to care for our daily body's needs and wives to bear us legitimate children and to be faithful guardians of our households."

So it continued in Rome, which became the source for many of our own marriage traditions the wedding ring on the third finger of the left hand, the bride in white and veiled, the hand-clasp of the married couple. The Romans too had various degrees of marriage: in one a woman lost the rights of inheritance from her father and gained them from her husband; in another the woman retained control of her own money, making divorce easier for her; in a third less binding form, a man could dispose of his wife by sending her a note saying "take your things away" as one Roman famously did, just because his wife went to the games without telling him.

It is hardly surprising then that serial monogamy which many assume we invented after the swinging Sixties was rife in the latter days of the Roman Republic and throughout the Empire. So much so that the Emperor Augustus tightened up the divorce laws fearing that the new trend would lead to low birth-rates and a population crisis.

Religion, surprisingly, kept a low profile in all this. The Jewish scriptures were full of stories of sexual liaisons which were, shall we say, unorthodox from the father of the faith, Abraham, having a child outside wedlock, to the great king Solomon with his 700 wives and 300 concubines. These were far from simple tales of ordinary family life, though in later Judaism monogamy came to be the ideal.

Christianity for centuries took its lead from St Paul's back-handed recommendation that it was "better to marry than to burn". The gospels were ambiguous on marriage; the first recorded miracle of Jesus was at a wedding, but St Luke's genealogy of Christ included only four women, all of whom had irregular sexual relationships. The early church fathers took the view that since the end of the world was looming, the faithful really had no time for sex, but should get on with preparing for Christ's imminent second coming.

The idea that celibacy, to allow a more single-minded devotion to God and his people, was the preferable option persisted within the church for centuries. The Romans saw early Christianity as decidedly not pro-marriage. And for hundreds of years thereafter the church did not concern itself overmuch with marriage, largely just accepting the marital practices of the societies into which it expanded.

It was not for a thousand years that the church began to claim exclusive jurisdiction over matrimonial cases. Even in the Middle Ages couples were betrothed not at the altar but merely in the porch of the church. It was only in the 1540s that Catholics were required to get married before a priest. And it was the 1750s before British Protestants had to wed in church, Luther having decreed that marriage was not a sacrament but a "worldly thing... subject to worldly authority".

It was only with the coming of the Industrial Revolution that marriage began to be legally codified. It was the Marriage Act of 1753 that demanded a formal ceremony of marriage, with the publishing of banns, and parental consent for minors. It outlawed common-law marriage (the notion that a couple living together were subject to the rights and obligations of a legal marriage). Within 80 years, civil marriages had been recognised as a legal alternative to church marriages under the Marriage Act 1836.

But what persisted through all this and where real change has since come in the modern era was the status of marriage as a social institution. Throughout the centuries, marriage had crucially been an economic arrangement between two families, though among poorer classes it was governed by social form and class more than financial advantage. This reached its high point in the Victorian era. As Charles Pickstone, whose book For Fear of the Angels is an intriguing study of shifting attitudes to sex and marriage, puts it: "The Victorian era, with its high moral standards, was, able to buttress the difficulties of marriage with a scaffolding of public blame and private licence (at least for men)." It was when that scaffolding gave way that marriage shifted from being a social institution to a vehicle for personal fulfilment.

Economics consolidated the shift. Life became more comfortable as the Industrial Revolution continued. The affluent began to have more time on their hands for "relationships". Individuals who in previous centuries would have been content to settle for second best now developed much higher demands of what marriage should deliver emotionally. As the younger generation moved away from home to go to college, and broke links with their extended family, they began to invest more emotionally in marriage and the bond of sexual fidelity.

Women going out to work provided another gear change in the process. A century earlier, John Stuart Mill in The Subjection of Women had pointed out that women's decisions to marry could scarcely be called "free" given their low wages, precarious employment situation and poor educational prospects. The choice to marry, he said, was a Hobson's choice. When women began to go out and earn decent money, things changed again.

Feminist critiques of marriage followed. In The Feminine Mystique in 1963, Betty Friedan criticised the idea that women could only find fulfillment through child rearing and home making. In Feminism: An Agenda, 20 years later, Andrea Dworkin likened marriage to prostitution. Sheila Cronan took the view that marriage constitutes slavery for women, and that liberation meant the abolition of marriage. There followed gender feminism, equity feminism and post-feminism with suggestions that "marriages" should be replaced by five-year rolling contracts or that domestic responsibilities should be set down in legally binding documents.

Most of the world could not quite come round to applying doctrines of jurisprudence to dishwashing and continued to see marriage as rooted in ties of love and affection rather than the principles of justice. But expectations of marriage continued to rise and with it rates of divorce as those elevated expectations were dashed. "If love goes, the marriage goes" became the new orthodoxy. In the past, divorce had been a luxury for the rich, but almost everyone in the West was rich by the end of the 20th century.

Even so, Aristotle's truth still obtains. Marriage remains the commonest form of partnership between men and women. In 2006, of the 17 million families in the UK, 70 per cent were headed by a married couple. And though the number of cohabiting couples has doubled in two decades to around 2.2 million couples in the UK more than half of those will go on to marry.

Wedding ceremonies that a generation earlier had marked the start of a new household within the community now were seen as consolidations of an established relationship. The children of the couple became the bridesmaids and pages. And since cohabiting couples are statistically twice as likely to split up as married ones the wedding ritual has become an expression of stability.

There are new variations on the old theme, with solid second marriages after the failure of a "starter marriage" earlier on. But still today 95 per cent of women and 91 per cent of men in the UK have been married by the age of 50. And divorces have fallen for the past three years.

If the external pressures to marry have declined, the inner ones remain strong. Marriage still has an enduring magic. Even in recessions a high proportion of income continues to be spent on weddings, and the fairytale elements of the veil and white dress persist across the social scale; indeed there is very little difference in how different classes celebrate their weddings, apart from in scale and cost. Four Weddings and a Funeral remains one of the most successful British films ever made.

The fact that a third of marriages now end in divorce seems to make little difference. Couples queue to make vows that are splendidly extravagant. "Love seeks not a promise of affection," as the philosopher Roger Scruton has noted, "but a vow of loyalty" unconditional, lifelong and extraordinarily ambitious.

"The marriage contract is unlike most contracts," writes the academic L J Weitzman in that most unromantic of titles, The Economic Consequences of Divorce. "Its provisions are unwritten, its penalties are unspecified, and the terms of the contract are typically unknown to the contracting parties... No one would sign it if they had read it first."

But we do. And we continue to.