FORGET the seven-year itch. Eleven years is the average time that divorced couples had stayed together before their marriage broke up, according to figures from the Office for National Statistics (ONS).

The finding has sparked a debate between supporters of marriage as a beacon of social stability and those who believe the expectation that couples should stay together for life runs counter to the lessons of history and evolution.

Instead, they suggest, marriage should be viewed as an economic partnership which couples should expect to fail when the emotional and economic benefits begin to decline.

“The costs and benefits of a relationship are more fluid than in the past,” said Malcolm Brynin, co-author of Changing Relationships, a new book based on five years of research into family life published by the Economic and Social Research Council. “People come together and stay together only when this is to their individual advantage.”

However, Frank Furedi, professor of sociology at Kent University and author of Paranoid Parenting, attacked such a mercenary approach to marriage.

“When you get married, if you make this kind of statistical calculation saying, ‘Well, I’m getting married. The chances are we’ll only get to 11 years’, the whole ritual becomes entirely pointless. If you adopt the idea, we might as well give up on the concept of durable relationships altogether.”

Brynin’s book follows ONS figures showing that the number of UK marriages in 2007 fell to 270,000, a 2.6% decline from 2006. In 1940 there were 426,100 weddings.

The 2007 figures also show that 144,220 couples were divorced and the average length of marriages ending in divorce was 11.5 years.

Brynin and others argue that the expected norm of marriages lasting for decades is a modern construct and social and economic pressure may be pushing the relationship beyond its “natural” duration.

In previous centuries, death rather than divorce was what cut marriages short. The average length of marriage of a modern couple who divorce is roughly the same as the 11-15 years spent together by late medieval English peasants, who stayed married until one of them died.

Michael Buchanan, the twice-divorced author of The Marriage Delusion, a new book, said: “In previous centuries people would get married early, have children and then be parted by the death of one or other, usually within a decade or two. “Nowadays they can expect to live for four to five decades after marriage. It’s unrealistic to expect most people to sustain love and interest in each other for such long periods, especially if their children have grown up and moved out.”

Some researchers suggest the short lifespan of early man meant he evolved to discard old partners and seek new ones to maximise potential offspring.

In his book Love Sick, Frank Tallis, a former neuroscientist at the Institute of Psychiatry in London, argues that falling in love is an evolutionary device to make men and women stick together for the few years necessary to produce children and see them through their most vulnerable years of growth.

After that, love slowly and inevitably dies, allowing both partners to move on to someone new. He said: “This duration corresponds exactly with the time it would have taken our ancestors to produce and wean children.”

Janice Hillier, a clinical psychologist, said her experience of dealing with relationships confirmed the 10 to 15-year mark could be pivotal — when tensions become insuperable in couples who married for love without rational reflection.

Iain Duncan Smith, the Tory MP who chairs the centre for social justice, said: “Marriage is on average not going to be for life, but that doesn’t mean those 11 years would be better spent apart.

“Marriage is good for society, good for children, good for binding together communities. All of that says we should value marriage and try to stabilise it.”

Sandy and Susannah Mackie from Warwickshire are currently divorcing amicably after 10 years. “After that time we just decided we weren’t meant to be together,” said Sandy.

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