When companies face financial hardship, they are often tempted to merge, splitting their costs and combining their customers. Couples do the opposite. The weight of unpaid bills and overdue loans can break a relationship: the unspoken vow that haunts many marriages is "till debt us do part".

But a demerger makes little economic sense. Ever since Gary Becker of the University of Chicago published "A Treatise on the Family" in 1981, economists have thought of the family as a factory, combining inputs of money and time to produce domestic bliss. This marital enterprise has obvious advantages over its competitor, the single-person household, just as the factory can outdo the lone artisan. Couples enjoy economies of scale--sharing a bed, a kitchen and perhaps a car--and can benefit from a division of labour between them.

It will surprise no one to learn that men devote more hours to paid employment, women to housework. But contrary to popular belief, these two contributions balance out, on average, in most rich countries. In America, according to a report by Michael Burda of the Humboldt University of Berlin and others, men do 313 minutes a day of market work and 163 minutes of housework (liberally defined as anything you could pay someone else to do for you), whereas women do 201 and 271 minutes, respectively. The great exception is Italy, where men, in the words of the novelist Ian McEwan, "expect their wives to replace their mothers, and iron their shirts and fret about their underwear".

The economic gains from coupledom are substantial. For middle-aged women, the cost of living in a married couple is 30% less than the cost of living alone, according to calculations by Arthur Lewbel of Boston College and Krishna Pendakur of Simon Fraser University, using Canadian prices. For men, it is about 20% less. Marriage, it seems, is a bargain.

But how do couples divide these spoils? Economists once touchingly assumed that spouses wanted the same things and acted as one to achieve them. Households were "glued together" in Amartya Sen's phrase. Now economists think of marriage as a bargain in a second sense. Husbands and wives haggle over cash, chores, care and consideration. By one recent estimate, the split in Britain is 40:60 in favour of the man. But these deals are not set in stone. When the government started paying child benefit to wives not husbands, in the 1970s, Britain's unglued households began spending less on booze and home-cooked food and more on restaurants and takeaways, according to Jennifer Ward-Batts of Claremont McKenna College. They also spent £6 more a year on toys.

Bad times can force a painful renegotiation. Of unemployed men consigned to household chores, Friedrich Engels wrote: "One may well imagine the righteous indignation of the workers at being virtually turned into eunuchs." That indignation has lingered in the 160 years since he wrote about it. Anne Solaz of France's National Institute for Demographic Studies (INED) has shown that when French women lose their jobs, they do more housework and their husbands do less, as one might expect. But when French husbands are laid off, their wives do no less housework, even though the man does more. Perhaps he doesn't do it very well.

These bargains may not be fair. But economists typically assume they are efficient. Between them, spouses create the biggest pie possible, even if they do not split it equitably. No other bargain could make one of the spouses better off, without making the other worse off. The traditional family, for example, strikes a bargain in which the wife sacrifices her career prospects to raise the children both spouses want, freeing the man to make more money. If the husband is sufficiently generous with his improved earnings, the bargain should make them both better off.

But this model of marriage is becoming obsolete, according to Betsey Stevenson and Justin Wolfers, an unmarried couple at the University of Pennsylvania's Wharton School. Because people live longer and breed less, couples spend a smaller fraction of their lives together with children. No need, then, for one to specialise in child-rearing, the most demanding of domestic tasks. Many other chores can now be automated or outsourced. Before electric appliances, it took four hours to wash 38lb of laundry, by stove-heating water, scrubbing on a washboard, then wringing by hand or mangle.

Households also now spend less time, and more money, feeding the family. But even in America, working couples still spend nine and a quarter hours a week putting food on the table. At America's average wage of $20 an hour, that time would be worth $185 in the labour market. People are under-using the take-out menu.

The marriage factory faces a second threat: couples now find it harder to make their bargains stick, because vows can now be broken cheaply and without stigma. If a woman agrees to sacrifice her career to make a home, her prospects outside marriage diminish, leaving her with less conjugal clout. The husband might therefore be tempted to renege on his commitment to share his earnings equitably. But if his wife anticipates this outcome, she won't make the bargain in the first place. The rise of easier, "no-fault" divorces, Betsey Stevenson shows, has made American women less willing to have children or to pay their husband's way through college. And no one takes home economics anymore.

Coupledom has changed, these two economists argue, from a mode of production to a mode of consumption. Instead of complementing each other in their work, spouses complement each other in their pleasures. The husband enjoys cinema-going more because he goes with his wife. In the traditional economic conception of the family, one spouse provides the things money can buy, the other provides the time good housekeeping demands. Now a spouse is someone to spend time and money with. That may be some consolation when economic misfortunes leave a couple with more time and less money than they'd bargained for.

Picture credit: korbatz/flickr

(Simon Cox is a business correspondent for The Economist.)