Tolstoy never wrote a duffer line than the one about all happy families resembling one another. Wife Swap (Tuesdays, 9pm), now in its third series, takes as its subject dissimilar families that, if not ecstatically happy, jog along well enough. The agony begins when Mum - despite the salacious title, this is a programme more about mothers than wives - is swapped. The arriving mater is shocked by the regime into which she is plunged and determines to change it. The receiving family is even more shocked - by her.
Because it is the mothers who must volunteer for this social experiment, they tend to have in common, if nothing else, a certain lippy bravado. Up in a Notting-ham suburb in last Tuesday's programme was Pat, a God-fearing prison officer who was described by her husband Spike, her senior at the local clink, as "strong, independent, exuberant and bossy". The day starts at 6.30am with chores, negotiates its way through a system of fines and punishments, and ends at 8.30pm with the children saying prayers in bed.
Down in a west London council estate lurked Pat's reverse image: Lucy, a mother of four (one daughter named Paris, one son called Tommy Lee). Her husband Tony, a van driver big on shades and boys' nights out, said she ran a house that was "mad, happy and you never know what will happen next". "Running it" was putting it too strongly - Lucy was merely the most senior unruly child. She had no money of her own, not even housekeeping, and no life outside the home. What she was permitted to do, like the kids, was "express herself" - that is, swear and whinge.
Lucy knew she wasn't in Feltham any more as she approached Pat's house and noticed the lack of kids setting fire to objects. Whereas her home was a kindergarten, Pat's was a prison (work must be a home from home for these screws). Being a child herself, Lucy found her sympathies were mainly with the children, who were not even allowed to beat each other up. Spike explained that he sees a lot of violent people at work and reckons it is down to boundaries. What are hers? "The boundaries," said Lucy impatiently, "are you can fight with your brother and sister indoors, but you don't go outside and batter some old granny for a tenner." Lucy had a plan: abolish all the rules, every one - and then throw a party.
On arrival in London, meanwhile, Pat removed a slipper from an electric fire on health and safety grounds, but was too late to prevent an infestation of nits. An educational game she bought ended up in the bin and a major investigation was launched. She identified 29 changes that needed to be made, "most of them quite severe". She thought the rules she Blu-Tacked to the kitchen wall would hit them like a bolt of lightning, but what followed was more like a five-day electrical storm.
Wife Swap will go down as the most important documentary series of the decade, not merely because of the huge ratings it gains for Channel 4, but because it is at heart a serious attempt to explore family life. Although it regularly reaches the pages of Heat magazine, it is in the business of delineating character rather than making celebrities. Inevitably, however, the exception to this rule made the best narrative in the documentary about the second series, Wife Swap Changed Our Marriage (Channel 4, 29 June). A foul-mouthed, green-eyed harridan called Lizzie was one Rochdale lass the show did make famous. This mother of eight, supported by £37,500 of state handouts, made it into the tabloids and on to Kilroy. She got her mammaries out for the Daily Sport and ended up on Channel 5's Big Brother-style show, Back to Reality. Wife Swap turned a domestic witch into national monster.
In contrast to her sorry tale, the reformation of Jason, also retold in the documentary, always brings a tear to my gills. Jason was a husband and father who believed that his duty, on his return from work, was to hand money over to his wife, Nicola, and spend the rest of the night in his games room, a private domain to which even his son had no access. After a week away, he realised the error of his ways, just as, in a fabu-lous coincidence, Nicola was preparing to demand a divorce. The documentary maintained that Jason returned a transformed man. Nicola said: "Wife Swap did not change my marriage. It saved it."
Wife Swap likes to think itself in the family therapy business, but its main job is sociological, charting how different each happy family is. Although children would love to watch, it is far too subversive for the likes of Pat and Spike to let them do so. A single episode would destroy a child's entrenched belief that their home life is "normal".
As for the wider picture, Wife Swap depicts our island peopled by the overweight, the overbearing and the self- righteous. Even the homes that run on welfare have most of the contents of Dixons in their living rooms. The F-word is entirely accepted. There is not a book to be seen. Yet, as Richard Curtis would say, there is a lot of love actually around. Unspoken, because it is not a prurient programme, that is what keeps most of these marriages going. The cheeky last moment of Tuesday's documentary recorded that in every case, when they were reunited, the couples bonked for England.
Andrew Billen is a staff writer on the Times