When the rollcall of men and women who shaped the 20th century is finally read, Benjamin Spock will be near the top. His has been a household name since Baby And Childcare outsold everything except the Bible in the 1950s and 1960s.

Spock has become a piece of public property, attracting instant opinion and sometimes dismissive scorn from those who these days think they know it all. If we do, it is in part thanks to him.

Dr Spock changed the upbringing of a generation. That is almost certainly true. More doubtful is the later accusation, that he formed a generation - that the permissive society, the drop-outs, the rebels without a cause and those stoned out of their heads were Spock babies and that the paediatrician who marched with them against the Vietnam war was to blame for a moral degeneration in the fabric of American youth.

But behind all this hype and fury is a fundamental truth - that millions of women were very, very grateful to Dr Spock. These were the new mothers of the 1950s, struggling between their instinct to cuddle, fuss over and feed their child, and the perceived wisdom of the time, as handed down by patriarchal doctors.

Babies were not born innocent but needed to be tamed and civilised, these august figures decreed. Luther Emmett Holt demanded four-hourly baby feedings. John Watson warned of the pernicious influence on children of cuddling: "Never hug or kiss them. Never let them sleep in your lap."

Steel yourself against the urge to embrace, mothers were told. Shake hands with your children in the morning. To curb the thumbsucking habit, parents were instructed to tie the offending digit to the cot.

Into this world of Victorian sado-masochism burst Ben Spock. If the opening words of his childcare manual are often quoted it is because the change in thinking they offered was so radical. It felt like freedom. "Trust yourself. You know more than you think you do," Spock wrote.

It was not just the words but the tone. For the first time, new mothers were being given advice by a doctor who treated them as colleagues in the child-rearing business. He did not patronise them. He encouraged the expression of motherly love, indulged thumbsucking and advised against smacking. This was revolution.

Some 50 years on, when the bookshop shelves are stacked with manuals on childcare, it is hard to understand the impact Spock had or the gratitude with which women grasped his ideas and clung to them. By the 1960s, every mother had her wellthumbed copy by the bed. There were lots of jokes about dropping the baby and then picking up book and baby at the same time.

"He seemed to take the lid off loving your children," said Carolyn Douglas, founding director of Exploring Parenthood. "People were in some sort of straitjacket paradigm of the time. You had to discipline your children and you had to control them and not feed them until exactly four hours had passed. The fact that you were dying of desperation as your child turned purple screaming from hunger did not matter."

Spock is a landmark. Nothing in parenting was the same again. For all the childcare volumes now produced each year, nothing as dramatically different as what he had to say has been offered since - in the mainstream at least.

Most of the childcare experts who offer guidance to parents have refined in some way on his advice. And Spock's own thinking changed with the times too, causing him to be accused of betraying his followers on more than once.

In Britain, another paediatrician, Hugh Jolly, was quickly dubbed "the British Spock". In one matter he went further than Spock, praising the comfort of the family bed, where children are tucked in for the night beside their parents. He encouraged mothers to breastfeed in hospitals at a time when the fashion was for bottles, and encouraged fathers to be at the birth, a cause Sheila Kitzinger later made her own. She was so successful that it has now become not a rarity but the norm.

Dr Penelope Leach is to some the inheritor of the Spock mantle. "If I used a book at all," she admitted, "it was Spock." But to his "you know more than you think you do" she added "and more than anyone else".

Spock, in the later revisions of his thinking, agreed with her. What the professionals, the baby doctors and the childcare gurus had to say was nowhere near as important as the gut feelings of the mother. Perhaps influenced by the feminist who became his second wife, but perhaps because he always had an ability to think radically, Spock also reshaped his parenting advice to include the father far more than his original volume did.

He had been hurt by the violent reaction against him of the women's movement in the 1970s. When he stood for president as the People's Party candidate in 1972, Gloria Steinem denounced him at the National Women's Political Caucus: "Dr Spock, I hope you realise you have been a major oppressor of women in the same category as Sigmund Freud." He publicly apologised for his sexist attitude. The next edition of his book appeared with every suspect reference edited out, such as his advice to fathers to compliment their daughters on their dresses.

"I think the one mistake he made was that he was too frightened by the reaction of the women's movement," says Leach. "I have great sympathy. It is awful to find that inadvertently one has been taken that way. It often happens when somebody is trying to put children first."

She believes fathers must shoulder half the burden of childcare to lessen the burden on women but Spock never got that far. "I don't think he really quite believed it. Remember, he took part in the Olympics in the 1920s. The man was almost as old as the century."

From Spock's advocacy of the truth of the parental instinct, thinking became more and more child-centred. Carolyn Douglas, while admiring Leach, worries that her totally non-confrontational attitude to children who misbehave - do not smack, but reason with the child and if you cannot control yourself leave the room - does not offer much to the parent who lacks the necessary self-belief.

Some find her too idealistic in an imperfect world. "If you like your child, if you are proud of him and pleased with yourselves as parents so far, you may be able to get right through his childhood without ever thinking about `discipline' as a topic at all," she writes in her bestselling, Baby And Child.

In the US, the child-centred approach has had many critics in recent years. John McEnroe, the spoilt brat of Wimbledon, became a sort of symbol as the typical product of a lax family regime where children are sacred, given everything and never disciplined.

The pendulum has been swinging back for a while now. The Belfast-born paediatrician Christopher Green, who practises in Sydney, Australia, is the bestknown of the new disciplinarians. His book, Toddler Taming, has become a bestseller in the UK.

Toddlers are miniature Idi Amins, he says, all "power without sense". While he believes the best approach is to ignore the screaming child, his book has a controversial chapter called "Smacking used correctly".

In the US, where opinion swings can be extreme, a fundamentalist Christian couple are suddenly all the rage. Gary and Anne Marie Ezzo say babies should be left to cry because "God did not intervene when His Son cried out on the cross". Their book, On Becoming Babywise, is now selling faster than Leach or Spock and a million babies are now being raised, it is said, on their extreme theories.

Extreme they are - a return to those Victorian ideas which Spock kicked into touch. Children as young as 18 months should be spanked for banging their highchair tables. Children up to three should be slapped up to five times.

Babies should not be carried in a sling - mothers are not marsupials. And most worryingly, babies should not be fed on demand during the night. Paediatricians have been warning that their theories are dangerous and there are reports of malnourished babies being admitted to clinics.

For the most part, however, childcare specialists tread moderate paths these days, provoking a stir usually only when they address a particular problem - like the under-achievement of boys, which the Australian guru, Steve Biddulph, has made his own. Positive masculine qualities do not emerge naturally, he says. Boys have to learn them from men. He advocates play-wrestling between fathers and their sons.

It seems that, while many childcare theories will vanish in the wind, the fundamental things remain. And despite the enemies he made over his stance on Vietnam and his attitude to women, Spock hit a fundamental truth or two. "Trust yourself," he said. Nobody will dare gainsay him for that.