STOCKHOLM • Something was wrong with the Penguins, the incoming class of toddlers at the Seafarer's Pre-school, in a wooded suburb south of Stockholm.

The boys were clamorous and physical. The girls held up their arms and whimpered to be picked up. The group of one-and two-year-olds had, in other words, split along traditional gender lines.

And at this school, that is not OK.

Their teachers cleared the room of cars and dolls. They put the boys in charge of the play kitchen. They made the girls practise shouting "No!" Then they decided to open a proper investigation, erecting video cameras in the classroom.

Science may still be divided over whether gender differences are rooted in biology or culture, but many of Sweden's government-funded pre-schools are doing what they can to deconstruct them. State curriculum urges teachers and principals to embrace their role as social engineers, requiring them to "counteract traditional gender roles and gender patterns". It is normal, in many Swedish pre-schools, for teachers to avoid referring to their pupils' gender - instead of "boys and girls", they say "friends", or call children by name. Play is organised to prevent children from sorting themselves by gender.

A gender-neutral pronoun, "hen", was introduced in 2012 and was swiftly absorbed into mainstream Swedish culture. Exactly how this teaching method affects children is still unclear. One of the few peer-reviewed efforts to examine the method's effects, published last year in the Journal of Experimental Child Psychology, concluded some behaviours do go away when children attend what the study called "gender-neutral" pre-schools.

For instance, the children at these schools do not show a strong preference for playmates of the same gender, and are less likely to make assumptions based on stereotypes. Yet, scientists found no difference at all in the children's tendency to notice gender, suggesting that may be under a genetic influence.

Recently in Hammarbyhojden, south of Stockholm, Seafarer's in-house gender expert Elis Storesund sat bent over a worksheet with two teachers of the four-and five-year-old Seagulls, reviewing their progress on gender objectives.

"When we are drawing," said Ms Melisa Esteka, 31, one of the teachers, "we see that the girls - they draw a lot - they draw girls with lots of makeup and long eyelashes. It's very clear that they are girls. We ask, 'Don't boys have eyelashes?' And they say, 'We know it is not like that in real life'."

Ms Storesund, 54, nodded thoughtfully: "They are trying to understand what it is to be a girl."

Ms Esteka looked frustrated. She had set a goal for herself: to stop the chldren from identifying things as "for girls" or "for boys". But lately, her pupils were absorbing stereotypes from billboards and cartoons, and sometimes it seemed like all the systematic work of Seafarer's was flying away overnight. She said: "They bring the whole world with them. We can't stop that from happening."

Sweden's experiment in gender-neutral pre-schools began in 1996 in Trodje, a small town near the Baltic Sea. The man who started it, Mr Ingemar Gens, was not an educator but a journalist who dabbled in anthropology and gender theory, having studied Swedish men seeking mail-order brides in Thailand. Newly appointed as a district "equal opportunity expert", Mr Gens wanted to break down the norm of stoic, unemotional Swedish masculinity.

Pre-school struck him as the right place to do this. Swedish children spend much of their early life in government-funded pre-schools, which offer care at nominal cost for up to 12 hours a day starting at the age of one. Two schools rolled out what was called a compensatory gender strategy. Boys and girls at the pre-schools were separated for part of the day and coached in traits associated with the other gender. Boys massaged each other's feet. Girls were led in barefoot walks in the snow, and told to throw open the window and scream. A wave of criticism broke.

"They said we were indoctrinating the kids," Mr Gens said. "I say we're always indoctrinating kids."

Teachers were required to review videotapes of themselves with the children, to identify subtle differences in the way they interacted with boys and girls. Many found that they used more words, and more complex sentences, with girls.

Ms Helena Baggstrom, who taught at one of the schools, recalled watching footage of herself in a cloakroom, attending to children as they bundled up to go outside. She saw, to her shock, that she had helped one boy after another get dressed and run out the door. The girls, she realised, were expected to dress themselves. "It was hard at first to see patterns," she said. "We saw more and more, and we were horrified at what we saw."

The strategy of separating boys and girls was later set aside in favour of a "gender neutral" approach intent on muting differences. Still, the spirit of Mr Gens' experiment had percolated through the government. In 1998, Sweden added new requirements to its national curriculum that all pre-schools "counteract traditional gender roles and gender patterns" and encourage children to explore "outside the limitations of stereotyped gender roles".

One of the Penguins' teachers, Ms Izabell Sandberg, 26, noticed a shift in a two-year-old girl whose parents dropped her off wearing tights and pale-pink dresses. The girl focused intently on staying clean. If another child took her toys, she would whimper. "She accepted everything," Ms Sandberg said. "And I thought this was very girlie."

Until, that is, a recent morning, when the girl had put a hat on and carefully arranged bags around herself, preparing to set off on an imaginary expedition. When a classmate tried to walk off with one of her bags, the girl held out the palm of her hand and shouted "No" at such a high volume that Ms Sandberg's head swivelled around. It was something they had been practising.

Later, the girl had become so loud she drowned out the boys in the class, Ms Sandberg said. At the end of the day, she was messy. The girl's parents were less than delighted, and reported that she had become cheeky and defiant at home.

But Ms Sandberg has plenty of experience explaining the mission to parents. "This is what we do here, and we are not going to stop it," she said.

NYTIMES