Chicago educator Vivian Gussin Paley has witnessed her share of rulers and bosses among her students -- kindergarteners. At the Laboratory School of the University of Chicago where she taught, Paley noticed that some children were always included in games, while others were left alone or in small groups.

Paley, a 1989 MacArthur Foundation "genius" grant winner, worried that the children were establishing lifelong patterns that would shape their relationships with others: The excluders would feel entitled to act superior, while the excluded would grow resigned to a status they felt powerless to change -- outsiders.

What Paley wanted her children to do was something preached by exasperated mothers everywhere: Play nice, and more specifically, play nice with everyone. To guide them toward this behavior, she imposed a deceptively simple rule: You can't say you can't play. The rule, which she had the children paint and post on the classroom wall, meant that Lisa, the most popular girl in class, had to play with Nelson or Angelo, two boys who often were shunned, if they asked her.

There was some debate and opposition, expressed mostly by the children who were seemingly in power. But within a week, life in the kindergarten went along as if the rule had been a part of the children's lives from the first day in the classroom.

Particularly for young children, a rule can be a comfort and, for parents and educators, an efficient way of calling attention to unacceptable behavior. Paley helped the children understand and adjust by telling them stories about a bird who is a friend to sad children.

"I wasted a lot of time before the rule," says Paley, who had taught for more than 30 years before she instituted the edict. "It really did make so much sense to the children. It just took me a little longer to realize the value of it."

Paley chronicled her experiences in a 1992 book, "You Can't Say You Can't Play" (Harvard University Press, paperback, $12). "The book most immediately touched people in the place that bothered them. I think the readers thought, `I'm here, too. I am struggling with this issue, too.' "

In instituting the rule, Paley was challenging the assumption that cruelty in childhood is to be expected and that children should fend for themselves when it happens -- notions she believes unfairly relieve adults of their duty to intervene. And she rejects the idea that children could benefit from such experiences.

"No one learns anything from getting hurt except thinking that it is a mean, cruel world," Paley says. "I don't believe children have the perspective and the control to let them fight it out themselves."

As they age, children grow more particular about their friends and associates, Paley says. By the time children reach middle school, they have significantly narrowed the criteria for judging who is acceptable. Research has shown that children as young as 3 pick up on racial and ethnic differences.

Before she imposed the rule, Paley shopped the idea around the upper grades in her school. She was shocked that even third-graders, who said the rule was admirable, felt that they were already too "mean" to comply with it themselves. Some children that Paley taught, however, have clung to the principle of inclusion as they moved on to other grades. "The children who . . . meet me in the grocery store . . . the first thing they ask me is, `How is the rule doing?' and they tell me how it works in their grade," says Paley, who retired five years ago.

"Vivian putting it like that and researching the topic really heightened everyone's sensitivity about it," says Carla Young, who was a Lab School teacher during that time and is currently principal of the nursery and kindergarten classes there. But, Young adds, "there is some disagreement about whether you should have that sort of flat rule. There are some children with whom it is difficult to play. If a child says, `I don't want to play,' we sort it out. I would want to know more. The bottom line is children's thoughts and feelings need to be taken seriously."

Says Paley, "You can't say you can't play" is appropriate even for much younger children, and could be applied in homes, backyards and neighborhood parks. "We are supposed to be teaching our children with the understanding that they will move on, taking positions of responsibility beside everyone of every background, every talent. We have the opportunity, from the very beginning, to build an egalitarian and empathetic society."