Here is an experiment you don't want to try at home.

Show a toy -- a doll, say, or a model boat -- to a toddler and explain that it it's something special you've had since you were little. Ask the child to be ''very careful'' with it. Hand over the toy, which appears to be in fine condition, except that you've secretly rigged it to break spectacularly as soon as the child handles it.

When your precious toy falls apart, express regret by mildly saying, ''Oh, my.'' Then sit still and observe the child.

The point is not to permanently traumatize anyone -- the researchers who performed this experiment quickly followed it with a ritual absolving the child of blame. But first, for 60 seconds after the toy broke, the psychologists recorded every reaction as the toddlers squirmed, avoided the experimenter's gaze, hunched their shoulders, hugged themselves and covered their faces with their hands.

It was part of a long-term study at the University of Iowa to isolate the effects of two distinct mechanisms that help children become considerate, conscientious adults. One mechanism, measured in other experiments testing toddlers' ability to resist temptations, is called effortful self-control -- how well you can think ahead and deliberately suppress impulsive behavior that hurts yourself and others.

The other mechanism is less rational and is especially valuable for children and adults with poor self-control. It's the feeling measured in that broken-toy experiment: guilt, or what children diagnose as a ''sinking feeling in the tummy.''

Guilt in its many varieties -- Puritan, Catholic, Jewish, etc. -- has often gotten a bad rap, but psychologists keep finding evidence of its usefulness. Too little guilt clearly has a downside -- most obviously in sociopaths who feel no remorse, but also in kindergartners who smack other children and snatch their toys. Children typically start to feel guilt in their second year of life, says Grazyna Kochanska, who has been tracking children's development for two decades in her laboratory at the University of Iowa. Some children's temperament makes them prone to guilt, she said, and some become more guilt-prone thanks to parents and other early influences.

''Children respond with acute and intense tension and negative emotions when they are tempted to misbehave, or even anticipate violating norms and rules,'' Dr. Kochanska said. ''They remember, often subconsciously, how awful they have felt in the past.''

In Dr. Kochanska's latest studies, published in the August issue of The Journal of Personality and Social Psychology, she and colleagues found that 2-year-olds who showed more chagrin during the broken-toy experiment went on to have fewer behavioral problems over the next five years. That was true even for the ones who scored low on tests measuring their ability to focus on tasks and suppress strong desires to act impulsively.

''If you have high guilt,'' Dr. Kochanska said, ''it's such a rapid response system, and the sensation is so incredibly unpleasant, that effortful control doesn't much matter.''

But self-control was critical to children in the studies who were low in guilt, because they still behaved well if they had high self-control.

''Even if you don't have that sinking feeling in the tummy, you can still suppress impulses,'' Dr. Kochanska said. ''You can stop and remember what your parents told you. You can stop and reflect on the consequences for others and yourself.''

But what if your child lacks both self-control and guilt? What can you do? And should you feel guilty for doing a lousy job of parenting?

Well, you could blame yourself, although researchers haven't been able to link any particular pattern of parenting to children's levels of guilt, says June Tangney, a psychologist at George Mason University. But Dr. Tangney, who has studied guilt extensively in both children and adults, including prison inmates, does have some advice for parents. (To offer your thoughts on parents and guilt, go to

''The key element is the difference between shame and guilt,'' Dr. Tangney says. Shame, the feeling that you're a bad person because of bad behavior, has repeatedly been found to be unhealthy, she says, whereas guilty feelings focused on the behavior itself can be productive. But it's not enough, Dr. Tangney says, for parents just to follow the old admonition to criticize the sin, not the sinner. ''Most young children,'' Dr. Tangney said, ''really don't hear the distinction between 'Johnny, you did a bad thing' versus 'Johnny, you're a bad boy.' They hear 'bad kid.' I think a more active, directive approach is needed.''

She recommends focusing not just on the bad deed, but more important, on how to make amends. ''Both children and adults can be surprisingly clueless about whether and how to make things right,'' Dr. Tangney said. ''Little kids are overwhelmed by the spilled mess of milk on the floor. Parents can teach and support them to say 'I'm sorry' and to clean it up, maybe leaving the kitchen a little cleaner than it was before.''

That was the same atonement strategy, by the way, followed by the experimenters in Iowa who tricked the children with the broken toy. After the 60 seconds of angst, the children were asked what had happened and then were told that the toy could be easily repaired. The researcher would then leave the room with the broken toy and return in half a minute with an intact replica of it. The experimenter took the blame for having caused the damage, reassuring the children that it wasn't their fault and that the toy was now as good as new anyway.

No harm, no foul, no guilt. If only the rest of their lives were so simple.