Attempt to determine what your daughter sees in her friend. "Ask her anything from 'What's your favorite thing to play with Janey?' to 'How does it make you feel when she gets in trouble?'" says Kathy Thornburg, Ph.D., director of the Child Development Laboratory at the University of Missouri-Columbia. If your child wants to experiment with misbehavior but lacks the temperamental moxie to do it alone, "she may be using this other little girl to do it vicariously," says Thornburg.

When Miss Misbehavior plays at your house or in your yard, feel free to enforce your rules. I used to feel awkward disciplining other people's children. Now I think of it as enforcing the laws of a host nation: While you are visiting my tiny republic on East Pleasant Street, if you are mean to others, you will be deported. Remind your daughter of your family's code of conduct and the good reasons behind it, and do so in front of her friend (and also later, after the girl has gone home).

Encourage other relationships, but don't try to forbid this one. And remember: Just as your child can grow out of a nasty stage, so can her friend. Besides, a heart of gold and an important friendship could be hiding behind the naughtiness.

Trisha Thompson is a contributing editor to PARENTING magazine and a former editor-in-chief of BabyTalk.

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