Megan Kanka, Jessica Lunsford, Shasta Groene.These girls, all believed to be sexually molested by men and two of them killed, have generated headlines, prompted states to stiffen penalties against offenders and caused many parents to fear their child could be next.Yet gut-wrenching as these high-profile cases are, they don't tell the whole story of sex offenders in America. They don't reflect the surprisingly good news: Sex crimes against children have dropped dramatically in the last decade. An online national sex-offender registry was launched in July. And recent research shows doctors can better predict which offenders may strike again. "There's a...