Richard and Maureen Kanka’s 7-year-old daughter Megan was raped and murdered in Hamilton, New Jersey, in 1994. The Kankas were infuriated when they discovered that the man accused of killing Megan was a twice-convicted child molester who had moved in across the street from their home. Their outrage led to the adoption of a state law—and in 1996 a federal law—known as “Megan’s Law,” which requires local police to notify communities when convicted sex offenders move into the neighborhood.

Critics of Megan’s Law maintain that such notification laws constitute an invasion of privacy and are unconstitutional because they impose punishments on people who have already served their sentences. For example, Alexander Artway, who served seventeen years for sexual assault, filed suit against New Jersey, claiming that the state’s disclosure...