A ruling handed down by New Hampshire's highest court is the latest illustration of how a defendant's web-browsing history can be turned into criminal evidence.

The case is a disturbing one: A young mother was convicted of manslaughter in the 2010 death of her 1-year-old son who drowned after she left him unattended in a bathtub. A forensic examination of her computer showed that after running a bath for her child and another young son, she spent 42 minutes viewing and writing posts on a website that helps women raise donations...