The husband, who like others in this article was reached through his lawyer, said the decision to invade his wife’s privacy was not an easy one. “If I were to tell you I have a pure ethical conscience over what I did, I’d be lying,” he said. But he also pointed to companies that have Internet policies giving them the right to read employee e-mail messages. “When you’re in a relationship like a marriage, which is emotional as well as, candidly, a business, I think you can look at it in the same way,” he said.

When considering invading their spouse’s privacy, husbands and wives cite an overriding desire to find out some secret. One woman described sensing last year that her husband, a Manhattan surgeon, was distant and overly obsessed with his BlackBerry.

She drew him a bubble bath on his birthday and then pounced on the device while he was in the tub. In his e-mail messages, she found evidence of an affair with a medical resident, including plans for them to meet that night.

Image Jolene Barten-Bolender says she discovered a tracking device in a wheel well of the family car. Credit... Melanie Fidler for The New York Times

A few weeks later, after the couple had tried to reconcile, the woman gained access to her husband’s America Online account (he had shared his password with her) and found messages from a mortgage company. It turned out he had purchased a $3 million Manhattan condominium, where he intended to continue his liaison.

“Every single time I looked at his e-mail I felt nervous,” the woman said. “But I did anyway because I wanted to know the truth.”

Being on the receiving end of electronic spying can be particularly disturbing. Jolene Barten-Bolender, a 45-year-old mother of three who lives in Dix Hills, N.Y., said that she was recently informed by AOL and Google, on the same day, that the passwords had been changed on two e-mail accounts she was using, suggesting that someone had gained access and was reading her messages. Last year, she discovered a Global Positioning System, or G.P.S., tracking device in a wheel well of the family car.