Few devices know more personal details about people than the smartphones in their pockets: phone numbers, current location, often the owner's real name—even a unique ID number that can never be changed or turned off. These phones don't keep secrets. They are sharing this personal data widely and regularly, a Wall Street Journal investigation has found.

An examination of 101 popular smartphone "apps"—games and other software applications for iPhone and Android phones—showed that 56 transmitted the phone's unique device ID to other companies without users' awareness or consent. Forty-seven apps transmitted the phone's location in some way. Five sent age, gender and other personal details to outsiders.

The findings reveal the intrusive effort by online-tracking companies to gather personal data about people in order to flesh out detailed dossiers on them.

Apps sharing the most information included TextPlus 4, a popular iPhone app for text messaging. It sent the phone's unique ID number to eight ad companies and the phone's zip code, along with the user's age and gender, to two of them.

Both the Android and iPhone versions of Pandora, a popular music app, sent age, gender, location and phone identifiers to various ad networks. iPhone and Android versions of a game called Paper Toss—players try to throw paper wads into a trash can—each sent the phone's ID number to at least five ad companies. Grindr, an iPhone app for meeting gay men, sent gender, location and phone ID to three ad companies.

Mobclix then matches that location with spending and demographic data from Nielsen Co. In roughly a quarter-second, Mobclix can place a user in one of 150 "segments" it offers to advertisers, from "green enthusiasts" to "soccer moms." For example, "die hard gamers" are 15-to-25-year-old males with more than 20 apps on their phones who use an app for more than 20 minutes at a time.