Stopping driver's license fraud is no laughing matter: Four states are ordering people to wipe the grins off their faces in their license photos. Neutral facial expressions" are required at departments of motor vehicles (DMVs) in Arkansas, Indiana, Nevada and Virginia. That means you can't smile, or smile very much. Other states may follow. The serious poses are urged by DMVs that have installed high-tech software that compares a new license photo with others that have already been shot. When a new photo seems to match an existing one, the software sends alarms that someone may be trying to assume another driver's identity. But there's a wrinkle in the technology: a person's grin. Face-recognition software can fail to match two photos of the same person if facial expressions differ in each photo, says Carnegie Mellon University robotics professor Takeo Kanade. Dull expressions "make the comparison process more accurate," says Karen Chappell, deputy commissioner of the Virginia DMV, whose no-smile policy took effect in March. Arkansas, Indiana and Nevada allow slight smiles. "You just can't grin really large," Arkansas driver services chief Tonie Shields says. A total of 31 states do computerized matching of driver's license photos and three others are considering it, says the American Association of Motor Vehicle Administrators. Most say their software matches faces regardless of expressions. "People can smile here in Pennsylvania," state Transportation Department spokesman Craig Yetter says.