RICHMOND, Va. - The Virginia Department of Motor Vehicles would like to clear this up: You really are allowed to smile for your driver's license photo. A little.

Like Mona Lisa, with an ever-so-slight upward curl on both corners of the lips.

Just don't bare your teeth.

Drivers in some other states are getting a similar message as a growing number of motor vehicle bureaus implement facial recognition software to combat license fraud and identity theft.

The American Association of Motor Vehicle Administrators says 31 states are using facial recognition technology.

Virginia hopes to join them in the near future, DMV spokeswoman Melanie Stokes said. In anticipation of adding the facial recognition technology, the agency is asking Virginians to strike a "neutral expression" when they are photographed for driver's licenses loaded with new security features.

The technology compares facial features to get a match and can be used to compare license photos over time. Stokes said it works best when a similar expression is maintained from photo to photo.

Indiana, which began using facial recognition in November, uses the same rationale to stifle excessive smiling. Dennis Rosebrough, spokesman for the Indiana Bureau of Motor Vehicles, said a few people have complained about the smile restriction, but they accept it more quickly once they're told it's a security measure, not a "bureaucrat's whim."

Authorities say Indiana's system already has led to the arrest of a convicted forger as he tried to establish a sixth fake identity just months after his parole.

In Nevada, the technology helped authorities nab a welfare cheat and a sex offender, DMV spokesman Kevin R. Malone said.

Virginia's DMV began requiring the neutral expression in March. The policy has been widely interpreted as a smile ban, but Stokes said that's not quite accurate.

"You can have a grin with your mouth closed," she said. "The main thing is, just don't open your mouth."

It's not clear how many states are restricting smiles because some systems are more sensitive to variations in expression than others, said Jason King, a spokesman for the American Association of Motor Vehicle Administrators.

Nevada is a little more flexible than some, allowing a glimpse of pearly whites - but no ear-to-ear beaming.

"If someone's got a Cheshire cat grin, the technician will tell them they can't do that," Malone said.

More important to Nevada's system, which was fully implemented in January: Customers must pull their hair behind their ears and not wear glasses, Malone said. He said he was not aware of any complaints about the requirements.

In Virginia, Patrick Gunn of Chesterfield County learned firsthand that you must keep your incisors to yourself when posing for the new, laser-engraved, black-and-white driver's license photos.

"They took my picture and said, 'Your mouth was open, we have to retake it.' I guess I was kind of cracking a smile," Gunn, 31, said last week as he waited at the DMV for a temporary permit that cannot be used for identification. The permanent license, with the new security features, would be processed and mailed from a central facility in Danville - another change intended to deter those who otherwise might try to get a fraudulent license over the counter.

Kathy Bard, a 53-year-old transplant from New York, said the bland look on her Virginia license seemed unnatural.

"It's just not me," she said. "I had a big ol' smile on my New York license. When my daughter saw my Virginia license she said, 'Mom, you look like a criminal.'"