Consider the driver's license. That unimpeachable document you show to enter government buildings, get on a plane or go anywhere sensitive in this post-9/11 world. It's the cornerstone of our security these days, used to weed out the dangerous terrorist from the benign traveler. It establishes that you are YOU and not some impostor. Because everything's true on a government-issued ID, right?

Name? Check. Address? Check. Birth date? Check. Sex? Check.

Weight? ...

Yeah, right.

Let's face it, most women in America lie about their weight on their driver's licenses.

There are blogs, Internet mailing lists and entire Web sites devoted to the topic, like the one on CalorieCounts.com titled "Want to get down to driver's license weight lol." One contributor named Lori explained her situation this way: "I did not lie on my license at the time I had the picture taken. But here in GA we can renew by mail for 10 years. At renewal time, I was at my highest weight which was 117 lbs more than my license."

I think I've lied on every driver's license application since I was 16 and never thought twice about it. So when I went to renew my Virginia driver's license not long ago, I was pleasantly surprised to see that my fictitious weight was nowhere to be seen on the face of my shiny new license.

Had Virginia's Department of Motor Vehicles finally come to the sad realization that women are big fat liars about their weight and that it was time to end the useless charade? Not exactly. "Uh, we discontinued showing weight on driver's licenses in 1985," said Pam Goheen, Virginia DMV director of communications. It was all part of a license redesign, she said. There wasn't enough space for all the usual physical identifiers, so they left out weight, eye and hair color.

Virginia still asks for weight, it just doesn't show it anymore. It's in the squiggly black lines of the bar code on the back of my license now, which can be read only by a little scanner in the car of the state trooper or police officer who pulls me over and runs my license through the Virginia Criminal Information Network.

"It is to provide additional information for law enforcement and to assist in the prevention of fraud," Goheen said.

So a law enforcement officer with a keen eye could still figure out I'd lied about my weight?

"I would remind everyone that the application to apply for or renew a driver's license or identification must be signed by penalty of perjury so the information presented is correct," she said. "It's a Class 2 misdemeanor to knowingly make a false statement on a driver's license."

(Can you IMAGINE what would happen if they started enforcing those perjury penalties? "Out of the car, ma'am. Step on this scale nice and easy now.")

So why is weight enshrined on so many driver's licenses? Why are state motor vehicle departments mindlessly promoting such widespread mendacity? I decided to investigate.

First, I called Jason King, spokesman for the American Association of Motor Vehicle Administrators, who said he had no idea why states ask for weight.

He promised to check into it. While I waited, I wondered if other countries were as weight-conscious as we. I Googled "International Driver's License" and called the first listing that popped up, Vimar International in Brooklyn, N.Y., and spoke to Helena Kairsa.

"We do not have weight. We have only height," she said.

"Why don't you include weight?"

"Why should we?"

Silence.

"The weight is variable," she said finally. "Today one weight. Another day another weight. So why do we have to include it?"

After a week or so, King e-mailed me a chart, "Items Included on a Drivers License." It's a survey of all 50 states and 12 Canadian provinces done in 1999, the most recent available. Turns out there are only 10 states in the country that do not show weight on driver's licenses: Arkansas, Connecticut, Florida, Michigan, New York, North Carolina, Pennsylvania, Tennessee, Texas and Virginia.

I asked them why not.

Folks at the North Carolina DMV don't remember ever showing weight on their licenses. "We don't even ask about it," said soft-spoken spokesperson Marge Howell. "Somebody suggested that could get us in a lot of trouble."

Florida took weight off their licenses along with hair and eye color in 1973, DMV officials there said, when they started using photos instead.

"Do we really not have the weight?" That was Laura McPherson, spokesperson for Tennessee's Department of Public Safety. She pulled out her wallet and looked. "You're right. They have my weight on my state ID, though." She let out a gasp. "Mine's totally wrong. I'm 15 pounds heavier. Wow, when did I ever weigh that much? And I know I lied about it then, too."

McPherson later e-mailed her official response: "In July of 1989, Tennessee implemented the Classified and Commercial Driver License Program. This law brought about many changes, several of which affected the design of the driver license itself. The decision was made to leave off the weight field in order to put other pertinent data on the face of the license."

Danielle Klinger, the Pennsylvania DMV spokesperson, said they dropped weight and hair color 20 years ago. Why? "They can change," she said. "Height, maybe, yeah, with your younger drivers. But something like height or even eye color is something that's not going to change, obviously, unless someone wears color contacts. Even so, that would be noted on the driver's license, that they wear corrective lenses. But one day you could be a brunette and the next you could be a blonde. Same with a person's weight."

New York, Michigan, Connecticut and Arkansas weren't sure why they didn't use weight.

"In the 10 years I've been doing this job, I've never been asked the question," said Bill Seymour, DMV spokesperson in Connecticut. "It's probably vanity. This is the Gold Coast. You know, Botox and all."

Of all the states, only flinty "live free or die" New Hampshire, which discontinued using weight in 1999, acknowledged the obvious. "People just aren't honest about their weight," said DMV spokesperson Katie Daley. "Our law enforcement has come to conclude that that's not a correct way to determine if that's the person you're looking at. Whereas people don't tend to lie about their height."

Well, apparently they do. MEN do.

Eric Ossiander, an epidemiologist with the Washington State Department of Health, decided to analyze driver's license information and let other researchers know whether it could be a valid source of data.

It wasn't.

Women lied about their weight, and men lied about their height, as anyone who's ever tried computer dating could probably tell you. (Cornell University researchers found 52.6 percent of the men in their 2007 computer dating study boosted their height an inch or more and 64.1 percent of the women trimmed their weight by at least five pounds.)

"You might expect that," Ossiander said. "Considering what human nature is."

A quick search of the medical literature on PubMed finds the same is true around the world. The Japanese can't be trusted to report their height and weight accurately. British researchers use a "predictive equation" to mathematically correct for our fibs. And French researchers warn, "Self-reported weight and height should be treated with caution."

P. Willey, a physical anthropologist at California State University in Chico, wanted to know whether information on a driver's license could be used to help forensic researchers identify skeletal remains. He and a colleague, Tony Falsetti, studied college students, in the prime of life, and found nearly everyone fudged. "People tended to round to an even digit. Every male was six foot. Hardly anybody was 5-11. One fella overreported his stature by six inches. And with weight they rounded down to the nearest five to 10 pounds," he said. "I certainly have."

So what does a driver's license really tell you?

"That we're all very tall and very thin," Falsetti said. "Psychologists would have a field day. Why would someone, on this inconsequential document, a driver's license, want to distort their body image? I supposed because what we all want to be is a Swedish model." (Which is probably true even in Sweden, where a 2007 study of 1,703 Swedes by the Skaraborg Institute in Skovde found nearly everybody lied about their weight and height.)

So why have weight and height on a driver's license at all?

"In our field, human identification, we use it, so do medical examiners and coroners, as a 'guideline,' " Willey said. "It's often the only information they have. They can run it more easily than tracking down the next of kin, and the memory of how much a loved one weighed or how tall they were can be really off. So they just tap into driver's licenses as a general practice. It's easy. It's simple. They know we're all lying. But taking that into account, they can kind of start to sort things out."