We'd been on the top deck of the ferry churning across Long Island Sound for just a few minutes last week when my son sidled over and whispered, "Why are so many people smoking? At home, we almost never see people smoking."

My 9-year-old wanted to know why smoking was so much more popular in one part of the country than in another.

As it turns out, I came home to find that our federal tax dollars have purchased a study of just such differences. For the first time, the government has analyzed the data on smoking, drinking and drug use all the way down to the county and ward level. The results are riveting.

My son's observation about smoking was on target: Cigarette smoking in Washington, Maryland and Virginia -- a home to the tobacco industry -- is well below the U.S. average. And our home, the District's Ward 3, west of Rock Creek Park, has the city's lowest smoking rate after Ward 4, right across the park (22 and 20 percent of adults surveyed, respectively). The region's lowest rate is in Northern Virginia, where only 18 percent said they smoke.

But before highly educated Washingtonians congratulate ourselves on our health consciousness, mark this: When the survey asked who had used alcohol in the past month, it found the highest rate of use in the District -- in staid, serious Ward 3. (A whopping 71 percent, compared with 47 percent nationwide and in Maryland, 55 percent in Northern Virginia, and 36 percent in Ward 8 in Southeast.) Ward 3's affinity for a drink -- just a glass of wine with dinner, of course -- aligns well with the results in similarly overeducated spots such as Boston's western suburbs and Boulder, Colo.

There appears to be an inverse relationship between smoking and drinking rates, says Douglas Wright, a research statistician at the Substance Abuse and Mental Health Services Administration, which compiled this study from three years of interviews with 207,000 Americans.

Ward 3's D.C. Council member, Kathy Patterson, embraced that news as another reason to support a proposed ban on smoking in bars and eateries. "So now I can tell tavern owners that as soon as we go smoke-free, everyone will drink more," she quipped.

Why do the feds think anyone would respond honestly to a survey about vices? To assure confidentiality, field interviewers show up at front doors, hand over a laptop with a headset and then go away for a while to allow private recollections of altered states. The feds sweeten the deal with a $30 gratuity. No questions asked about how you use the cash.

While Washington's well-heeled set may argue that its drinking rate reflects a concern for cardiac health or a penchant for dinner parties, it's harder to explain why Ward 2 -- Georgetown, Foggy Bottom, Dupont Circle -- has by far the region's highest marijuana usage. Fully 8.5 percent of ward residents surveyed said they'd smoked pot in the past month, as opposed to 4.2 percent in Ward 4, 3.6 percent in Northern Virginia, 4.5 percent in Maryland and 5 percent nationwide. (Each state defines its regions as it wishes. Maryland divided itself so oddly as to be useless for pop sociology.)

Ward 2 is also the city's leader in use of illicit drugs other than pot. Wards 7 and 8 east of the Anacostia River report the least drug use in the city; they, like Northern Virginia and Maryland, are below the U.S. average.

You could explain Ward 2's daze, as its council member, Jack Evans, did, by pointing to college kids at Georgetown and George Washington. But plenty of places with lots of colleges don't come close to those numbers. Ward 2 is right up there with the top marijuana usage spots in the nation -- the island of Hawaii, Portland, Ore., Boulder, Vermont's Champlain Valley, Northern California, Boston and, for that matter, virtually the entire state of Massachusetts. (Lowest pot use: Iowa.)

Evans suggests another explanation: "We have a huge influx of single, young, affluent people. Other wards may be more suburban, more family-oriented."

Wright, the statistician, offers a variation on that idea: "Generally, drug use, particularly marijuana, is higher in places with higher incomes."

It's an old story: The police focus on drug markets in rough neighborhoods, while buyers commute carefree from more affluent spots. Is that fair?

Join me at noon today for "Potomac Confidential" at www.washingtonpost.com/liveonline.