Weaker earnings are being divided among ever multiplying numbers of casinos. Baltimore’s casino will be the fourth to open in Maryland, with a fifth soon to rise down the Potomac from Washington, DC. Maryland’s casinos compete with a clutch of new casinos in Philadelphia and Delaware.

Why so much building? Cities are authorizing more casinos for exactly the same reason that the existing casinos are losing business: the weak national economy. Casinos promise a new and easy flow of revenues to hard pressed local governments.

The promise however comes increasingly hedged with fine print.

The casino market is nearing saturation, if it is not already saturated. Two casinos have closed in Mississippi this year. Four have closed or will soon close in Atlantic City, including the glitziest hotel on the boardwalk, Revel.

Casinos that do stay in business yield less to their towns and states. Revenues from Maryland’s first casino, in Perryville, at the northern tip of Chesapeake Bay, have already dropped 30 percent from their peak in 2008, and are expected to decline even more rapidly in future as competitors proliferate.

Yet the truly bad news about casinos is not found in the tax receipts. It’s found in the casinos' economic and social impact on the towns that welcome them.

Until the late 1970s, no state except Nevada permitted casino gambling. Then Atlantic City persuaded its state legislature to allow casinos, in hope of reviving the prosperity of the battered resort town. Hotels sprung up along the seafront. Thousands of people were hired. And the rest of Atlantic City … saw no benefits at all. All these years later, it still has desperate trouble sustaining even a single grocery store.

No one should look to casinos to revive cities, “because that’s not what casinos do.” So explained the project manager for a new Wynn casino rising near Philadelphia. He’s right, but it has taken a surprisingly long time for city governments to acknowledge a fact that was well understood by the 19th-century Americans who suppressed gambling in the decades after the Civil War.

The impact of casinos on neighboring property values is “unambiguously negative,” according to the economists at the National Association of Realtors. Casinos don’t encourage non-gaming businesses to open nearby, because the people who most often visit casinos do not wander out to visit other shops and businesses. A casino is not like a movie theater or a sports stadium, offering a time-limited amusement. It is designed to be an all-absorbing environment that does not release its customers until they have exhausted their money.

The Institute for American Values has gathered the best evidence on the social consequences of casinos. That evidence should worry any responsible city government.

People who live close to a casino are twice as likely to become problem gamblers as people who live more than 10 miles away. As casinos have become more prevalent, so has problem gambling: in some states, the evidence suggests a tripling or even quadrupling of the number of problem gamblers.