More casinos do not necessarily mean more casino-goers, according to a study released by the University of Iowa.

It's a finding Maryland may have to grapple with as its casinos double in number over the next four years.

The survey looked at how much and how often Iowa residents reported using the state's 21 casinos and compared the results to similar surveys conducted in 1995 and 1989. While the number of Iowa casinos has more than doubled since 1995, the percentage of Iowans who gamble dropped from 23 percent to 14 percent over that time. The number of residents who refrained from gambling grew from 72 percent in 1995 to the current 83 percent.

Psychiatry professor Donald Black, who co-authored the study, said its findings apply to other states.

"It seems society reaches a saturation point beyond which additional gambling opportunities won't capture more people," Black said. "And that applies to problem gamblers, too. They all seem to adjust to it."

Casino saturation was a key point of debate during this year's gambling expansion fight in Maryland. While proponents looked at Maryland's rising gambling revenue as evidence that more casinos would bring more money, critics of expansion said six casinos might be too many for the region.

In the lead-up to Election Day, casino operators like the Cordish Cos., which owns Maryland Live! Casino in Anne Arundel County, and Penn National Gaming, which owns Hollywood Casino in Perryville, fretted about more facilities cutting into their market share.

Cordish Cos. Managing Partner Joe Weinberg said casino saturation was "already a serious problem for Maryland," and Penn National launched a $41.5 million fight against the expansion. But voters in November still approved a new casino in Prince George's County, table games for existing casinos and all-day operation.

The new casino is likely to be an $800 million luxury complex operated by MGM Resorts International and located at National Harbor, though bidding isn't expected to start until early next year.

Supporters say losses at other in-state casinos will be more than outweighed by the draw from D.C. and Northern Virginia, along with a National Harbor casino's likelihood of tearing Marylanders away from Penn National's casino in Charles Town, W.Va.

"It's possible we could see revenues of $40 million to $60 million a year to Prince George's County," County Executive Rushern Baker said, adding that much of the revenue will come from restaurants, shopping and entertainment rather than gambling.

The state currently has three casinos, with the Casino at Ocean Downs near Ocean City operating along with Maryland Live! and Hollywood Casino. While the Prince George's casino can't open until 2016, Caesars Entertainment is opening a casino in Baltimore in 2014, and a Rocky Gap casino is expected to open in June.