Jan. 23 (Bloomberg) -- Concern that short-sellers accelerate stock declines may prompt the Securities and Exchange Commission to adopt a rule next month aimed at curbing bearish bets when equities are plunging.

The regulation would require the trades be executed above the best existing bid in the market when shares fall 10 percent in a day, said Brian Hyndman, the senior vice president in transaction services at Nasdaq OMX Group Inc. In a short sale, an investor borrows an asset and sells it, hoping to profit from a decrease by repurchasing it later at a lower price.

Forcing short sellers to wait for a stock to rise above the best price bid may prevent them from flooding the market with sell orders and causing losses to multiply. Some exchange officials say the restrictions known as uptick rules don’t work, citing studies that show they may be less effective during panics that drive prices down and volatility up.

“There is no empirical data to support the introduction of a new rule,” Hyndman said yesterday at a securities industry conference in Chicago. “But this is the least intrusive of the proposals the SEC was considering.”

Hyndman expects the SEC to adopt a so-called alternative uptick rule that includes a 10 percent trigger, changing regulations that were eliminated from U.S. markets in 2007. The commission asked the public last April to comment on strategies to cushion the impact of short selling following criticism that hedge funds and other speculators used trading tactics to deepen market retreats that began in 2008.

SEC spokesman John Heine declined to comment.

Computer Upgrades

The Standard & Poor’s 500 Index dropped 9.1 percent in September 2008 after New York-based Lehman Brothers Holdings Inc. filed the biggest-ever bankruptcy. The SEC implemented a ban on short selling more than 900 financial stocks that month after Morgan Stanley Chief Executive Officer John Mack and New York Senator Charles Schumer blamed the practice for driving companies to the brink of collapse.

The implementation date for the new rule is likely to be later in the year, according to Hyndman, who didn’t say what he was basing his estimate on. He said exchanges and brokers will probably have 180 days to upgrade their computer systems to accommodate the regulation.

Nasdaq in New York, Kansas City-based Bats Exchange and Jersey City, New Jersey-based Direct Edge Holdings LLC, which operates two alternative trading centers, have told the SEC that no new restrictions on short selling are needed. Paul Adcock, executive vice president in charge of trading at NYSE Arca, a unit of New York-based NYSE Euronext, said that while most exchanges oppose a new regulation, it’s probably inevitable.

Potential Impact

“Because the politicians and the public are all banging the drums, we’re not going to get away with this one,” Adcock said about the reluctance of exchanges to support new short- selling restrictions.

The SEC discussed the potential impact of such a rule when it proposed the alternative uptick last August. Because it would restrict short selling more than other proposals being considered, the regulation might “lessen some of the benefits of legitimate short selling, including market liquidity and pricing efficiency,” the commission said.

When the SEC proposed the alternative uptick rule, it said it would be easier for exchanges and brokers to implement than the former regulation that operated on the New York Stock Exchange for almost 70 years before its removal in 2007. That rule would no longer make sense in a marketplace of automated trading, the commission said.

No Trigger

The rule was proposed to the SEC last March by NYSE Euronext, Nasdaq, Bats and the Chicago-based National Stock Exchange. NYSE Euronext last June said it preferred a different bid test with no 10 percent threshold.

NYSE Euronext’s Adcock raised concern at yesterday’s conference that so-called circuit breakers setting off the restriction might keep stocks from falling as much as they should when a company reports bad news.

“Do you trigger the 10 percent when the stock should be trading down?” Adcock said. The trigger would be mandated uniformly across trading venues when a stock declines by the specified percentage.

Daniel Aromi and Cecilia Caglio, economists at the SEC in Washington, said in a December 2008 report to former Chairman Christopher Cox that even with uptick rules in place, short sellers in a simulation executed trades 25 percent faster on average when stocks plunged than when prices were steady.

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