Mud-Slinging Enters Debate Over Ethanol's Impact on Gasoline Prices Viewed from outside the ivory tower, a dispute among economists over ethanol's impact on gasoline prices might be expected to proceed in a genteel manner. Instead, the debate has bubbled over. Christopher Knittel, an energy economist at the M.I.T. Sloan School of Management, and Aaron Smith, an agricultural economist at the University of California, Davis, took issue in a recent paper with a claim, made in advertisements by the Renewable Fuels Association, a trade group, that ethanol production reduced gas prices nationally by 89 cents a gallon in 2010, and by more than $1 a gallon in 2011. Agriculture Secretary Tom Vilsack has also cited the 2010 figure in his defense of the biofuel. The trade group's claims are based on academic work by Dermot J. Hayes of Iowa State University and Xiaodong Du of the University of Wisconsin, Madison. The professors" original peer-reviewed work on the subject, published in 2009, assigned a more modest amount of credit to the biofuel for its ability to reduce gasoline prices. That work, however, was updated in May - with financial support from the trade group - for Iowa State's Center for Agricultural and Rural Development, and the findings ascribed greater price-reducing powers to ethanol. The updated report included the assertion that ethanol production in 2011 had an average marginal effect of reducing gasoline prices by $1.09 a gallon, with regional impacts ranging from 73 cents a gallon on the Gulf Coast to $1.69 a gallon in the Midwest. Mr. Knittel and Mr. Smith claimed that the scholarship was "driven by implausible economic assumptions and spurious statistical correlations." To illustrate their critique, the professors employed the same statistical models used in the Iowa State report to draw intentionally absurd conclusions, among them that ethanol production depresses the price of natural gas and increases unemployment in the United States and Europe. "We also used their model to show that ethanol production is causing my daughter to grow older, and if it was taken away she'd get younger," Mr. Smith said in a telephone interview. We know that's not true, or at least we hope it isn't true. Matt Hartwig, a spokesman for the Renewable Fuels Association, defended the scholarship represented in the Iowa State report, noting it was based on a peer-reviewed model, and was updated annually. "I would be surprised if this paper from M.I.T. were to hold up under such scrutiny," he said in a telephone interview. It doesn't read like a scientific paper, but more like two professors with an ax to grind taking potshots at the secretary of agriculture and R.F.A. A report by the National Renewable Energy Laboratory in 2008 cited various estimates for ethanol's monetary impact on the price of a gallon of gasoline. "Without ethanol, today's gasoline prices would be 20 to 35 cents higher," the report said, citing the Energy and Agriculture Departments. Ethanol has long been added to gasoline in 10 percent concentrations, but a richer formulation known as E15, having been approved by the Environmental Protection Agency, began making its way to filling stations this month. The cost savings for the motorist who chooses E15 over E10, however, are not so clear, as explored recently by Wheels. Mr. Knittel of M.I.T. said in an interview that ethanol most likely reduced gasoline prices, at most, by 10 cents a gallon, after adjustments were made to account for the biofuel's lower energy potential relative to pure gasoline. He described claims made by the trade group, which have appeared at bus shelters and billboards, particularly around Washington, as "false advertising." For his part, Mr. Hayes, one of the authors of the Iowa State study, said in an e-mail that the marginal effect on gasoline prices was difficult to calculate precisely. Even so, he was confident that the report released in May would stand up to scrutiny. "We will have to let the academic process play its way out," he wrote.