The Philadelphia City Council’s recent rejection of Mayor Michael Nutter’s new push for a soda tax may signal (most consumers hope) the end of a mostly fruitless year for “Twinkie tax” advocates. Despite activists’ contentions, this mask for a municipal money-grab has long faced public doubts of its alleged effectiveness as a population-wide weight loss plan. New research finding that a soda tax wouldn’t work is adding to fat-taxers’ frustrations. Why? Obese people generally prefer to consume diet soda, which has no calories. They’re also less price-sensitive, making them less likely to modify behaviors when faced with an increase in the price of sugar-sweetened pop from a new tax.

TIME magazine asked Yale professor (and Twinkie tax creator) Kelly Brownell for his reaction to the latest research:

[Brownell] has doubts about the accuracy … Simulations of the potential impact of public health actions such as a soda tax are based on a huge number of assumptions — about consumption, spending behavior, weight change — that are, in reality, difficult to make accurately, he explains.

Fair enough. It is just one study. But Brownell isn’t so cautious when it comes to other studies of soda taxes — namely his own.

In a 2009 article Brownell (with fellow food cop Thomas Frieden) wrote that soda taxes “could become a key tool in efforts to improve health.” He also claimed a soda tax would lower health care costs significantly.

But Brownell seems to make many of the same kinds of assumptions about consumption, spending behavior, and weight changes that he now criticizes. After a December study found that not even considerable taxes on soft drinks (of 20 and 40 percent) would be effective, Brownell insisted that “nobody has been able to see how people will really respond under these conditions.”

Kelly Brownell seems to be saying that he doesn’t know any more than the next researcher what will happen under a “Twinkie tax” regime. And “nobody knows” isn’t an appropriate burden of proof for new laws, much less wide-ranging social engineering.