Reason links to a new study purporting to show that increased fast food consumption is not linked to poverty. 

Unfortunately, the study uses consumption figures from 1994-96. That wouldn't be a big deal, except that McDonald's didn't launch its Dollar Menu until 2002, at which point McDonald's consumption skyrocketed.

Reacting to the success of McDonald's Dollar Menu, Wendy's and Burger King both started promoting their versions of low-priced deals. Wendy's, which in 1989 was the first burger chain to experiment with menu items for $1, lowered prices on its Super Value Menu to 99 cents in January. And in February, Burger King started offering its own version of a dollar menu, including the Whopper Jr. and cheeseburgers.

The Dollar Menu became a permanent part of McDonald's menu in the United States in late 2002. It offers items like a double cheeseburger, the fried McChicken sandwich, French fries, a hot fudge sundae, pies, a side salad, a yogurt parfait and a 16-ounce soda.

Since McDonald's started advertising the Dollar Menu nationally, the double cheeseburger has become the chain's most ordered item. Even priced at $1, double cheeseburgers bring in more revenue than salads or the chicken sandwiches, which cost $3.19 to $4.29.

McDonald's executives say the Dollar Menu has driven enormous additional traffic into the stores, primarily young men and women aged 18 to 24. "The Dollar Menu appeals to lower-income, ethnic consumers," said Steve Levigne, vice president for United States business research at McDonald's. "It's people who don't always have $6 in their pocket."

You would think that a study analyzing the consumption of fast food by poor people would take into account a massive sea change in the way fast food was consumed by poor people.  But then again, I'm not a science-type person.

UPDATE: Jacob Sullum notes my objection, and also points out that the upward tick in obesity predates the advent of the Dollar Menu.  That's true, but not particularly relevant.  The purpose of the study (and the Reason post) was to point out that fast food didn't contribute to the rise in obesity because of consumption patterns when fast food was relatively more expensive. We live in a different environment now where consumption patterns for the poor have shifted toward fast food.

The other issue is that the advent of the Dollar Menu reflected a wider societal move toward cheap, crappy convenience food that predates 2002.  My wager is that fast food post-Dollar Menu supplanted other junk food choices - chief among them the recent McDonald's decision to sell 44 ounces of soda for a buck, which is a way better deal than buying a 20-ounce bottle for $1.50.  Fast food is the current culprit, but it's not like there haven't been other ones.