evidence: By 2009, the amount of people who flew a commercial airliner in the previous year had risen to 39.85 percent in the Omnibus Household Survey. Gallup polls give numbers that are a little higher. In 2012, 52 percent of respondents said they had flown at least once in the past year, the highest number in a decade. Of course, these are only U.S. flyers, and for many parts of the world, information is even scarcer. A consumer survey conducted by Credit Suisse First Boston in 2004 found that 47 percent of respondents in eight large Chinese cities had ever flown in an airplane. Air travel is on the rise in China, as well as in other parts of Asia and Latin America. "As gross domestic product increases the number of trips per capita increases," Heimlich says, citing numbers produced by airplane manufacturers. "What they show is, to a point, air travel increases exponentially. Places with large populations like China and India are of great interest." [...] What’s not clear from these statistics is whether more people were flying, or the same people were flying more often, or both. It seems likely that the percentage of global citizens who have ever flown is growing, and will continue to grow. We just don’t have the data to back it up.