Past surveys on the demographics on Mechanical Turk users indicated that most of the workers come from the US, are younger and more educated than the general population, and work on MTurk as a way to get some spare cash.
Since the last survey, a few things have changed. First, Amazon allows now workers in India to get paid in cash in rupees, essentially encouraging many people from India to start using Mechanical Turk as workers. Second, the recession has affected many households, leaving many people at home looking for cash to cover their needs. These two forces has changed the demographics of the participants, so a new survey was needed to capture the new demographics of the Mechanical Turk workers.
