I think something else to consider is the fact that the show is not paying for any travel expenses. If you are only a twenty minute subway ride away from the studio, you don't have nearly as much to lose as a person who had to shell out a couple hundred dollars for plane tickets, a place to stay, food, etc. It might be interesting to see if there is a correlation between where a contestant is from and whether or not they use a lifeline in the first five questions (not that it would prove anything, but it would still be interesting).