Why are megachurches (at least 2,000 attendees in weekend services) and Meetup groups (www.meetup.com) more popular in some cities or states than others? Our answer: residential mobility (when people move a lot, they like groups that are easy to join and easy to leave; they might move again soon, so they cannot commit to 1 group). As predicted, we found that there are more megachurches in residentially mobile states than in stable states. We also found that there are more Meetup groups in residentially mobile cities than stable cities (population and median income being equal). We also found that low-commitment, short-term Internet plans (low initial set-up fee, low penalty of breaking the contract, but higher monthly fee) are more popular in residentially mobile areas than in stable areas. In the final study, we also found that college students who had moved a lot joined low-commitment student clubs more than students who had not moved.