In addition to the economic, social and cultural, and political push and pull forces directly caused by humans, environmental conditions drive migration. In the most benign form of environmental push and pull, people can be pushed away from areas with uncomfortable climates and pulled to places with pleasant climates. For instance, since the 1970s, population growth in the US southern and western Sun Belt states has increased at a faster rate than in the Snow Belt states of New England and the Midwest.