In the literature on property rights, there is general consensus framed by people such as Boserup and Demsetz that
common property regimes will change to private property regimes when resources are scarce and population increases.
This is mostly correct where land tenure is concerned. However, there are many cases where common property
regimes continue even though the population has grown and resources have become scarcer. Ocean areas are almost
always held as common property. This article analyzes property regimes that develop for land and ocean and the
conditions under which they do so. The analysis is based on worldwide ethnographic case studies. I argue that the
form property regimes take is determined by economic defendability. Many variables influence economic defendability,
ranging from technical change and physical attributes of the resource to government action and cultural
factors. That arable land is usually held privately suggests this solution is adaptive for land with high economic
defendability. However, the fact that many ocean areas, forests, pastures, and swidden lands are held as common
property suggests that this type of tenure may be an optimal solution when the value of resources are low relative
to defense costs.