Fence-in laws in most states require ranchers to pay for fences to keep their livestock from trespassing onto others' property. Some states, or jurisdictions within states, have a fence-out rule that requires ranchers' neighbors to pay for fences to keep livestock out. Both rules are Pareto optimal. Using a potential Pareto criterion, we show that a preference for fence-out in some areas may end as conditions change, such as increased nonranching land uses. Changed conditions may have legal consequences. Specific fence-out and fence cost-sharing provisions may be potentially Pareto inefficient and may be challenged for being unconstitutional under the due process clause.