You have entered the Pathfinder's decision tree for public measures for influencing collective adaptation.

If interdependence does exist between private actors, a collective action situation is at hand and a different set of decision nodes become relevant for the public actor to consider. In many collective AS, interdependence gives rise to conflicts between the individual preferences of private actors and social welfare. Examples of such interdependence are:

• Environmental pollution: If an actor pollutes the environment and does not suffer from the pollution herself than it is not rational for the private actor to stop polluting.
• Over-exploitation of a common pool resource. For scarce resources which any actor can access and use it is not rational from the perspective of one single actor to preserve the resource. From the social welfare perspective, however, it may be. An example of such a situation would be a common groundwater stock that is declining under climate change and is used by a group of farmers to irrigate their fields.
• Under-provisioning of a public good and free-riding. For actors that consume a freely available public good it is not individually rational to contribute to the maintenance or the provisioning of the public good. An example of such an AS is a community of private actors facing increasing risks of flooding but not contributing to the maintenance of the dike that protects them.

In order to identify appropriate policy measures, understanding the nature of these interdependence and conflicts is required.

The first decision node concerns what type of interdependence is present. One-way interdependence means that the action of one actor influences another actor but not vice versa. In the economics literature, this is called a unilateral externality (Dombrowski 2007). Examples of such challenges include pollution problems and upstream-downstream situations in shared river basins. Prominent examples of one-way interdependence in adaptation include the provisioning of urban flood risk reduction by private upstream farmers, and the establishment and maintenance of biodiversity migration corridors by private farmers (Bisaro and Hinkel 2013). The design and appraisal of these options may be addressed through methods for formal appraisal of options (see the Pathfinder's section on Appraising adaptation options).

When interdependence is one-way, the public actor needs to find a normative agreement between the interests of the upstream and downstream actors and may achieve this through regulation and economic incentives.

When interdependence is two-way, the decision node to consider concerns whether a coordination solution is available.

If it is unknown which type of interdependence exists, the relevant task is a description of governance arrangements, which involves identifying relevant actors and their preferences (see Toolbox section on Institutional analysis).