Many situations require the action of a single volunteer to create a benefit for a larger group. For example, programmers provide others with open source software, interested writers publish articles on Wikipedia, or people bring wine-openers to a party in the park. These actions often happen in fairly unstructured, spontaneous and rapidly changing environments. And very naturally, individual information about personal costs and benefits, ties between actors, or the dimensions of problems entailed in the system is limited in those situations.

Maybe most importantly, we often have no or only partial information about how many other individuals are willing and able to engage. While we may have some rough feeling over the number of other potential volunteers, the exact number is usually unknown. We call this feature population uncertainty.

Volunteering situations give rise to the so-called bystander erffect - the observation that the presence of multiple potential volunteer's decreases the individual probability to act. One prominent line of reasoning or "narrative" that these defectors use in these situations is the (potentially biased belief) that somebody else will do it. In other words, decision makers might have a situation in mind where one ore more specific others - a "designated volunteer" - does the job.

As we will argue in the project, population uncertainty might
hinder this mechanism because the imagined designated volunteers or specific others might just not be present. Or, alternatively, it is harder to keep up the narrative that someone else will do the job if the group and designated volunteers are not tangible. Consequently, being less able to focus on the actions of others, population uncertainty might focus decision makers on their own actions and activate
a volunteering norm. Interestingly, population uncertainty might thus activate the actual norm to volunteer. Having a certain group size would then actually be a special case allowing to put blame on others.

We analyse these questions theoretically and experimentally. In particular, we exogenously generate designated volunteers by introducing heterogeneity in the costs to volunteer. Making the group size uncertain - introducing population uncertainty thus generates the possibility that these designated volunteers are not present. First results show the important impact of population uncertainty on volunteering behavior.