Garden clubs and social tipping points

Voluntary contribution clubs have been increasingly proposed as a way to address social dilemmas of cooperation, which arise from free-riding and limited observability of individual actions. However, a club good does not necessarily ensure large scale cooperation, because of multiple equilibria. Our hypothesis is that a club good presents a social tipping point working as an endogenous threshold of cooperation. We use a discrete choice model in a population framework, the Garden Model, to describe and characterise contribution equilibria. The model provides a number of behavioural hypotheses that are tested in laboratory experiments. Aggregate experimental results show that subjects contribute more than they would if they were fully rational and contribution rates decrease on average, in agreement with the literature. However, disaggregate figures show equilibrium selection patterns: subjects can converge either to high or low rates of cooperation, which indicates an endogenous threshold of contribution. As the basin of attraction of high-contribution is smaller, early decisions drive the group to this desirable equilibrium less frequently.