Whether in business ventures or academic research, voluntary participation is at the heart of
collaboration. Yet, previous studies concerning collaboration have primarily focused on the freeriding
problem, largely overlooking this mundane but critical aspect. We address this gap by
modeling collaborative situations as people’s voluntary choices between joining an uncertain
public goods provisioning in groups and pursuing a certain but less profitable individual option.
Theoretically and experimentally, we show that this voluntary nature in collaboration increases the
likelihood of group success by filtering out otherwise pessimistic defectors or encouraging them
toward cooperation. Further, we reconcile our findings with the existing literature highlighting
detrimental effects of individual alternatives to collaborative solutions. We argue that whether
individual options aid or hinder collaboration hinges on the degree of the “externality” of loners
(who pursue the individual option) to the group. As long as the collaboration pertains to local
public goods, the externality remains limited, ensuring that the existence of an individual option
robustly aids collaborative success by fostering cooperation through improved optimism within
groups.