  Human social dilemmas are often shaped by actions involving uncertain goals
and returns that may only be achieved in the future. Climate action, voluntary
vaccination and other prospective choices stand as paramount examples of this
setting. In this context, as well as in many other social dilemmas, uncertainty
may produce non-trivial effects. Whereas uncertainty about collective targets
and their impact were shown to negatively affect group coordination and
success, no information is available about timing uncertainty, i.e. how
uncertainty about when the target needs to be reached affects the outcome as
well as the decision-making. Here we show experimentally, through a collective
dilemma wherein groups of participants need to avoid a tipping point under the
risk of collective loss, that timing uncertainty prompts not only early
generosity but also polarized contributions, in which participants' total
contributions are distributed more unfairly than when there is no uncertainty.
Analyzing participant behavior reveals, under uncertainty, an increase in
reciprocal strategies wherein contributions are conditional on the previous
donations of the other participants, a group analogue of the well-known
Tit-for-Tat strategy. Although large timing uncertainty appears to reduce
collective success, groups that successfully collect the required amount show
strong reciprocal coordination. This conclusion is supported by a game
theoretic model examining the dominance of behaviors in case of timing
uncertainty. In general, timing uncertainty casts a shadow on the future that
leads participants to respond early, encouraging reciprocal behaviors, and
unequal contributions.
