Coincidence judgment in causal reasoning: How coincidental is this?

•Coincidences are an under-assessed but fundamental part of causal reasoning.•Coincidentality (how coincidental is this?) is a stable psychological construct.•Coincidentality is predicted by the ratio of support for chance to causal hypotheses.•Coincidentality is also predicted by data surprise from causal and chance hypotheses.•Coincidentality is a cognitive marker that new causal knowledge may be available.