Work on analogical problem solving has found that people often fail to spontaneously notice the relevance of a source analog when solving a target problem, although they can form mappings and derive inferences when given a hint to recall the source. To investigate the determinants of spontaneous analogical transfer, solution rates to Duncker’s radiation problem were measured across varying source presentation conditions, and participants’ understanding of the source material was assessed. Supplemental animations increased both comprehension of the source analog and spontaneous transfer to the radiation problem. Supplemental diagrams yielded lesser improvement in participants’ understanding of source material and did not increase solution rates to the target problem. To investigate individual differences in spontaneous transfer, fluid intelligence was measured for each participant using an abridged version of the Raven’s Progressive Matrices (RPM) test. Animated source depictions were most beneficial in facilitating spontaneous transfer for participants with low RPM scores.