Numerous observations substantiate a pronounced contrast in lightning activity between continents and oceans. The traditional explanation for continental dominance is based on a contrast in thermal properties of land and sea. A more recent idea is based on the contrast in boundary layer aerosol concentration between land and sea. This study makes use of islands as miniature continents of varying area to distinguish between these two hypotheses. Scaling law analysis is used to predict transitional island areas for the two hypotheses. NASA Tropical Rainfall Measuring Mission satellite observations provide a uniform data set on island activity. The island area dependences of lightning activity are more consistent with the thermal hypothesis than the aerosol hypothesis, but this conclusion must be tempered with the extreme simplification of the theoretical predictions.