The total fertility rate is lower in island nations than non-island nations, and in small land-area nations than large land-area nations. Because islands tend to be much smaller than non-islands, these phenomena raise the question of whether there are two distinct effects or two manifestations of a single effect. This paper uses 1990 cross-sectional data on 198 countries to estimate the independent effects of islands and land-area on fertility. We find evidence that there are two separate effects. One possible explanation for the island and small-country fertility effects can be drawn from the theory of the commons, but another possible explanation is offered by the new theory of economic growth. We explore each of these possible explanations and conclude that the small-country effect is consistent with new theories of economic growth while the island effect is likely the result of a ‘commons effect’.