How trade/GNP ratio decreases with country size

The ratio of a country's foreign trade (i.e., exports plus imports) to its GNP has a known tendency to decrease with country size. Previous studies have used a single year's data; but trade fluctuates greatly from year to year. This paper makes available a compilation of 1953–1972 export/GNP and import/GNP figures for 110 countries. The average import/GNP figure is found to correlate strongly with population size; the simple expression, Imports/GNP = 40 $\text{P}{}^{\mathrm{<mtext>?1</mtext><mtext>3</mtext>}}$, applies, within a factor of 2, in 94% of cases. No correlation with development level can be seen. The United States data throughout its history (1799–1972) follow the same inverse cube root pattern, but with a constant of 20 instead of 40. Correlation is much poorer in the case of export/GNP ration. Export and import figures are only marginally correlated to each other.