One of the most striking features of economic development is the relative decline of the primary sector in growing economies. Also typical, particularly of densely populated countries, is a decline in their agricultural comparative advantage as industrialization proceeds. Whether that leads to declines in food self-sufficiency and the value of net imports of agricultural products are moot points: it depends in part on policy trends, which happen often to gradually change from disfavouring to favouring agriculture relative to other tradable sectors over the long term. This Appendix seeks to explain these trends.

Why the primary sector declines relatively as an economy grows

A primitive economy with few trading opportunities necessarily has to devote most of its resources to the provision of food. Agriculture's shares of national output and employment therefore start at high levels. As economic development proceeds, however, agriculture's shares of GDP and employment typically fall. This has commonly been attributed to two phenomena: the slow rise in the demand for food as compared with other goods and services as incomes rise (that is, relatively low price and income elasticities of demand); and the more rapid development (at least historically – see Martin and Mitra 1998) of new technologies for primary sectors, relative to those for other sectors, which leads to expanding food supplies per hectare and per worker. Some of those new technologies can be imported by a late-developing economy from those more-advanced economies that were similarly endowed in earlier decades with a scarcity (or abundance) of land per worker, and then adapted relatively easily to local conditions.