Elasticities YED < 0: Inferior goods, demand decreases with more income YED < 1: Inelastic normal goods, demand increases, but not faster than growth of the economy YED > 1: Elastic normal goods, demand increases faster than the growth of the economy.

Primary commodities are inelastic normal goods because there is a certain level of consumption that is satisfactory to consumers. They only need so much food. The decreasing marginal utility of each unit of food causes the YED to be less than 1.

So as economy increases, the sectors with higher YED increases at the expense of sectors with YED < 0; service sectors increase at the expense of agricultural sectors.

Note that total output is still growing. It’s just that primary sectors take a smaller portion overall.