Subsequently, basic education may become popularized, so that even an unskilled worker may have to have some basic education. Similarly, it might have required someone to acquire only a basic education by today’s standards to be a skilled worker in the early phases of development, but subsequently it would require a person to receive a lot more education—often tertiary—in order to qualify as a skilled worker. First, earlier literature on development (Lewis, 1954; Sen, 1966) stressed the importance of rural surplus labour, meaning labour that produces a zero or close-to-zero agricultural marginal product.