The first form of intensification is generally labour intensification (Box 4.4I.? A survey of three Nigerian villages in 1973, one with high population density, one with medium population density and one with low population density, showed that yields were higher where population density was lower. The same three villages where analysed again 20 years later. By then, yields in the high-density village had doubled, and they had declined by 15% in the low-density village.