Traditional agricultural systems in less developed countries (LDCs) maintain not only the germplasm of local varieties of ancestral crop populations (landraces) but also the human knowledge and behavioral practices that reflect a long coevolution between crops and human populations. The germplasm of crop landraces provides the material from which high-yielding seed varieties (HYVs) have been developed by international and national agricultural research centers. Farmers' knowledge and behavioral practices, in turn, shape the manner and extent to which these new varieties are adopted and integrated back into the traditional farming system. Although farmers in traditional agricultural systems frequently adopt new seed varieties and other modern inputs, many adopters also continue to cultivate local varieties. High-yielding varieties are introduced into agricultural systems where high levels of crop diversity exist and where different varieties are selected to fill specific ecological (e.g., land-quality) niches. Small farmers in the tropics are often confronted with complex and heterogeneous environments.' A household may own or work several plots, usually scattered in different physiographic zones and made up of different recognized soil qualities.2 Once farmers understand the principal characteristics of new HYVs, usually after a period of observation and experimentation, a "technology equilibrium" is eventually reached.3 This equilibrium may entail the complete displacement of local varieties, but frequently it does not.4 Instead, HYVs may simply be incorporated into one or more agroecological niches on farms. Efforts to explain partial adoption on farms have focused overwhelmingly on considerations of risk and farmer risk aversion.5 How