The on going quest for a better understanding and prediction of adoption behaviour through the identification and analysis of the most influential behaviour determinants gave rise to this study. It was especially designed with the main objective of determining the comparative role of independent and intervening variables on the adoption of seed spacing among maize growers in the Njombe district of Tanzania. A pre-tested structured questionnaire was used to collect data from 113 farmers randomly selected to represent 5 percent samples of four villages selected to represent the biggest variation in terms of the climatic conditions in the district. The outstanding finding of this research is the much stronger and more consistent relationship that intervening variables have compared to the independent variables, which is in support of the research hypothesis. The total contribution of intervening variables to the variation of adoption behaviour is as much as 93 percent, which far exceeds that of the independent variables contributing only 6 percent. This supports the assumption that the intervening variables are the direct precursors of adoption behaviour and that the influence of independent variable becomes manifested in adoption behaviour via the intervening variables. From this emerge exciting possibilities for behaviour interventions of development programmes, but more research is necessary to verify the findings in different countries and cultures and to refine the selection of the most relevant intervening variables.