This paper addresses the interactions between development policies and upland agriculture expansion with a model of household-level responses to economic and technical factors, based on characteristics of the household and farming system. The authors assess the validity of the model with data from farm households in the region bordering the Mt Kitanglad Range National Park in southern Mindanao, using results of surveying low income corn and vegetable farms. Crop prices and productivity influenced land allocation within a farm, though the different crops generated different influences. Crops were expanded primarily through land substitution and increased input levels, but when prices or productivity of other crops changed, the expansion occurred in the total farm area. The constraints of land and family labor also have varying influences for different crops. The complex interactions and influences of different factors and policies suggest that environmental policies must correspondingly include multiple conditions if they are to effectively mitigate incentives for land expansion.