A necessary pre-requisite to the development of effective food and development policies involves understanding the intermediate steps and relationships between income and nutritional status. This study focuses on the social and cultural factors which are of importance in the determination of dietary patterns under normal circumstances. Four populations were studied (a total of 105 households) in Sonora, Mexico, an area of prevalent mild to moderate malnutrition: rural residents of Arroyo Lindo, migrants from there to Hermosillo, households in which both husband and wife were born in Hermosillo, and households with an American wife. The social and cultural variables investigated included: income, subsistence pattern, prestige, material culture, ethnicity, women's roles, intrahousehold food distribution, rural-urban residence, and nutritional knowledge. Data were collected over a 4-8 day period for each household on both actual food consumption (3 1-day recalls for each member of the household) and ethnographic aspects of dietary patterns. It was found that all of the variables interact through the "invisible variable" of available income. This refers to income which is available to those responsible for household expenditures to spend on houshold needs, including food. In Sonora, it includes the husband's and/or wife's earnings, plus that portion of any other household wage earner's income which is given to either the husband or wife. Husbands and wives use their incomes primarily for household needs, while other wage earners do not contribute equivalent proportions of their earnings to the household. This results in differences between total household income and available income. While husbands (and men in general) tend to retain more of their earnings for personal expenses than do wives (and women in general), calculation of available income focused on role, rather than gender of the wage earner. Using the concept of available income makes it clear that the interaction of the other variables and their effect on dietary patterns is through affecting the amount of available income, or how this amount is allocated. The importance of social and cultural variables in affecting dietary patterns is demonstrated by considering the ways their manipulation might ameliorate Sonoran nutritional problems.