BMI, income, and social capital in a native Amazonian society: interaction between relative and community variables

Researchers have shown interest in the relation between (a) social capital and individual income and (b) the individual health of people of industrial nations. The socioeconomic complexity of industrial nations makes it difficult to arrive at firm conclusions. We circumvent the obstacle by using data from a small-scale rural society of foragers-farmers in the Bolivian Amazon (Tsimane'). We examine the interactions between the outcome (BMI) and relative income, relative social capital, village income, and village social capital. We test three hypotheses: people in villages with more social capital should have higher BMI, the positive association between social capital and BMI will be more marked among the less well-off, and better-off people who display generosity will have higher BMI than better-off people who do not. On the methodological side we show the importance of: focusing on relative measures of income and social capital, estimating interaction between community and relative measures of income and social capital, and showing results through contour plots that summarize the relation between BMI and pairs of explanatory variables. On the substantive side we find evidence that village social capital and village income complement each other and are associated with higher BMI, the rich who are stingy have lower BMI than the rich who display generosity, and increase in village income might reduce individual incentives to invest in social capital. We explore interactions between explanatory variables and their influence on BMI, and end by recommending the use of an experimental research design to obtain unbiased estimates of causal effects.