Measuring beliefs in quality of life research requires researchers to deal with endogeneity and social desirability issues. The connectionist theory of cognition suggests that individuals have mental models composed of associations between concepts. Moreover, structuralist perspectives consider beliefs form an interrelated system. Therefore, the estimation of causal effects of beliefs is undermined by these interdependent relationships of beliefs. This article introduces factorial surveys (FS) as a useful method to solve these potential problems. To demonstrate the appropriateness of this method, this study presents a design and application of FS to a representative sample of Chileans and their beliefs about what determines happiness. The findings provide a folk theory (or a lay theory) of happiness determinants, where health is the most important determinant of happiness, followed by income and lifestyles. In addition, the reliability and cognitive bias of FSs are addressed, finding support for their applicability in developing countries. A potential reduction of social desirability bias in FSs is discussed in the conclusions.