Using a mixture model to detect son preference in Vietnam.

Son preference is strong in Vietnam, according to attitudinal surveys and studies of contraceptive prevalence and birth hazards. These techniques assume a single model is valid for all families, but it is more plausible that son preference is found for some, but not all, families. Heterogeneous preferences may be addressed with a mixture model. This paper specifies and estimates a two-Weibull regression model, applied to the interval between the second and third births. The data come from the Vietnam Living Standards Survey of 1992-93. Applying information criteria, graphs, and martingale-based residuals, the two-Weibull model is found to fit better than a one-Weibull model. Roughly half of parents have son preference and, curiously, a propensity for fewer children. The other group has more children, no son preference, and is colourless in the sense that the birth interval is difficult to predict on the basis of the regressors used.