The recently developed computer program BayesX provides a Bayesian approach to the estimation of non-parametric additive models. Such models can be useful in applications when the effect of metrical covariates (such as time) are to be estimated while controlling for other factors. In an application of this methodology, trends in the height of West Point cadets in the 19th century are estimated. The results indicate that the biological standard of living of the "middle class" increased relative to the rest of the American society during the Antebellum years.