Sample-selection biases and the historical growth pattern of children

Bodenhorn et al. (2017) have sparked considerable controversy by arguing that the fall in adult stature observed in military samples in the United States and Britain during industrialization was a figment of selection on unobservables in the samples. While subsequent papers have questioned the extent of the bias (Komlos and A'Hearn 2019; Zimran 2019), there is renewed concern about selection bias in historical anthropometric datasets. Therefore, this article extends Bodenhorn et al.'s discussion of selection bias on unobservables to sources of children's growth, specifically focusing on biases that could distort the age pattern of growth. Understanding how the growth pattern of children has changed is important because these changes underpinned the secular increase in adult stature and are related to child stunting observed in developing countries today. However, there are significant sources of unobserved selection in historical datasets containing children's and adolescents' height and weight. This article highlights, among others, three common sources of bias: (1) positive selection of children into secondary school in the late nineteenth and early twentieth centuries; (2) distorted height by age profiles created by age thresholds for enlistment in the military; and (3) changing institutional ecology that determines to which institutions children are sent. Accounting for these biases adjusts the literature in two ways: evidence of a strong pubertal growth spurt in the nineteenth century is weaker than formerly acknowledged and some long-run analyses of changes in children's growth are too biased to be informative, especially for Japan.