Use of joinpoint regressions to evaluate changes over time in conscript height

Objectives: Cohort variation in adult height expresses both the impact of socio-economic change on human biology in a wide temporal perspective and social inequalities within populations. We aimed to test the use of joinpoint regressions to identify periods in which changes in height trends were statistically significant. Methods: Data correspond to the height recorded in Madrid City (Spain) for 65 313 conscripts between 1936 and 1974 (cohorts from 1915 to 1953), a period of social and political turmoil. Secular trends in height were analyzed in eight districts with contrasting socio-economic conditions, grouped in two categories, lower-class and middle- and upper-class. Trends in height were evaluated by quadratic regressions and by joinpoint regressions to identify the cut-off years when trends changed significantly. Results: Height increased in both socio-economic categories of districts, more among conscripts from the lower-class ones. However, results clearly show differences in trends according to district of residence. Whereas the increase in height in conscripts from the middle- and upper-class districts was steady, it was slower in those from the lower classes, with declines in height during the Civil War and first years of the Franco dictatorship. Conclusions: Joinpoint analysis reveals the association between urban living conditions and adult height, and that the disparities intensified during critical historical periods of Spain.