The Plains Paradox: Secular Trends in Stature in 19^th Century Nomadic Plains Equestrian Indians

This study documents the occurrence of secular trends in height in an historic population of 19^th century nomadic Plains equestrian Indians. The eight tribal samples utilized are a subset of the Boas North American Indian anthropometric data set. A cross-sectional design was used to examine the span of years from 1800 to 1870 for adult individuals over 20 years of age, sexes analyzed separately, male n=1,123 and female n=362. Adult heights were adjusted for aging effects on three variables: standing height; sitting height; and sitting height/subischial length ratio. Combined with an unadjusted subischial length, these variables were used to examine each of the Plains tribes for secular trends. Each variable was then regressed onto year of birth using a quadratic model. Tribal samples showing significant differences (p ≤ 0.05) among the means of the birth cohorts, and hence secular change through time, included five of eight male tribal samples, but only two of the female tribal samples. Significant regressions for adjusted standing height were seen for Sioux males and females, Crow females, and Comanche and Kiowa males. The variable adjusted sitting height suggested secular trends for the Sioux males and females, Crow females, and Assiniboin, Comanche and Kiowa males. Sioux males and females as well as Crow males showed significant trends for the variable ratio. Only Crow males showed any significant change for the subischial dimension. There do seem to be common trends in heights seen for the majority of the tribes represented in the male Plains sample, with an early decline in heights through the 1830s to the 1850s, and then a recovery in heights in the 1860s and 1870s. This pattern is also seen for the Sioux and Crow females and is reflected in an analysis of the entire sample by sex, where each of the three age adjusted variables yielded significant results for both sexes. Comparison with contemporary populations of White Americans also shows that most of these Plains Indians remained tall at the start of the traumatic reservation period.