Secular changes in childhood, adolescent and adult stature.

This essay provides a brief history of the etymology and usage of the phrase 'secular change' followed by a description of secular changes in height and relative leg length in childhood, adolescence, and adulthood. Both positive and negative changes are described. Possible causes are reviewed, with an emphasis on nutrition, infection and social-economic-political (SEP) environments. The case of the Maya people living in Mexico, Guatemala, and the United States is given, which shows that intergenerational changes in stature and its components - leg length and upper body length - may occur in different directions and at different rates. The deleterious consequences of rapid catch-up growth after birth have been proposed as a hypothesis to explain the 150 years of positive secular change in height of populations in the richer nations. That hypothesis is found to be an incomplete explanation. Growth changes better track the rate of change in SEP factors. Epigenetic assimilation is a new hypothesis, which focuses on those epigenetic processes regulating gene expression, metabolic function, physiology, and behavior. Epigenetic assimilation shows promise to account for plasticity and intergenerational changes in human growth and development phenotypes.