Urbanization and the growth of trade gained considerable momentum in the 16th and 17th centuries.
Both brought people together, which encouraged the spread of disease. And global exploration and trade led to the worldwide diffusion of many diseases into previously isolated areas.
"Height studies for the late 18th and early 19th centuries show that large cities were particularly hazardous for health," Steckel said. "Urban centers were reservoirs for the spread of communicable diseases."
Inequality in Europe grew considerably during the 16th century and stayed high until the 20th century the rich grew richer from soaring land rents while the poor paid higher prices for food, housing and land.
"In poor countries, or among the poor in moderate-income nations, large numbers of people are biologically stressed or deprived, which can lead to stunted growth," Steckel said. "It's plausible that growing inequality could have increased stress in ways that reduced average heights in the centuries immediately following the Middle Ages."
Political changes and strife also brought people together as well as put demand on resources.
"Wars decreased population density, which could be credited with improving health, but at a large cost of disrupting production and spreading disease," Steckel said. "Also, urbanization and inequality put increasing pressure on resources, which may have helped lead to a smaller stature."
Exactly why average height began to increase during the 18th and 19th centuries isn't completely clear, but Steckel surmises that climate change as well as improvements in agriculture helped.'"/>