Steckel analyzed height data from thousands of skeletons excavated from burial sites in northern Europe and dating from the ninth to the 19th centuries. Average height declined slightly during the 12th through 16th centuries, and hit an all-time low during the 17th and 18th centuries.
"Height is an indicator of overall health and economic well-being, and learning that people were so well-off 1,000 to 1,200 years ago was surprising," he said.
Northern European men had lost an average 2.5 inches of height by the 1700s, a loss that was not fully recovered until the first half of the 20th century.
Steckel believes a variety of factors contributed to the drop - and subsequent regain - in average height during the last millennium. These factors include climate change; the growth of cities and the resulting spread of communicable diseases; changes in political structures; and changes in agricultural production.
"Average height is a good way to measure the availability and consumption of basic necessities such as food, clothing, shelter, medical care and exposure to disease," Steckel said. "Height is also sensitive to the degree of inequality between populations."
The study appears in a recent issue of the journal Social Science History.
Steckel analyzed skeletal data from 30 previous studies. The bones had been excavated from burial sites in northern European countries, including Iceland, Sweden, Norway, Great Britain and Denmark. In most cases, the length of the femur, or thighbone, was used to estimate skeletal height. The longest bone in the body, the femur comprises about a quarter of a person's height.
According to Steckel's analysis, heights decreased from an average of 68.27 inches (173.4 centimeters) in the early Middle Ages to an average low of roughly 65.75 inches (167 cm) during the 17th and 18th centuries.
"This decline of two-and-a-half inches substantially exceeds any height fluctuations seen during the various industrial revolutions of the 19th century," Steckel said.
Reasons for such tall heights during the early Middle Ages may have to do with climate. Steckel points out that agriculture from 900 to 1300 benefited from a warm period - temperatures were as much as 2 to 3 degrees warmer than subsequent centuries. Theoretically, smaller populations had more land to choose from when producing crops and raising livestock.
"The temperature difference was enough to extend the growing season by three to four weeks in many settled regions of northern Europe," Steckel said. "It also allowed for cultivation of previously unavailable land at higher elevations."
Also, populations were relatively isolated during the Middle Ages - large cities were absent from northern Europe until the late Middle Ages. This isolation in the era before effective public health measures probably helped to protect people from communicable diseases, Steckel said.
"It is notable that bubonic plague made its dramatic appearance in the late Middle Ages, when trade really took off," he said.
Steckel cites several possible reasons why height declined toward the end of the Middle Ages:
The climate changed rather dramatically in the 1300s, when the Little Ice Age triggered a cooling trend that wreaked havoc on northern Europe for the following 400 to 500 years.
Colder temperatures meant lower food production as well as greater use of resources for heating. But many temperature fluctuations, ranging in length from about 15 to 40 years, kept people from fully adapting to a colder climate, Steckel said.
"These brief periods of warming disguised the long-term trend of cooler temperatures, so people were less likely to move to warmer regions and were more likely to stick with traditional farming methods that ultimately failed," he said. "Climate change was likely to have imposed serious economic and health costs on northern Europeans, which in turn may have caused a downward trend in average height."
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.
"Increased height may have been due partly to the retreat of the Little Ice Age, which would have contributed to higher yields in agriculture. Also improvements in agricultural productivity that began in the 18th century made food more plentiful to more people.
This study is part of the Global History of Health Project, an initiative funded by the National Science Foundation to analyze human health throughout the past 10,000 years.
Steckel wants to continue looking at, and interpreting, fluctuations in height across thousands of years "I want to go much further back in time and look at more diverse populations to see if this general relationship holds over 10,000 years," he said.