"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.