Western Europe nearly tripled its population in the period from about 1050 to 1300 and its economy grew at a similar rate. There are many factors that contributed to this demographic and economic boom, and they are mostly interconnected factors, ranging from improvements in agricultural production, the birth and expansion of a silver coinage based economy, and the establishment of regional and long-distance trade routes. By the beginning of the period, western Europe was far behind the two other major powers in the lands formerly of the Roman empire, the Byzantines and the Abbasid Caliphate, but by the end of the thirteenth century it had matched or surpassed them in terms of population density and economy.

Improvements in agricultural efficiency were one of the main factors. The amount of land cultivated was increased, including land in parts of eastern Europe that was newly cultivated as a result of migration from other parts of Europe. (By the end of the period the population density had reached its limit in parts of Europe and this led to an increase in migration towards less populated areas.) Three field crop rotation and improvements to the plow were introduced and made it possible to farm a larger area with a smaller workforce per unit area. Crop specialization also comes into play in this period. While during the early middle ages farms had to produce everything needed for their own survival, now there was more specialization, and you could farm what was best for the local land and climate, do it on a larger scale, and trade with other areas that specialized in different crops and products. This was made possible by the birth and growth of the silver economy and of the trade networks, which I will touch upon later. So now England was producing a significant part of the wool used to make cloth in the Flanders and elsewhere in Europe, and Sicily was once again producing large amounts of grains to feed the urban areas of northern Italy, as it had done once before during the days of the Roman Empire. The urban areas in western Europe also contributed to the greater efficiency of agriculture by producing tools, cloth and other goods that had previously been made locally or on the farm itself, freeing agricultural labor for other needs. Ultimately, all those factors allowed the agricultural land in western Europe to feed three times as many people, those living in the countryside and those living in towns and cities.

Trade was another important factor in the sustained demographic and economic boom of the central middle ages. There was always some small amount of trade in the early middle ages, but in the central middle ages we see the development of regional and international trading fairs in several places in Europe. An example are the Champagne fairs in northern France. Tied to the growth in commerce as well as the growth in the number of people traveling as pilgrims, crusaders, functionaries, preachers, you have the development of an improved travel infrastructure of roads and hostels.

The use of silver coinage also contributed to the economic and demographic boom. You could not have agricultural specialization without it, and you could not run any long distance commerce without it. Having a silver currency throughout Europe also allowed the birth of a financial industry that allowed people to borrow to invest in the capital equipment needed to upgrade from a local industry to one that could produce for a larger market.

There are a large number of factors that contributed to the population expansion in the central middle ages. And most of them were interconnected and building on each other.

In Wickham's recent book Medieval Europe, he has a chapter on "The Long Economic Boom" that gives some more detail on the demographic and economic growth in the central middle ages.