Introduction

It happened on October 12, 1999 – at least, according to the United Nations (UN). That was the day the human population of planet Earth officially reached 6 billion. Of course, given the inaccuracy of the data, the UN could have been off by 100 million people or so. Every day some 100 million billion sperm are released and 400,000 babies are born, whereas “only” 140,000 persons die. Consequently, the world population is growing rapidly, especially since the second half of the twentieth century.

Given the average population density in the world, of about fifty people per square kilometer (Km2), if you are part of a family with two children, your family could have about eight hectares (or twenty acres) at its disposal. The great majority of our readers will probably look around in amazement as they realize that they do not own an area close to this size. The reason is simple: the world population is unevenly distributed. But why?

There may be many reasons why people cluster together. Sociological: you like to interact with other human beings. Psychological: you are afraid of being alone. Historical: your grandfather used to live where you live now. Cultural: the atmosphere here is unlike anywhere else in the world. Geographical: the scenery is breathtaking and the beach is wonderful. We will at best cursorily discuss the above reasons for clustering. Instead, we focus attention in this book on the economic rationale behind clustering, known technically as agglomeration.