I often ask my first class of postgraduate students each year to answer what in general is special about this circle on a map. The answer is very simple, yet surprisingly unfamiliar to most people from Europe and the Americas. Half of the human race lives within this 4,000-kilometre (2,500-mile) radius.

So, half of everything that occurs to people occurs inside this simple circle. Enlarge its radius by only a kilometre and the majority of whatever happens to people happens within it.