  We investigate a relationship network of humans located in a metric space
where relationships are drawn according to a distance-dependent probability
density. The obtained spatial graph allows us to calculate the average
separation of people in a very simple manner. The acquired results agree with
the well-known presence of the small-world phenomenon in human relationships.
They indicate that this feature can be understood merely as a consequence of
the probability composition. We also examine how this phenomenon evolves with
the development of human society.
