Trust Propagation

How often do you not hear someone exclaiming “it’s a small world”, astonished at bumping into somebody he/she thought almost impossible to meet, or at discovering they have a mutual acquaintance. This ‘small world’ idea dates back ages, and has ever since found its way into our daily lives and even our popular culture; think for example of the movies1 and the song lyrics2. And it is not only an established expression in the English language: the Spanish compare the world to a handkerchief3, and the Namibians have an old, even more poetic proverb saying that it is only the mountains that never meet4. Despite their old age, these sayings are still remarkably up-to-date and widely applicable; not only in our everyday life, but also online.