“Other studies have looked at the relationship between population density and more everyday examples of altruistic behaviour. A common way of investigating this area is the ‘lost letter’ method, in which researchers leave stamped, addressed letters in public locations and wait to find out how many are eventually posted by members of the public. Since picking up the letter and taking it to a post office is a minor inconvenience for which a person will never be recompensed this method is considered a good way of comparing rates of altruism between different areas.”