American Heritage からの引用です。
between の使い方についての説明：
http://www.bartleby.com/61/88/B0218800.html より引用：
According to a widely repeated but unjustified tradition, “between is used for two, and among for more than two.” It is true that between is the only choice when exactly two entities are specified: the choice between (not among) good and evil, the rivalry between (not among) Great Britain and France. When more than two entities are involved, however, or when the number of entities is unspecified, the choice of one or the other word depends on the intended sense. Between is used when the entities are considered as distinct individuals; among, when they are considered as a mass or collectivity. Thus in the sentence The bomb landed between the houses, the houses are seen as points that define the boundaries of the area of impact (so that we presume that none of the individual houses was hit). In The bomb landed among the houses, the area of impact is considered to be the general location of the houses, taken together (in which case it is left open whether any houses were hit). By the same token, we may speak of a series of wars between the Greek cities, which suggests that each city was an independent participant in the hostilities, or of a series of wars among the Greek cities, which allows for the possibility that the participants were shifting alliances of cities. For this reason, among is used to indicate inclusion in a group: She is among the best of our young sculptors. There is a spy among you. Use between when the entities are seen as determining the limits or endpoints of a range: They searched the area between the river, the farmhouse, and the woods. The truck driver had obviously been drinking between stops.
つまり、二者間というよりも、単に、境界がはっきりしているかどうか、一つ一つを個として意識しているかで、between かamong かを使い分けると書いています。
