Preferential attachment

A preferential attachment process is any of a class of processes in which some quantity, typically some form of wealth or credit, is distributed among a number of individuals or objects according to how much they already have, so that those who are already wealthy receive more than those who are not. 'Preferential attachment' is only the most recent of many names that have been given to such processes. They are also referred to under the names 'Yule process', 'cumulative advantage', 'the rich get richer', and, less correctly, the 'Matthew effect'. They are also related to Gibrat's law. The principal reason for scientific interest in preferential attachment is that it can, under suitable circumstances, generate power law distributions. A preferential attachment process is any of a class of processes in which some quantity, typically some form of wealth or credit, is distributed among a number of individuals or objects according to how much they already have, so that those who are already wealthy receive more than those who are not. 'Preferential attachment' is only the most recent of many names that have been given to such processes. They are also referred to under the names 'Yule process', 'cumulative advantage', 'the rich get richer', and, less correctly, the 'Matthew effect'. They are also related to Gibrat's law. The principal reason for scientific interest in preferential attachment is that it can, under suitable circumstances, generate power law distributions. A preferential attachment process is a stochastic urn process, meaning a process in which discrete units of wealth, usually called 'balls', are added in a random or partly random fashion to a set of objects or containers, usually called 'urns'. A preferential attachment process is an urn process in which additional balls are added continuously to the system and are distributed among the urns as an increasing function of the number of balls the urns already have. In the most commonly studied examples, the number of urns also increases continuously, although this is not a necessary condition for preferential attachment and examples have been studied with constant or even decreasing numbers of urns. A classic example of a preferential attachment process is the growth in the number of species per genus in some higher taxon of biotic organisms. New genera ('urns') are added to a taxon whenever a newly appearing species is considered sufficiently different from its predecessors that it does not belong in any of the current genera. New species ('balls') are added as old ones speciate (i.e., split in two) and, assuming that new species belong to the same genus as their parent (except for those that start new genera), the probability that a new species is added to a genus will be proportional to the number of species the genus already has. This process, first studied by Yule, is a linear preferential attachment process, since the rate at which genera accrue new species is linear in the number they already have. Linear preferential attachment processes in which the number of urns increases are known to produce a distribution of balls over the urns following the so-called Yule distribution. In the most general form of the process, balls are added to the system at an overall rate of m new balls for each new urn. Each newly created urn starts out with k0 balls and further balls are added to urns at a rate proportional to the number k that they already have plus a constant a > −k0. With these definitions, the fraction P(k) of urns having k balls in the limit of long time is given by for k ≥ k0 (and zero otherwise), where B(x, y) is the Euler beta function: with Γ(x) being the standard gamma function, and The beta function behaves asymptotically as B(x, y) ~ x−y for large x and fixed y, which implies that for large values of k we have In other words, the preferential attachment process generates a 'long-tailed' distribution following a Pareto distribution or power law in its tail. This is the primary reason for the historical interest in preferential attachment: the species distribution and many other phenomena are observed empirically to follow power laws and the preferential attachment process is a leading candidate mechanism to explain this behavior. Preferential attachment is considered a possible candidate for, among other things, the distribution of the sizes of cities, the wealth of extremely wealthy individuals, the number of citations received by learned publications, and the number of links to pages on the World Wide Web. The general model described here includes many other specific models as special cases. In the species/genus example above, for instance, each genus starts out with a single species (k0 = 1) and gains new species in direct proportion to the number it already has (a = 0), and hence P(k) = B(k, γ)/B(k0, γ − 1) with γ=2 + 1/m. Similarly the Price model for scientific citations corresponds to the case k0 = 0, a  = 1 and the widely studied Barabási-Albert model corresponds to k0 = m, a = 0.