  We show how reciprocal arcs significantly influence the structural
organization of Wikipedias, online encyclopedias. It is shown that random
addition of reciprocal arcs in the static network cannot explain the observed
reciprocity of Wikipedias. A model of Wikipedia growth based on preferential
attachment and on information exchange via reciprocal arcs is presented. An
excellent agreement between in-degree distributions of our model and real
Wikipedia networks is achieved without fitting the distributions, but by merely
extracting a small number of model parameters from the measurement of real
networks.
