Communication in nature is not restricted to the transmitter-receiver pair. Unintended listeners, or eavesdroppers, can intercept the signal and possibly utilize the received information to their benefit, which may confer a certain cost to the communicating pair. In this paper we explore (computationally and mathematically) such situations with the goal of uncovering their effect on language evolution. We find that in the presence of eavesdropping, languages exhibit a tendency to become more complex. On the other hand, if eavesdroppers belong to a different (competing) population, the languages used by the two populations tend to converge, if the cost of eavesdropping is sufficiently high; otherwise the languages synchronize. These findings are discussed in the context of animal communication and human language. In particular, the emergence of synonyms is predicted. We demonstrate that a small associated cost can suppress synonyms in the absence of eavesdropping, but that their likelihood increases strongly with the probability of eavesdropping.
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