Rarissima and rara, features and properties found only in one or very few languages, tell us as much about the capacities and limits of human language(s) as do universals. Explaining the existence of such rare phenomena on the one hand, and the fact of their rareness or uniqueness on the other, proves a reasonable and interesting challenge to any theory of how human language works.

Jan Wohlgemuth and Michael Cysouw, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany.