Much modern linguistic research, in particular within the paradigm of generative grammar, has concerned itself with trying to account for differences between languages of the world. This has worked on the assumption that, if human linguistic ability is narrowly constrained by human biology, then all languages must share certain fundamental properties.
In generativist theory, the collection of fundamental properties all languages share are referred to as universal grammar (UG). The specific characteristics of this universal grammar are a much debated topic. Typologists and non-generativist linguists usually refer simply to language universals, or universals of language.
Similarities between languages can have a number of different origins. In the simplest case, universal properties may be due to universal aspects of human experience. For example, all humans experience water, and all human languages have a word for water. Other similarities may be due to common descent: The Latin language spoken by the Ancient Romans developed into Spanish in Spain and Italian in Italy; similarities between Spanish and Italian are, thus, in many cases due to their both having descended from Latin. In other cases, contact between languages — in particular where many speakers are bilingual — can lead to much borrowing of structures, as well as words. Similarity may also, of course, be due to coincidence. English much and Spanish mucho are not descended from the same form or borrowed from one language to the other;^[13] nor is the similarity due to innate linguistic knowledge (see False cognate).
Arguments in favor of language universals have also come from documented cases of sign languages (such as Al-Sayyid Bedouin Sign Language) developing in communities of congenitally deaf people, independently of spoken language. In general, the properties of these sign languages conform to many of the properties of spoken languages. Other known and suspected sign language isolates include Kata Kolok, Nicaraguan Sign Language, and Providence Island Sign Language.