His 1964 paper "The Capacity of Language Acquisition" sets for the seminal arguments picked up and popularized later by Noam Chomsky in his famous arguments for the innate "language organ".

He presents four arguments for biological innateness of psychological capacities, as constructed in parallel to arguments in biology for the innateness of physical traits:

1. Universal appearance of a trait at a single time across a species. "Species typical" traits.
2. Universal appearance across time for a group. Not just an artifact of cultural history. Again, "species typical" diagnostic feature.
3. No learning of the trait is possible.
4. Individual development of a trait rigidly follows a given schedule regardless of the particular experience of the organism.