Suggested that children would not develop until particular stages of cognitive development had been reached.
Outlined four cognitive stages : Sensorimotor (0-2 years), Pre-operational (2-6/7 years), Concrete operational (6/7-11/12 years), Formal operational (11-16+ years)
Focuses on the need for conceptual understanding to be present before language can reflect it.
For example, a child would not be able to use comparative adjectives (small, smaller, smallest etc.) unless they had first understood the concepts of size and scale. Use of the passive voice and of subordination in complex sentences also suggest progression in conceptual level.