Grammatical Categories and Cognition uses original, empirical data to examine the Sapir-Whorf linguistic relativity hypothesis: the proposal that the grammar of the particular language we speak affects the way we think about reality. The author compares the grammar of American English with that of Yucatec Maya, an indigenous language spoken in south-eastern Mexico, focusing on differences in the number marking patterns of the two languages. He then identifies distinctive patterns of thought relating to these differences by means of a systematic assessment of memory and classification preferences among speakers of both languages. The study illustrates the distinct approach to empirical research on the linguistic relativity hypothesis which Lucy develops in a companion volume Language Diversity and Thought.