How Many Words Do We Know? Practical Estimates of Vocabulary Size Dependent on Word Definition, the Degree of Language Input and the Participant's Age.
Level 4 - case-series / case-control
Cross-sectional crowdsourced observational study and literature analysis without a control group.
PubMed 27524974 · doi:10.3389/fpsyg.2016.01116
What was done
The authors conducted a literature review combined with a large-scale crowdsourcing experiment to estimate vocabulary size in native speakers of American English across different ages and proficiency levels, evaluating lemmas, non-transparent multiword expressions, and word families.
What was found
An average 20-year-old native speaker of American English was estimated to know 42,000 lemmas and 4,200 non-transparent multiword expressions (derived from 11,100 word families). Vocabulary sizes ranged from 27,000 lemmas in the bottom 5% to 52,000 lemmas in the top 5%. Between ages 20 and 60, individuals acquired an estimated 6,000 additional lemmas (approximately one lemma every two days). Inflected forms and proper nouns accounted for much higher figures cited in previous literature.
Why it matters
This study provides standardized benchmarks for native English vocabulary size and clarifies how variations in word definitions (such as counting lemmas versus inflected forms or proper nouns) lead to widely disparate estimates across literature.
Limits
The abstract does not report the exact sample size, participant demographics, or testing methodology. Vocabulary knowledge was defined broadly to include shallow recognition (simply knowing a word exists), which may overestimate functional or productive vocabulary capacity.