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

Figure 3 shows the percentage of lemmas known to native speakers of American English as a function of age and education level (see Keuleers et al., 2015, for similar findings in Dutch). As can be seen, the knowledge of words increases with age and education. As a matter of fact, the effect of age very strongly resembles the power function that would be expected on the basis of Herdan’s law (Figure 1): Because older people have come across more words than younger people, they also know more words. This suggests that people rarely forget words, even when they don’t know the (exact) meaning any more. Indeed, a test in which persons are asked to spot a word among non-words is thought to be a good estimate of premorbid intelligence in elderly people and patients with dementia (Baddeley et al., 1993; Vanderploeg and Schinka, 2004; but also see Cuetos et al., 2015 for evidence that dementia patients are deficient at spotting words among non-words, in particular words that are low in frequency, low in imageability, and acquired later in life).

The median score of 20-year-olds is 68.0% or 42,000 lemmas; that of 60-year-olds 78.0% or 48,200 lemmas. This corresponds to a difference of 6,200 lemmas in 40 years’ time (or about one new lemma every 2 days). The difference between education levels is also substantial and likely illustrates the impact of reading and studying on vocabulary knowledge. Indeed most of the difference between the education levels seems to originate during the years of study.

Figure 4 shows the same information for the base words (the word families). An interesting observation here is that the overall levels of word knowledge are lower. A 20-year-old knows 60.8% of the base words (for a total of 11,100 base words), and a 60-year-old knows 73.2% (or 13,400 base words). The lower percentage of base word knowledge is to be expected, because well-known lemmas tend to come from large word families. So, the 325 lemmas including ‘man’ (craftsmanship, repairman, congressman, …) are better known than the four lemmas including ‘foramen’ (foramen, foraminiferous, foraminifera, and foraminifer). The former add much more weight to the tally of lemmas known (325 vs. 4) than to the tally of base word known (1 vs. 1). As a result, quite a lot of lemmas can be known based on the mastery of a limited number of prolific base words.