Contrary to lay opinion, the size of a man’s vocabulary is not only an index of his schooling, but an excellent measure of his general intelligence. Its excellence as a test of intelligence may stem from the fact that the number of words a man knows is at once a measure of his learning ability, his fund of general information and of the general range of his ideas.
From the Measurement and Appraisal of ADULT INTELLIGENCE by David Wechsler 4th edition 1958
One of the great things about using vocabulary as a measure of IQ (or at least verbal IQ) is that like physical measurements, it’s a true ratio scale with an actual zero point.
So how many words does the median young adult know and how does this map to IQ?
According to the above study, among young U.S. adults, the 5th percentile, 50th percentile and 95th percentile, know 27,100, 42,000, and 51,700 lemmas respectively. These percentiles equate to verbal IQ equivalents of 75, 100, and 125 allowing me to equate total vocabulary to the IQ scale.
Notice how linear the relationship is? Verbal IQ appears to be a true interval scale, at least within 2 SDs from the mean.
Verbal IQ is almost a true ratio scale too because notice how 51,700 (verbal IQ 125), is roughly 125% as large as 42,000 (verbal IQ 100). On the other hand, a vocabulary of no words equates to a verbal IQ of 19, and not zero.
It is interesting to ask what would be the verbal IQ equivalent of someone who knew every word in the English language. According to the study data cited above, the maximum number of lemmas is 61,800 which would equate to an IQ of 143. Although 143 is an exceptionally high IQ, it’s absurd to think one in several hundred Americans knows every single word (even on a very superficial level).
Reading the above study further I find:
A first limitation is the list of 61,800 lemmas we used. Although we are reasonably sure the list contains the vast majority of words people are likely to know, there are ample opportunities to increase the list. As indicated above, the Collins scrabble list could be used to more than double the number of entries. We are fairly confident, however, that such an increase will not change much in the words known by the participants (see also Goulden et al., 1990). The words we are most likely to have missed are regionally used common words and recently introduced words.
So if the maximum number of lemmas could be doubled to 123,600, that raises the ceiling of verbal IQ (as measured by total vocabulary) to 267! I doubt anyone could ever score that, unless they have some kind of autistic obsession with reading scrabble dictionaries, in which case the test would be invalid for them.
Pumpkin Person 
“We can put numbers on some”thing” therefore it’s a measurement “is ridiculous. What’s the specified measured object, the object of measurement and the measurement unit for vocabulary?
Oh shut up RR! There are 4 levels of measurement: Nominal, ordinal, interval, and ratio. Vocabulary is the highest level because it has both equal intervals and a true zero point making it a ratio scale. You can literally say one person’s vocab is twice as big as someone else’s.
IQ is ordinal. You then “map” vocabulary to “IQ”—as if the conceptual objections on IQ have been addressed. So if “one person’s vocab is twice as big as someone else’s”, then are they “twice as intelligent”? How does this address the conceptual arguments against IQ? You’re just assuming that since you can put numbers to something that there is a “thing” being measured. That’s fallacious.
Twice as good at learning words. If I say your vocab is at the 50th percentile, that’s an ordinal scale. If I say your vocab equates to an IQ of 100, that’s an interval scale. If I say your vocab is 42,000 words, that’s a ratio scale. You’re lumping all psychometric measures together without appreciating the subtle differences.
You’re assuming that IQ is a “measure.”
nah. english has lots of synonymy like the old story that eskimo has 200 words for snow. the real point of vocab isn’t how many words you know for a few things. it’s how many things you have words for…and your ability to define them. people can use words passably yet have great difficulty defining them. defining well requires like a whole theoretical structure in your head
.https://www.youtube.com/watch?v=hOSYiT2iG08
Duh look at [redacted by pp, 2021-05-11]. He probably does read dictionaries. He’s a good example of autosm obessession leading to high vocab
Interesting !
I would have liked to know who is philo’s example of a high vocabulary holding person due more to an autistic like obsession than to a general aptitude …
I doubt that outside technical matters, vocabulary would be anything other than a deeply automated process like vision. Even 42 000 is too much words for it to be learned by a mere voluntary process. It’s like walking or running or flying for most birds. Great deal of improvement possible, and learning and environmental conditions essential, but the basic necessary process is automated.
I wonder if the size of vocabulary vary greatly between natural languages. Colour, sounds, reaction time and vocabulary are 4 great ratio scale available for IQ investigations .
if your memory is good you do not need to be autistic to reach a high vocab by reading dic. 170 IQ probably is a super memorizer of vocab.
Hey Pumpkin. Are base words and word families effectively the same thing? I ask because I recently took an online test that measured the amount of word families I knew.