How many words do most adults know?

In the largest study of its kind ever attempted, new research published in the journal Frontiers in Psychology suggests that by the age of twenty, on average, a native speaker of American English knows around 42,000 words.

The study was based on an online vocabulary test which was initially designed for people in the Netherlands to take, but its popularity soon grew.

"Our research got a huge push when a television station in the Netherlands asked us to organize a nation-wide study on vocabulary knowledge," said Professor Marc Brysbaert of Ghent University in Belgium, the leader of the study. "The test we developed was featured on TV and, in the first weekend, over 300 thousand Dutch speakers had done it - it really went viral."

The researchers began to realize that many people were interested in finding out the size of their vocabulary so they subsequently developed English and Spanish versions of the test. The English version alone has been taken nearly one million times after being widely shared on social media.

"At the Centre of Reading Research we are investigating what determines the ease with which words are recognized;" Brysbaert said. The test is based on a list of 62,000 words that his team compiled themselves. "As we made the list ourselves and have not used a commercially available dictionary list with copyright restrictions, it can be made available to everyone, and all researchers can access it."

The test works by asking the participant whether or not the word that appears on the screen is an existing word. Each test is made up of 70 real words as well as 30 sequences of letters which look like words, but aren't.

When the English test results were combined with personal information from the participants, such as age, gender and education level, the team were able to suggest that the average twenty-year-old native speaker of American English knows 42,000 dictionary words. It is thought that as we get older, we learn one new word every two days, meaning that by the age of 60, the same native speaker would know about 48,000 words.

"As a researcher, I am most interested in what this data can tell us about word prevalence, i.e. how well each word is known in a language;" Brysbaert said.

"In Dutch, we have seen that this explains a lot about word processing times. People respond much faster to words known by all people than to words known by 95% of the population, even if the words used with the same frequency. We are convinced that word prevalence will become an important variable in word recognition research."

As part of the study, the team were able to collect data from around 200,000 people for whom English is a second language. If the team were to examine how well people knew certain words, it could have implications for language learning research.

Looking forward, Brysbaert plans to enhance the accuracy of the test and extend the number of words on the list to more than 75,000.

"This work is part of the big data movement in research, where big datasets are collected to be mined;" he said.

"It also gives us a snapshot of English word knowledge at the beginning of the 21st century. I can imagine future language researchers will be interested in this database to see how English has evolved over 100 years, 1000 years and maybe even longer".

You can take the test here.