What Is a Word Family?

A word family is a base word plus all the forms built on it with common prefixes and suffixes. Develop, develops, developed, developing, developer, development, developmental and undeveloped all belong to one family. Vocabulary researchers count word families rather than individual word forms because once you know the base and the affixes, understanding the other members takes little additional learning.

Examples of word families

How word families are defined: Bauer and Nation (1993)

Where should a family stop? Is react really part of the act family? Laurie Bauer and Paul Nation (1993) answered this with a scale of affix levels, ordered by how frequent, regular and predictable each affix is:

• Level 1: every form counts as a separate word (no family).
• Level 2: inflections only: plural -s, past -ed, -ing, comparative -er, superlative -est, possessive 's.
• Level 3: the most frequent and regular derivational affixes, such as -able, -er, -less, -ly, -ness, -y, un- and non-.
• Level 4: frequent, regular affixes such as -al, -ation, -ful, -ism, -ist, -ity, -ize, -ment, -ous and in-.
• Levels 5–7: progressively less common or less transparent affixes and classical roots.

Most word lists used in research, including the Academic Word List, define a family at about Level 6. That is a generous definition, which matters when you read claims about how many words people know.

Word family vs lemma vs word form

The same text gives very different counts depending on the unit:

This is why estimates of vocabulary size vary so much. Goulden, Nation and Read (1990) put educated native speakers at around 17,000 word families, while Brysbaert and colleagues (2016) counted about 42,000 lemmas for a typical 20-year-old American. Both can be roughly right because they count different things.

Why word families matter for learners

The family concept is the basis of the best-known coverage figures. Nation (2006) estimated that 8,000–9,000 word families are needed to know 98 percent of the words in novels and newspapers, and 6,000–7,000 for spoken English. Those numbers sound manageable precisely because each family includes many forms. You can see what they mean for your own reading in how many words you need to be fluent.

The catch: family membership is only "free" if you know the affixes. A learner who does not know that -ity makes nouns from adjectives may not connect scarce and scarcity. Research by Schmitt and Zimmerman (2002) found that learners often knew some members of a family but could not produce others.

How to use word families in study

1. Learn the family, not the form. When you save ambiguous, also note ambiguity and unambiguous.
2. Study common affixes. Twenty or so prefixes and suffixes unlock thousands of derived forms. See morpheme.
3. Practice production. Make cards that ask for the noun or adverb form: reliable to reliability, reliably.
4. Watch for meaning shifts. Some members drift: business is no longer simply "being busy."

Sources: Bauer, L. & Nation, P. (1993), Word families, International Journal of Lexicography; Goulden, R., Nation, P. & Read, J. (1990), How large can a receptive vocabulary be?, Applied Linguistics; Schmitt, N. & Zimmerman, C. (2002), Derivative word forms: what do learners know?, TESOL Quarterly; Nation, P. (2006), How large a vocabulary is needed for reading and listening?, Canadian Modern Language Review; Brysbaert, M. et al. (2016), Frontiers in Psychology.