If Lexical Knowledge (understanding of words and their uses) is simply memorizing the definitions of fancy words, then, at best, it is a trivial ability valued by academics, pedants, and fuddy-duddies. At worst, its elevation by elitists is a tool of oppression. There is some truth to these views of Lexical Knowledge but they are myopic. I will argue that vocabulary tests are rightfully at the center of most assessments of language and crystallized intelligence. Some words have the power to open up new vistas of human experience. For example, when I was thirteen, learning the word “ambivalence” clarified many aspects of interpersonal relationships that were previously baffling.
A word is an abstraction. The need for labels of simple categories is perfectly clear. Knowing the word anger (or its equivalent in any other language) frees us from having to treat each encounter with the emotion as a unique experience. Being able to communicate with others about this abstract category of experience facilitates self-awareness and the understanding of interpersonal relations. We can build up a knowledge base of the sorts of things that typically make people angry and the kinds of reactions to expect from angry people.
It is less obvious why anger has so many synonyms and near-synonyms, some of which are a bit obscure (e.g., iracund, furibund, and zowerswopped!). Would it not be easier to communicate if there were just one word for every concept? It is worthwhile to consider the question of why words are invented. At some point in the history of a language, a person thought that it would be important to distinguish one category of experience from others and that this distinction merited a single word. Although most neologisms are outlived even by their inventors, a few of them are so useful that they catch on and are used by enough people for enough time that they are considered “official words” and are then taken for granted as if they had always existed. That is, people do not adopt new words with the primary goal of impressing one another. They do it because the word succinctly captures an idea or a distinction that would otherwise be difficult or tiresome to describe indirectly. Rather than saying, “Because Shelly became suddenly angry, her sympathetic nervous system directed her blood away from her extremities toward her large muscles. One highly visible consequence of this redirection of blood flow was that her face turned white for a moment and then became discolored with splotches of red.” It is simply more economical to say that “Shelly was livid with rage.” By convention, the use of the word livid signals that Shelly is probably not thinking too clearly at the moment and that the next thing that Shelly says or does is probably going to be impulsive and possibly hurtful.
Using near synonyms interchangeably is not merely offensive to word nerds and the grammar police. It reflects, and possibly leads to, an impoverishment of thought and a less nuanced understanding of the world. For example, jealousy is often used as a substitute for envy. They are clearly related words but they are not at all the same. In fact, in a sense, they tend to be experienced by people on opposite sides of a conflicted relationship. Envy is the painful, angry awareness that someone else enjoys some (probably undeserved) advantage that we covet. Jealousy is the angry, often vigilant, suspicion we may lose our beloved to a rival. Unaware of this distinction, it would be difficult to benefit from or even make sense of the wisdom of Rochefoucauld’s observation that “Jealousy is born with love, but does not die with it.”
Lexical Knowledge is obviously important for reading decoding. If you are familiar with a word, it is easier to decode. It is also obviously important for reading comprehension. If you know what a word means, it is easier to comprehend the sentences in which it appears. It is probably the case that reading comprehension also influences Lexical Knowledge. Children who comprehend what they read are more likely to enjoy reading and thus read more. Children who read more expose themselves to words that rarely occur in casual speech but the meaning of which can be inferred from how it is used in the text. Finally, Lexical Knowledge is important for writing. Children with a rich understanding of the distinctions between words will not only be able to express what they mean more precisely, but their knowledge of certain words will enable them to express thoughts that they might not otherwise have had. For example, it seems to me unlikely that a student unfamiliar with the word “paradox” would be able to write an essay about two ideas that appear to be contradictory at first glance but at a deeper level are consistent with each other.
 Of course, dictionaries abound with antique words that were useful for a time but now languish in obscurity. For example, in our more egalitarian age, calling someone a cur (an inferior dog because it is of mixed breed) is not the insult that it once was. It is now used mostly for comedic effect when someone affects an aristocratic air. My favorite example of a possibly soon-to-be antique word is decadent, which is nowadays almost exclusively associated with chocolate.
This post is an excerpt from:
Schneider, W. J. (2013). Principles of assessment of aptitude and achievement. In D. Saklofske, C. Reynolds, & V. Schwean (Eds.), Oxford handbook of psychological assessment of children and adolescents (pp. 286–330). New York: Oxford.