Efficient Communication in Word Formation: How Syntactic and Lexical Surprisal Jointly Shape English Conversion Over the Past Century

Conversion degree, defined as the proportion of converted-form use out of total lemma use, has notably increased in English over the past century. For instance, track had a conversion degree of only 5% in the 1920s (95% noun, 5% verb) but rose to 35% by the 2010s (65% noun, 35% verb). This historical trend presents a communicative paradox: it favors speaker economy (articulatory ease via form reuse) seemingly at the expense of listener economy (increased ambiguity). The present study investigates this paradox through the framework of efficient communication, which predicts that ambiguous forms like conversion should be licensed in predictable contexts. We tested this prediction by analyzing 639 noun-to-verb and 443 verb-to-noun conversion words from the 1920s to the 2010s. Generalized linear mixed models were fitted with surprisal estimates from three architectures: local 5-grams, syntactic POS-grams, and broad-context GPT-2. Results indicate that the historical rise in conversion degree is accompanied by increasingly predictable contexts, suggesting that contextual predictability compensates for the absence of explicit morphological marking. We further demonstrate that syntactic and lexical surprisal exert independent inhibitory effects on conversion degree. Crucially, their positive interaction indicates that the constraining effect of lexical surprisal is stronger when syntactic surprisal is low and attenuated when syntactic surprisal is high, consistent with a resource competition account. These findings portray the evolution of English conversion not as random drift, but as a dynamic optimization reflecting a negotiation between communicative efficiency and cognitive resource limitations, offering a novel information-theoretic perspective on language change.