It has been proposed that language evolves under the joint con-straints of communicative expressivity and cognitive ease. Weexplore this idea in the historical emergence of word forms.We hypothesize that new word forms that enter the lexiconshould reflect a trade-off between predictability and distinc-tiveness. An emergent word form can be highly predictable ifit efficiently reuses elements from the existing word forms, re-sulting in low cognitive load. An emergent word form shouldalso be sufficiently distinctive from the existing lexicon, facil-itating communicative expressivity. We test our hypothesis byexamining the properties of 34,478 emergent word forms overthe past 200 years of Modern English. We show how wordforms at future time t + 1 are bounded statistically betweenn-gram generated word forms (highly predictable) and slangwords that are outside the standard lexicon (highly distinctive)at time t. Our work supports the view of cognitive economy inlexical emergence.

Words frequently acquire new senses, but the mental processthat underlies the historical emergence of these senses is oftenopaque. Many have suggested that word meanings develop innon-arbitrary ways, but no attempt has been made to formalizethese proposals and test them against historical data at scale.We propose that word meaning extension should reflect a drivetowards cognitive economy. We test this proposal by exploringa family of computational models that predict the evolution ofword senses, evaluated against a large digitized lexicon thatdates back 1000 years in English language history. Our find-ings suggest that word meanings not only extend in predictableways, but also that they do so following an historical path thattends to minimize cognitive cost - through a process of nearest-neighbor chaining. Our work contributes a formal approach toreverse-engineering mental algorithms of the human lexicon.