We estimate the degree to which languages resort to borrowing as a means of lexical replacement, within a group of neighbouring languages of southwestern Cape York Peninsula, using several methods: (1) sound correspondences and correspondence mimicry; (2) the proportion of "local" words in singlelanguage lists; and (3) the creation of the vocabulary of special registers. We find that borrowing accounts for at most half of lexical replacement in these languages, and most usually is well below half. We demonstrate that this rate is crucial in the prediction of what fraction of vocabulary might in the long term be common to two neighbouring languages (the 'equilibrium percentage') in a model of lexical similarity that does not distinguish borrowings from common retentions. We then apply these findings to the case study, and compare the methods of lexicostatistical subgrouping (with and without recognition of loans), to results from classification by classic means. We find substantial agreement, and that the effect of borrowing to equilibrium9 on lexicostatistical subgrouping is tolerably small.