UC San Diego

We present four computational experiments that investigate the impact of null subjects (pro-drop) on the learnability of languages with a basic Subject-Verb-Object (SVO) word order and varying amounts of morphological marking on their nouns and verbs. The simulations show that the effect of pro-drop on language learnability is limited as long as some morphological marking is present. Contrary to expectation, rich agreement markers are no more useful in the simulations than nominal case markers or verbal Tense/ Aspect/Modality markers. In the absence of morphological marking, however, pro-drop leads to severe learnability problems in the simulations: overall performance on this language type is significantly worse (Experiment 1); additional exposure to language data is not as useful as with other types (Experiment 2); novel words are more problematic in this type (Experiment 3); and noun/verb homonyms also decrease performance for this type (Experiment 4). An analysis of the simulations shows that the main problem is accurately distinguishing nouns from verbs. These results suggests that the combination of pro- drop and no morphological marking should be unattested among natural languages. To test this hypothesis we first survey various creole languages as they are SVO and typically lack morphological markers. However, cross- linguistic data shows that creole languages do not allow pro-drop unless they have also developed agreement markers. We then discuss Mandarin Chinese because it allows widespread pro-drop and features only minimal morphological marking. A closer look at the language reveals that Mandarin provides quite reliable cues for identifying nouns and verbs in the language. Crucially, these cues are acquired very early by children learning Mandarin. Similarly, children only very rarely use nouns as verbs (or vice versa) - unlike in English where pro- drop is not possible. Two other unusual properties of Mandarin Chinese that are also compatible with our experimental results are the relatively early acquisition of verbs and the presence of relatively frequent noun/verb homonymy. Mandarin is thus not a counter-example to the results of the simulations. We end by situating our work in relation to various other approaches, such as the Competition Model, Optimality Theory, and probabilistic linguistics