This dissertation concerns one of the long-term debates about whether an innate grammar, i.e. Universal Grammar, is necessary for a child to acquire a language. The researchers who do not agree that an innate grammar needs to exist argue that the input or the child's experience of language is sufficient to acquire a language. In my work, I examine the relative clause, which is a complex structure cross-linguistically, as a case study to investigate the controversy between the role of innate grammar and the role of input in language acquisition. Based on both the comprehension data of Chinese-speaking children that I collected and the statistical analyses of corpora that I completed, I suggest that a satisfactory theory of language acquisition needs to still incorporate an innate grammar in order to accommodate the various and sometimes inconsistent input properties I document. Nevertheless, the acquisition pattern cannot be explained well without a good understanding of language-specific properties and of their potential interactions with language-external factors such as the comprehension system that may be at work in acquisition.