In typological literature, there has been disagreement as to whether there should be distinction between relative clauses (RCs) and nominal sentential complements (NSCs) in pro-drop languages such as Japanese, Chinese, Korean, Khmer and Thai. In pro-drop languages, nouns can be dropped when its reference can be retrieved from context. Therefore, what has been defined as a gap in RCs (i.e., the controversial element hereafter) and has often been used for differentiating RCs from NSCs might actually be a zero pronoun, making syntactic structures of RC and NSCs indistinguishable. This article reviews the study of the processing of RC attachment in Thai and discusses how its results provide preliminary evidence supporting that RCs and NSCs in Thai are syntactically different constructions and the controversial element in RCs is not a zero pronoun. The article suggests a new promising tool for investigating an unresolved typological problem.