Recursion is the nature of human natural language. Since Chomsky proposed generative grammar, many scholars have studied recursion either theoretically or empirically. However, by observing children's acquisition of tail recursion sequences, we can verify the nativism of language supported by universal grammar and reveal the cognitive mechanism of human brain. To date, our understanding of children's acquisition path of recursion and influencing factors still remain controversial. This systematic review summarizes the research of tail recursive sequence by taking possessive recursion and locative recursion as examples, focusing on the experimental methods, acquisition paths, and influencing factors of tail recursive sequence. The current behavioural experiments reveal that, the debate about children's performance revolves around: 1) Gradual acquisition or synchronous acquisition. 2) symmetry or asymmetry between the acquisition of locative recursion sequences and possessive recursion sequences. We presume that children can acquire recursion quickly in a short period of time thanks to the language acquisition device, though there are also scholars who believe that a third factor also plays a role.

Language is the cornerstone of human communication, and the complexity of language lies in the diversity and recursion of its structure. Chomsky (1957) introduced the concept of recursion into natural language, arguing that the grammar in human natural language was a finite set of recursive rules by which an infinite number of linguistic expressions could be generated. In Corballis' (2014) words, the claim that recursion is the essence of natural language has been a continuing theme of Chomsky's work since his 1957 book Syntactic Structures. This theme is reiterated in Hauser et al. (2002), proposing that the faculty of language in the narrow sense only includes recursion, the only uniquely human component of the faculty of language. This proposal is summarized as the "recursion-only hypothesis" in Jackendoff and Pinker (2005: 212), which highlights the importance of recursion in linguistics. In spited of the lack of a consistent definition of (linguistic) recursion in the literature, most literature involves category recursion, which is defined as the "embedding of a category inside another of the same category". For instance, Martins and Fitch (2014) claim that recursion has been used to characterize the process of embedding a constituent of a certain kind of category inside another constituent of the same kind. This "embedding" process naturally generates hierarchical structures that display similar properties across different levels of embedding, and, thus, the feature of "self-similarity" is a signature of recursive structures. To illustrate that, they hold that the compound noun [[student] committee] (which has the structure [[[A]A] ]) is recursive since a noun phrase (NP) is embedded inside another NP, while a sentence with a noun plus a verb such as [[trees] grow] (which has the structure [[[A]B] ]) is non-recursive since a constituent of a given type of category is not embedded within a constituent of that same type.

There have been apparently conflicting claims over the syntax-semantics relationship in child acquisition. However, few of them have assessed the child's path toward the acquisition of recursive relative clauses (RRCs). The authors of the current paper did experiments to investigate 3- to 11-year-olds' most-structured elicited production of eight Mandarin RRCs in a 4 (syntactic types)*2 (semantic conditions) design. The four syntactic types were RRCs with a subject-gapped RC embedded in an object-gapped RC (SORRCs), RRCs with an object-gapped RC embedded in another object-gapped RC (OORRCs), RRCs with an object-gapped RC embedded in a subject-gapped RC (OSRRCs), and RRCs with a subject-gapped RC embedded in another subject-gapped RC (SSRRCs). Each syntactic type was put in two conditions differing in internal semantics: irreversible internal semantics (IIS) and reversible internal semantics (RIS). For example, "the balloon that [the girl that _ eats the banana] holds _" is SORRCs in the IIS condition; "the monkey that [the dog that _ bites the pig] hits_" is SORRCs in the RIS condition. For each target, the participants were provided with a speech-visual stimulus constructing a condition of irreversible external semantics (IES). The results showed that SSRRCs, OSRRCs and SORRCs in the IIS-IES condition were produced two years earlier than their counterparts in the RIS-IES condition. Thus, a 2-stage development path is proposed: the language acquisition device starts with the interface between (irreversible) syntax and IIS, and ends with the interface between syntax and IES, both abiding by the syntax-semantic interface principle.