Recent work by Hewitt et al. (2020) provides a possible interpretation of the empirical success of recurrent neural networks (RNNs) as language models (LMs). It shows that RNNs can efficiently represent bounded hierarchical structures that are prevalent in human language. This suggests that RNNs' success might be linked to their ability to model hierarchy. However, a closer inspection of Hewitt et al.'s (2020) construction shows that it is not limited to hierarchical LMs, posing the question of what \emph{other classes} of LMs can be efficiently represented by RNNs. To this end, we generalize their construction to show that RNNs can efficiently represent a larger class of LMs: Those that can be represented by a pushdown automaton with a bounded stack and a generalized stack update function. This is analogous to an automaton that keeps a memory of a fixed number of symbols and updates the memory with a simple update mechanism. Altogether, the efficiency in representing a diverse class of non-hierarchical LMs posits a lack of concrete cognitive and human-language-centered inductive biases in RNNs.

循环神经网络（RNNs）作为语言模型（LMs）的经验成功可能与其能够有效地表示人类语言中的有界分层结构有关，并且可以推广其构造以表示更大类别的LMs，即可以用带有边界堆栈和广义堆栈更新函数的推挤自动机来表示。然而，RNNs在表示多样化的非分层LM类别时的效率表明其缺乏具体的认知和以人类语言为中心的归纳偏见。

关于RNN语言模型归纳偏差的理论结果