Learning Syntax Without Planting Trees: Understanding When and Why Transformers Generalize Hierarchically (2404.16367v2)

Abstract: Transformers trained on natural language data have been shown to learn its hierarchical structure and generalize to sentences with unseen syntactic structures without explicitly encoding any structural bias. In this work, we investigate sources of inductive bias in transformer models and their training that could cause such generalization behavior to emerge. We extensively experiment with transformer models trained on multiple synthetic datasets and with different training objectives and show that while other objectives e.g. sequence-to-sequence modeling, prefix LLMing, often failed to lead to hierarchical generalization, models trained with the LLMing objective consistently learned to generalize hierarchically. We then conduct pruning experiments to study how transformers trained with the LLMing objective encode hierarchical structure. When pruned, we find joint existence of subnetworks within the model with different generalization behaviors (subnetworks corresponding to hierarchical structure and linear order). Finally, we take a Bayesian perspective to further uncover transformers' preference for hierarchical generalization: We establish a correlation between whether transformers generalize hierarchically on a dataset and whether the simplest explanation of that dataset is provided by a hierarchical grammar compared to regular grammars exhibiting linear generalization.

An Analysis of Hierarchical Generalization in Transformers

The paper "Learning Syntax Without Planting Trees: Understanding When and Why Transformers Generalize Hierarchically" provides an in-depth examination of how transformer models trained with various objectives exhibit hierarchical generalization when interpreting syntax in NLP tasks. The authors challenge the notion that hierarchical structure is explicitly necessary for language understanding by investigating potential sources of inductive bias within transformers that encourage them to generalize hierarchically.

Key Findings

1. Inductive Bias and Training Objectives: Through comprehensive experimentation with transformer models on multiple synthetic datasets and varying training objectives, the authors discover that the LLMing (LM) objective uniquely encourages hierarchical generalization across diverse linguistic tasks. Other training objectives, such as sequence-to-sequence (seq2seq) modeling and prefix LLMing, frequently failed to foster hierarchical generalization. This distinction underscored the importance of modeling the entire sequence of tokens to learn hierarchical structures effectively.
2. Subnetwork Generalization: Using attention head pruning, the authors uncover subnetworks within transformer models that are indicative of distinct generalization behaviors. These subnetworks reveal differential generalization patterns associated with hierarchical and linear order principles. Interestingly, these subnetworks coexist and remain distinguishable at various training phases, even after the overall model's performance aligns closely with one type of generalization over another.
3. Bayesian Perspective: Adopting a Bayesian framework, the paper correlates transformer models' propensity for hierarchical generalization with a preference for hierarchical grammars offering simpler explanations of datasets compared to regular grammars adhering to linear generalization. Experiments reveal instances where transformers fail hierarchical generalization in scenarios where regular grammars possess higher posterior probabilities than hierarchical grammars, emphasizing the role of simplicity in guiding model generalization choices.

Implications and Speculation for Future Research

The authors' findings have significant theoretical implications regarding how transformer models can be optimized for linguistic tasks requiring syntactic understanding. The realization that LLMing objectives naturally encourage hierarchical generalization suggests leveraging these objectives more extensively in training regimes where syntactical nuance is crucial. Additionally, the identified feasibility of manipulating subnetwork behavior provides a novel approach to tuning transformer models' generalization strategies by potentially introducing biases towards desired linguistic structures at fine-grained levels.

For future developments in AI, these insights could refine transformer architectures to improve robustness in handling varied linguistic contexts with minimal explicit syntactic cues. The Bayesian approach could be instrumental in formalizing how model architecture and objective choices affect learning patterns, providing a more directed pathway for models that robustly handle ambiguous or under-specified linguistic data as seen in human-like language proficiency.

Overall, this paper broadens our understanding of transformers' syntactic learning capabilities, presenting compelling evidence that simpler, non-hierarchical representations can under certain conditions serve as viable predictors for complex linguistic patterns. The combination of rigorous objective comparisons, explorations of latent subnetwork discovery, and probabilistic semantics sets a foundational step for advancing transformer integrative capabilities in syntactic comprehension and beyond.