Expand Horizon: Graph Out-of-Distribution Generalization via Multi-Level Environment Inference

Graph neural networks (GNNs) are widely used for node classification tasks, but when encountering distribution shifts due to environmental change in real-world scenarios, they tend to learn unstable correlations between features and labels. To overcome this dilemma, a powerful class of approaches views the environment as the root cause of those unstable correlations, thereby their key focus is to infer the environment involved, enabling the model to avoid capturing environment-sensitive correlations. However, their inferences rely solely on the single-level information from one low-hop ego-graph, neglecting both global information and multi-granularity information in local ego-graphs with different hops. Although applying deeper GNNs on the high-hop ego-graph could capture global information, it will bring the side effect of over-smoothing node representations. To tackle these issues, we propose a novel Multi-Level Environment Inference model named MLEI, which effectively broadens the horizon of training GNNs under node-level distribution shifts. Specifically, MLEI first leverages a linear graph transformer to surpass the scope of ego-graph, efficiently enabling high-level global environment inference. This global environment is in turn used as an overview to assist layer-by-layer environment inference on local multi-hop ego-graphs. Finally, we combine the environment from global and local views and utilize the designed objective function to capture stable predictive patterns. Extensive experiments on real-world datasets demonstrate that our model achieves satisfactory performance compared with the state-of-the-art methods under various distribution shifts.