Title: An Investigation of Domain Generalization with Rademacher Complexity. Abstract: The domain generalization (DG) setting challenges a model trained on multiple known data distributions to generalise well on unseen data distributions. Due to its practical importance, many methods have been proposed to address this challenge. 
However much work in general purpose DG is heuristically motivated, 
as the DG problem is hard to model formally; and recent evaluations have cast doubt on existing methods’ practical efficacy -- in particular compared to a well tuned empirical risk minimisation baseline. 
We present a novel learning-theoretic generalisation bound for DG that bounds unseen domain generalisation performance in terms of the model’s empirical risk and Rademacher complexity -- providing a sufficient condition for DG. Based on this insight, we empirically analyze the performance of several methods and show that their performance is indeed influenced by  model complexity in practice. 
Algorithmically, our analysis suggests that tuning for domain generalisation should be achieved by simply performing regularised ERM with a leave-one-domain-out cross-validation objective. Empirical results on the DomainBed benchmark corroborate this.