  Open-domain Question Answering (OpenQA) aims at answering factual questions
with an external large-scale knowledge corpus. However, real-world knowledge is
not static; it updates and evolves continually. Such a dynamic characteristic
of knowledge poses a vital challenge for these models, as the trained models
need to constantly adapt to the latest information to make sure that the
answers remain accurate. In addition, it is still unclear how well an OpenQA
model can transfer to completely new knowledge domains. In this paper, we
investigate the generalization performance of a retrieval-augmented QA model in
two specific scenarios: 1) adapting to updated versions of the same knowledge
corpus; 2) switching to completely different knowledge domains. We observe that
the generalization challenges of OpenQA models stem from the reader's
over-reliance on memorizing the knowledge from the external corpus, which
hinders the model from generalizing to a new knowledge corpus. We introduce
Corpus-Invariant Tuning (CIT), a simple but effective training strategy, to
mitigate the knowledge over-memorization by controlling the likelihood of
retrieved contexts during training. Extensive experimental results on multiple
OpenQA benchmarks show that CIT achieves significantly better generalizability
without compromising the model's performance in its original corpus and domain.
