Understanding In-Context Learning via Supportive Pretraining Data
Authors: name: Xiaochuang Han, name: Daniel Simig, name: Todor Mihaylov, name: Yulia Tsvetkov, name: Asli Celikyilmaz, name: Tianlu Wang

In-context learning (ICL) improves language models' performance on a variety
of NLP tasks by simply demonstrating a handful of examples at inference time.
It is not well understood why ICL ability emerges, as the model has never been
specifically trained on such demonstrations. Unlike prior work that explores
implicit mechanisms behind ICL, we study ICL via investigating the pretraining
data. Specifically, we first adapt an iterative, gradient-based approach to
find a small subset of pretraining data that supports ICL. We observe that a
continued pretraining on this small subset significantly improves the model's
ICL ability, by up to 18%. We then compare the supportive subset constrastively
with random subsets of pretraining data and discover: (1) The supportive
pretraining data to ICL do not have a higher domain relevance to downstream
tasks. (2) The supportive pretraining data have a higher mass of rarely
occurring, long-tail tokens. (3) The supportive pretraining data are
challenging examples where the information gain from long-range context is
below average, indicating learning to incorporate difficult long-range context
encourages ICL. Our work takes a first step towards understanding ICL via
analyzing instance-level pretraining data. Our insights have a potential to
enhance the ICL ability of language models by actively guiding the construction
of pretraining data in the future.