Why do language models struggle with backward reasoning compared to forward?

This explores why models that reason fluently forward (problem to answer) struggle to run the same logic backward — and the corpus points to a single root cause: language models don't learn symmetric relationships, they learn directional ones. The cleanest evidence is the reversal curse: a model trained on 'A is B' often cannot answer 'B is A,' even though to a human those are the same fact Why can't language models reverse learned facts?. The reason is baked into how autoregressive training works — it encodes the order in which tokens appeared, so knowledge ends up format-bound rather than abstractly relational. Backward reasoning fails not because it's 'harder' but because the model never stored the inverse path in the first place.

That framing connects to a broader finding about what reasoning success actually depends on. Models don't break at some complexity threshold; they break at unfamiliarity — they succeed on chains that resemble their training instances and fail on ones that don't, because they're pattern-matching to seen examples rather than running a general algorithm Do language models fail at reasoning due to complexity or novelty?. Forward reasoning is the direction the training data ran, so it's the familiar one. Backward reasoning is the unfamiliar inverse, so the same instance-based machinery has nothing to lean on.

The most interesting twist is that you can fix this — and the fix reveals the mechanism. When you train a model to also generate backward questions and reason in reverse, its forward performance jumps ~13.5% across a dozen datasets Can backward reasoning during training improve forward reasoning?. Forcing the model to understand the inverse relationship between a problem and its solution deepens its grasp of both directions. In other words, the asymmetry isn't a hard architectural wall — it's a gap in what the training exposed, and exposing the reverse direction closes it.

Worth knowing too: some apparent reasoning failures aren't reasoning failures at all. Models often compute correct answers in early layers and then overwrite them to satisfy output format Do transformers hide reasoning before producing filler tokens?, and 'collapses' on long procedures can be execution bandwidth limits rather than logic limits — give the model a tool and the cliff disappears Are reasoning model collapses really failures of reasoning?. The takeaway for backward reasoning: before assuming the model 'can't reason in reverse,' it's worth asking whether the reverse path was ever encoded, or whether the right representation is present but getting suppressed.

So the surprise isn't that backward reasoning is intrinsically difficult — it's that for a system trained to predict the next token, forward and backward are not two views of one fact. They're two separate facts, and the model only learned one of them.