A reaction function is the informal map traders use for how a central bank tends to respond to data. It is not a published formula. It is a working hypothesis: if inflation stays sticky, language leans hawkish; if growth softens cleanly, language can ease. For UK beginners, the point is literacy — how desks talk — not a promise the next decision will follow your sketch.
What it is — and is not
After policy weeks, speakers often restate what they care about most: services inflation, wages, energy floors, labour tightness and financial conditions. The reaction function is the ranking of those inputs. When oil stays elevated, some speakers emphasise inflation risks more than growth softness. That split is the map markets trade between meetings.
Samuel & Co Trading’s assessment is that beginners should write the bank’s stated mandate first, then the two or three data series the last statement emphasised. That beats inventing a mechanical if-CPI-then-hike rule that no committee actually publishes.
Why UK desks care now
Once decisions are out, markets trade the residual reaction function. A hawkish hold can leave a hike minority live. A priced hike can leave the path open. Speeches fill colour without a new vote. Oil floors and soft patches can pull speakers in different directions inside the same week.
How to read it in practice
Map BoE language onto UK labour and services CPI colour. Map Fed language onto US core inflation and labour resilience. Map BoJ language onto underlying inflation and FX comments. Keep oil in a separate column when energy competes with wages. When speakers diverge, mark which asset believed which speaker.
Worked example for a UK desk
Two speakers can both say inflation risks remain. One ranks oil first; the other ranks wages first. If crude softens and front-end yields fall while sterling barely moves, the market may be weighting the oil rank more than the wage rank that day — a reaction-function read, not a trade order.
What it does not prove
A reaction-function sketch does not forecast the next vote. It does not override the published statement. It does not justify oversized FX positions. Educational framing only — no buy or sell recommendation.
Beginner checklist
After each decision, list decision, vote split, key path sentence, and the data series speakers said they are watching. Update when a speech shifts emphasis. Prefer official transcripts over paraphrases that flatten nuance into hawkish or dovish stamps.
Common mix-ups
Do not treat one speaker as the whole committee. Do not ignore minority votes. Do not collapse oil shocks and wage stickiness into one inflation number. Do not assume every bank shares the same ranking of risks. Do not overwrite last week’s statement with one lunchtime speech.
Putting it next to the tape
When a speech hits, mark whether front-end yields, the currency and equity futures moved together. Agreement suggests a reaction-function shift; disagreement suggests noise or a competing oil or risk channel.
Second-order links for UK traders
Reaction-function talk connects directly to speech days and to data surprises. A soft labour print matters more if wages were the top-ranked input in the last statement; an oil spike matters more if energy floors were explicitly named. UK traders should keep BoE, Fed and BoJ maps separate even in a triple week, because the ranking of risks is not identical across committees. Pair this literacy with path-versus-one-off-hike explainers so you do not confuse a restated ranking with a new decision.
If you want a structured check on how you process this map, a free traders assessment can highlight sizing and timing habits without turning this explainer into personal advice.
Conclusion
A central-bank reaction function is a desk map of data-to-language priorities, not a crystal ball. UK beginners gain more by tracking stated emphases and minority votes than by inventing a mechanical rule. Educational only, not a forecast.
