Direct answer
There is no single forex pair that “determines” the future outcome of EUR/USD. EUR/USD is defined as the value of one euro in U.S. dollars, so its changes are fundamentally driven by relative movement between EUR and USD. Other pairs can be used in analysis because they include USD (or EUR) in their price, but they do not guarantee or directly fix what EUR/USD will do next.
Within the canonical scope you requested (AUD/USD), the most relevant link is that AUD/USD also contains USD. As a result, movements in AUD/USD can sometimes provide information about how USD is moving relative to another currency, which can be related to EUR/USD changes. This is an informational relationship, not a deterministic one.
Explanation: inputs, relationships, and comparisons
What “outcome” means (and what it doesn’t) In forex terminology, “outcome” commonly refers to where a pair’s price ends up over some period (for example, higher or lower than a starting price). This kind of outcome cannot be inferred with certainty from any other pair because markets can react to new information at any time.
Which pairs are mechanically connected to EUR/USD Because EUR/USD uses EUR and USD, pairs that contain those currencies are the most mechanically connected for analysis:
- Pairs with USD can reflect broad USD strength/weakness.
- Pairs with EUR can reflect euro strength/weakness.
Where AUD/USD fits (canonical scope) AUD/USD quotes how many U.S. dollars (USD) are needed for one Australian dollar (AUD). If USD weakens generally across the market, both EUR/USD and AUD/USD can rise at the same time; if USD strengthens, both can fall. However, AUD/USD also depends on AUD-specific factors, so its movement is not a pure USD signal.
Example or checks: how to verify relationships without predicting
If you want an independent way to check whether a “relationship” is present, use a simple historical comparison for the period you care about:
- Check whether EUR/USD and AUD/USD have tended to move in the same direction during some historical windows.
- Compare different time ranges (for example, weeks vs. months), because relationships often change.
- Separate observation from inference: even if they move together in the past, that does not imply they will determine EUR/USD in the future.
A practical interpretation consistent with uncertainty is:
- Both pairs include USD, so they may co-move when USD-driven forces dominate.
- Only EUR/USD directly measures EUR vs USD, so AUD/USD cannot replace direct observation of EUR/USD.
Limitations and risks (uncertainty you must account for)
- No deterministic “decider” exists: correlations and relationships are statistical, not causal guarantees.
- Time variation: any linkage between EUR/USD and AUD/USD can weaken or reverse as market conditions shift.
- Currency-specific effects: AUD/USD reflects AUD as well as USD, which can cause divergence even when USD is moving.
- No future inference: historical overlap cannot confirm what will happen next for EUR/USD.
Because your question asks specifically about pairs that determine EUR/USD outcomes, the safe, verifiable conclusion is that EUR/USD is determined by EUR vs USD price changes, while AUD/USD (within the AUD/USD scope) can only be one of several potentially related inputs for analysis—not a reliable determinant of future outcomes.