Direct answer
The spread you see in FX pair session behaviour is mainly shaped by how easy it is to buy and sell the pair (liquidity), how much the price is moving (volatility), where and how trades are executed (execution venue conditions), and how a provider manages quoting and order handling (broker-policy effects). These factors can change from one session to another, which is why spreads are not constant.
Mechanism or definition: what “spread” means in this context
In FX trading, the spread is the difference between the quoted buy (ask) and sell (bid) prices for a currency pair at a given moment. Pair session behaviour refers to how trading activity and market conditions vary across different market hours and overlapping sessions.
Think of the quoted spread as a practical cost and risk buffer. If many participants are willing to trade near the current price, a provider can quote a tighter bid–ask spread. If fewer participants are present or prices move quickly, the provider may widen the spread to cover execution risk and uncertainty.
What affects the spread: stable mechanics vs variable conditions
1) Liquidity (how easily orders match)
Liquidity tends to be higher when more market participants are active and when major trading desks overlap. Higher liquidity usually reduces the time and price distance needed to fill an order, which supports tighter spreads. Lower liquidity makes order matching harder; the same trade size may require moving the effective price further, which often results in wider quoted spreads.
Assumption for examples: Suppose you compare the same pair at two different times, with identical order size and the same overall market direction. If one time has less liquidity, spreads are more likely to widen.
2) Volatility (how fast prices change)
Volatility describes how quickly and how far prices can move in a short period. During more volatile conditions, a provider’s inventory risk and hedging speed requirements rise. One common outcome is that the quoted spread widens to reflect the higher uncertainty between bid and ask.
Assumption for examples: If volatility rises but liquidity stays the same, spreads can still widen because the provider expects more rapid price changes.
3) Execution venue and pricing path
Even when the “spread” is quoted as a bid–ask difference, actual trading cost can shift depending on the execution venue and how price information is accessed. Two traders can see different realized costs for the same displayed spread if:
- their orders interact differently with available counterparties,
- the system routes orders differently,
- or the price path moves between quote and execution.
This is why two people may describe the “spread” differently: some focus on the quoted difference, others experience total cost through slippage or fees.
4) Provider policies (how bids/asks are set and how orders are handled)
Provider-policy effects include how a platform or provider:
- sets or updates quotes,
- manages risk limits and hedging behaviour,
- and handles order execution when conditions are thin.
For example, during low-liquidity or fast markets, some systems may quote wider levels to reduce the chance of adverse execution. Others may keep quotes tighter but then execution may occur at worse effective prices when the market moves before the order can be filled.
Evidence or example (independent reasoning, not live prices)
Consider three generic time windows for the same currency pair:
- High-liquidity window: many participants active; prices change more gradually.
- Low-liquidity window: fewer participants; the book is thinner.
- Fast-move window: volatility increases due to sudden repricing.
Without using any real-time numbers, you can still predict the direction of risk:
- In the low-liquidity window, spreads are more likely to widen because it is harder to find matching liquidity.
- In the fast-move window, spreads are more likely to widen because quote uncertainty rises.
- In the high-liquidity window, spreads are more likely to be tighter because execution risk is lower.
Limitations and risks (material failure modes)
A key limitation is that spread changes are not a standalone indicator of direction or future returns. Spread widening can be caused by normal liquidity thinning, not by an imminent price trend.
Material failure modes include:
- Thin-book effects: spreads widen sharply when depth is low.