DISCUSSION OF 'AN EFFICIENT LIQUIDITY SAVINGS MECHANISM' BY DARRELL DUFFIE, SRISHT FATEH SINGH AND CHAOJUN WANG
The following information was released by the
I. Introduction
I would like to thank the workshop organizers for the opportunity to discuss this fascinating and innovative paper by Professor
The paper uses economic theory to find a way to process interbank payments efficiently in a real-time gross settlement (RTGS) system. An RTGS system settles each payment fully and finally when the bank sends it, simultaneously debiting the account of the sending bank and crediting the account of the receiving bank. Globally, RTGS systems replaced an earlier generation of deferred net settlement (DNS) systems that exposed banks to more risk. A DNS system accumulates interbank payment orders throughout the day and settles them all at once, on a net basis, at day's end. Much less cash than the gross amount of payments moves each day in a DNS system because a bank's incoming and outgoing payments tend to offset each other. However, if a bank fails during the day, other banks do not receive money they expected, and the shock can spread across the banking system.
RTGS systems require more liquidity than DNS systems and can be more sensitive to shortfalls of aggregate liquidity in the banking system. If a bank has abundant cash on hand, it will be willing to send outgoing payments early in the day, even before receiving incoming payments. But if a bank has relatively little cash, it may delay outgoing payments until more funds arrive. Payment delays can propagate as more banks find themselves low on funds, and the system can experience gridlock (Bech and Garratt, 2003; McAndrews and Kroeger, 2016).
In recent years, payment system operators have sought to combine the risk-reducing benefits of RTGS and the liquidity savings of DNS by matching offsetting payments while settling payments finally throughout the day. It is intuitively obvious how to create an algorithm that saves liquidity while settling payments in close to real time. The algorithm should queue up payment messages and, when the queue contains matching payments in opposite directions, settle both at once. A more sophisticated algorithm could also look for multilateral matching opportunities, such as instances when A sends to B, who sends to C, who sends to A. There are also choices to be made about how to prioritize payments in the queue, whether to search for matches continuously or at defined intervals, and so on. Liquidity savings mechanisms (LSMs) now exist within RTGS systems in many countries, including
But while it is straightforward to construct algorithms that can save some liquidity, it is less obvious what algorithm would save the most liquidity or how payment system operators should choose among candidate algorithms. This is where Duffie and co-authors come in. Instead of inventing an algorithm, they propose a mechanism for processing payments.
II. Mechanism design
When
The word "mechanism" is a term of art. As formulated by
Mechanism designers seek to create mechanisms that will generate optimal resource allocations. Such an exercise has three components. First, designers must examine the constraints on feasible resource allocations. The auctioneer can't give the painting to more than one bidder. Second, designers must consider the participants' incentives, both those the mechanism creates and those in the broader economic environment. Will bidders offer as much for the painting as they are willing to pay, or do they have a reason to shade their bids? What induces them to attend the auction? Third, designers must specify their own objectives. By what yardstick do we judge whether a resource allocation is optimal? Should the auction aim to raise as much money as possible, or to deliver the painting to whoever most enjoys seeing it on the wall?
I applaud Darrell and co-authors for bringing the mechanism design approach to LSM design. Thinking about constraints, incentives and objectives beautifully clarifies what LSMs do, in a way that does not come out when one starts by discussing a specific algorithm for stepping through a queue, processing payments from smallest to largest, or what have you. The paper carefully identifies three mechanisms that are optimal in the sense of both minimizing banks' costs and maximizing payment flow, under different constraints on the use of overdrafts and reserve balances. This work simultaneously advances our theoretical understanding of LSMs and provides practical ideas for improving wholesale payment systems.
The baseline mechanism Darrell and co-authors propose is straightforward. At the start of the day, each bank has a list of payments it needs to make and some money in its account at the central bank. Each bank tells the payment system operator which payments it is willing to make through the LSM and a limit on the net debit the LSM can make to the bank's account. The LSM finds the combination of submitted payments that maximizes the gross total value of payments processed through the LSM. A bank that receives a net inflow from the LSM pays a fee, while a bank that has a net outflow receives a reward. These fees and rewards encourage banks to send money through the LSM. Finally, payments that the LSM did not select or that banks did not submit to it flow through an ordinary RTGS system.
Behind the elegant mechanism is a lot of careful work. The proof of efficiency has to keep track of all the details of how payments flow through a network of many banks, and how making or stopping one payment would influence all the others.
The paper also considers alternatives in which the LSM cannot debit banks' reserves at all or, to the contrary, is allowed to overdraft banks' accounts. The authors view their baseline mechanism as most relevant for practical purposes. However, I found all three mechanisms potentially relevant in practice, depending on the institutional context. A private-sector operator that implements an LSM in a shared account, along the lines of CHIPS, will have limited access to reserve balances and cannot allow overdrafts without reintroducing settlement risk. If the payments operator is distinct from the central bank but does not use a shared account, as in the case of
I will organize my comments in those same three categories relevant to all mechanism designs: constraints, incentives and objectives.
III. Constraints
Relative to real-world payment systems, the paper relaxes one important constraint and tightens two others.
All-or-none payments
It is axiomatic that payments settle in their entirety or not at all. If you are buying a house, you or your mortgage lender must wire the full purchase price to the seller. If you wire half the money, you will create a mess. The seller is not going to give you the deed but has a lot of your money. It is better to delay sending any money until you can send all of it.
However, searching for the optimal combination of complete payments is an integer programming problem, which is computationally extremely difficult. Therefore, the paper allows the system to send partial payments.
There are three ways to think about a mechanism with partial payments. One possibility would be to change real-world systems to allow partial payments. The authors note this would require changes to payment message formats and accounting systems. It might also require changes to contracts between counterparties, so the implementation effort could extend well beyond the payment system. Second, the paper suggests that if each payment represents a collection of sub-payments, a partial payment could come close to satisfying the all-or-none constraint with respect to the sub-payments. But
Time of day
The paper assumes all payments are known to banks and potentially available to process at the same time. In practice, payment needs arise throughout the operating day (for
Budget balance
The paper requires the mechanism's fees and rewards to be budget balanced. This is restrictive for two reasons. First, a payment system needs to cover its capital and operating costs. Second, budget-balanced fees and rewards can address externalities between banks that participate in the payment system but cannot address externalities vis-à-vis the rest of the economy. I'll come back to this when I discuss objectives below.
IV. Incentives
We can think of banks' incentives in two stages. The second stage is the one modeled in most of the paper. A bank begins the day with cash on hand and a list of payments and decides which payments to send to the LSM. The first stage is whatever happened before today that led to the bank having this much cash, needing to make these payments and participating in the LSM.
The incentives in both stages matter to the mechanism's effects and efficiency. I'll take them in reverse order.
Second-stage incentives
In deciding which payments to send to the LSM, the bank balances the fees and rewards in the LSM against the costs of late payments and overdrafts.
If overdrawing the central bank account were costless, banks would have less need to economize on liquidity. In practice, in
In the paper's model, the cost of late payments must be smaller than the LSM's rewards and fees to induce banks to use the LSM optimally. The paper assumes all payments are equally costly to delay. In practice, some payments are much more time-critical than others. Derivatives market participants must make margin payments at precise moments in the day (Marshall and Steigerwald, 2013), while a manufacturer's invoice can typically be paid anytime within 30 days.
Banks know the time-criticality of each payment; the payment system operator may not. If the LSM sets identical rewards and fees for all payments, banks have an incentive to submit only relatively less time-critical payments to the LSM. Selective submissions may prevent the LSM from generating an efficient allocation. The standard tool for solving this private-information problem in mechanism design is the revelation principle (Myerson, 1981): Ask banks to report the time-criticality of each payment, use this information to decide which payments to process and create incentives for banks to make truthful reports. It would be valuable for the authors to extend their analysis to private information about time-criticality.
Table 1:
System Operator Participants Average daily volume Average daily transactions Average transaction amount
CHIPS The
Fedwire
In
Table 2 shows that CHIPS' efficiency is more than double the average efficiency of the most efficient international systems with LSMs studied by Kabadjova et al. (2023). The combined efficiency of the
In effect, then, CHIPS functions as an LSM for the
Table 2: Efficiency of selected payment systems
Jurisdiction System Has LSM? Efficiency
First-stage incentives
Banks' first-stage choices include whether to participate in the LSM, whether to try to influence the mix of payments that will need to be sent and received on future days, and how much cash to hold overnight and begin the next day with.
The authors analyze the first of those first-stage choices. They show that, for the version of the LSM that can overdraw banks' accounts, each bank finds it rational to join, assuming all others do so as well.
Banks' business models and customers are not static. Banks can choose to enter, exit, scale up or scale down lines of business depending on costs and profits from those activities. One relevant cost is for the intraday liquidity a business requires. When the only available payment system uses RTGS, all intraday liquidity needs are equally costly. Once an LSM is available, intraday liquidity needs that can offset inside the LSM are less expensive than intraday liquidity needs that will not offset. On the margin, then, an LSM could give banks an incentive to pursue activities in which payment flows are more nettable and avoid those in which payment flows are less nettable. For example, businesses that generate large, unpredictable and time-sensitive payment flows, such as derivatives and securities clearing, might become relatively less attractive in the presence of an LSM. Whether this incentive is efficient for the economy depends on the social costs of intraday liquidity. If intraday liquidity is socially costly, encouraging banks to use it in nettable ways is desirable. However, if society can provide intraday liquidity at low cost, deterring the use of intraday liquidity for non-nettable activities could inefficiently distort banks' business models.
The presence of the LSM also reduces the marginal value of cash on hand at the start of the day. Banks could respond by reducing their start-of-day cash balances. Reducing banks' desired cash balances might be one objective of an LSM, a point I'll return to in a moment, but this incentive also affects how well the LSM might achieve other objectives.
V. Objectives
The authors take as their objective to minimize the cost of overdrafts and late payments in the banking system. It turns out the way to do this, in the baseline mechanism, is to maximize the value of payments the LSM can process given banks' start-of-day central bank balances. Equivalently, since every optimization problem has a dual, the mechanism minimizes the aggregate bank reserves needed to process a given volume of payments.
The quest to reduce the payments demand for bank reserves originates in recent discussions of options and tradeoffs associated with reducing the size of
I would make three points here. One, given the other drivers of reserve demand and the presence of CHIPS, it is uncertain how much adding an LSM to Fedwire can reduce reserve demand. Second, reducing the steady-state demand for reserves is not the same as making the system more resilient. Third, as discussed by Logan (2026),
Potential effect of a Fedwire LSM on reserve demand
Banks hold reserves not only to make payments under ordinary circumstances but also to manage the risk of unexpected outflows, to meet regulatory and supervisory requirements (which relate partly to the risk of unexpected outflows), and potentially as investments (Logan and Schulhofer-Wohl, 2026). An LSM can reduce business-as-usual (BAU) reserve needs. It is less likely to reduce reserve needs in stress scenarios, because stress scenarios may involve one-way outflows, like a run on the bank, that a netting mechanism cannot offset.
Banks do not publicly report how much reserve demand comes from BAU needs versus liquidity risk management. However, in the
As Professor
Another conceivable use of the authors' mechanism in the
Resilience and elasticity
There are occasional days when payment flows surge or bank reserve balances dip, such as in connection with tax payment deadlines and
However, the starting level of reserves is not exogenous. With an LSM in place, the marginal value of start-of-day reserves is lower, incentivizing banks to hold fewer reserves. This is exactly the means by which an LSM can help reduce the central bank's balance sheet. But it implies the system would begin each day closer to the level where stress would emerge. In equilibrium, then, an LSM may or may not make the system more elastic. It would be useful to extend the paper's model to analyze this effect.
Public benefits and costs
The paper defines efficiency as minimizing banks' total costs for overdrafts and late payments. On its face, this definition seeks an LSM that is efficient for banks.
When is the LSM also efficient for the economy as a whole? I'll highlight three considerations: whether banks' costs for overdrafts and late payments match the costs of banks' counterparties; whether there are other externalities between the banking system and the rest of the economy; and whether it is costly to implement the LSM.
At minimum, for an LSM that minimizes banks' late payment and overdraft costs to be efficient economywide, the cost of late payments to banks must equal the cost of late payments to banks' customers, and the cost of overdrafts to banks must equal the central bank's cost of providing intraday credit. The condition on late payment costs appears reasonable. The condition on overdraft costs is more challenging. As discussed above, some banks reportedly treat collateralized intraday credit as costly, even though
The paper's LSM is efficient for banks because its rewards and fees offset the externalities that banks create for each other when they send or delay payments. To be efficient for the economy, the LSM would also need to offset any externalities the banking system creates for the rest of the economy in processing payments. These externalities could be positive or negative: Faster payments might foster innovation and benefit consumers and businesses but could also facilitate fraud (
Lastly, adding an LSM to Fedwire would have costs. Developers would need to produce, test and maintain the computer code, both at
Depending on the magnitude of implementation costs and the perceived benefits of reducing liquidity needs, it might not be efficient, in this broader sense, to wring out every dollar of payments from the available supply of reserves. A simpler mechanism might be easier to deploy robustly and deliver "good enough" payments efficiency. In that case, the mechanism in this paper would represent a benchmark for how many payments can be processed if processing payments is the only object, but other mechanisms might remain attractive in practice.
In my view, the implications for small banks are particularly relevant in analyzing these trade-offs for
The paper suggests that "an LSM would likely only be used by a subset of large
Table 3 shows that
Table 3: Number of depository institutions in selected economies
Jurisdiction Depository institutions Depository institutions per million population Depository institutions per
The benefits and costs of an LSM may vary with bank size. Small banks may have fewer resources to devote to analyzing payment choices and may be more sensitive to the risk of paying large fees to an LSM. On the other hand, small banks do not face the same liquidity regulations as large banks and might have more flexibility to reduce reserve holdings if the payment system were more efficient. It would be valuable to investigate how adding an LSM to Fedwire would affect payment system access and costs as a function of bank size. Would an LSM in Fedwire support a level playing field for banks of all sizes (Logan, 2024), or would it tilt the playing field one way or another? If there is a tilt, would allowing banks to opt out of the LSM solve the problem, or is a different remedy needed?
What's so valuable about the authors' research strategy is that it opens up a clear avenue for addressing questions of this type. Tell Darrell and his colleagues the constraints, incentives and objectives, and they can work out the optimal mechanism. Without mechanism design, it would be much more difficult to know what each LSM algorithm achieves and what challenges it may create.
VI. Conclusion
I applaud the authors for bringing an innovative and clarifying theoretical approach to the design of LSMs. The paper provides an elegant benchmark for efficient LSM algorithms and demonstrates the power of mechanism design in this space. If I have raised some questions about the mechanism, it is mainly because the clarity of the authors' approach makes it possible to ask better questions in the first place. The authors' strategy opens up possibilities for using mechanism design to create payment systems that address different constraints, incentives and objectives. I look forward to reading the literature that I hope this paper launches.
Notes
I thank
References
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