Paired Switching — SPY/TLT (PAIRED)
Developed by Lewis Glenn · Pairs Rotation · Med Risk
Paired Switching was developed by Lewis Glenn and published as SSRN paper #2437049, introducing one of the simplest and most elegant concepts in tactical allocation: a continuous binary rotation between two complementary assets based solely on which one has shown stronger recent momentum. The strategy holds either US equities (SPY) or long-term Treasuries (TLT) — never both, never cash — creating a portfolio that is always fully invested but continuously pivoting between the two major asset classes that tend to move in opposite directions during market stress.
The theoretical foundation rests on the historically negative correlation between stocks and long-term bonds during periods of market stress. When equity markets decline due to economic weakness or risk aversion, investors typically flee to the safety of government bonds, driving bond prices up. Conversely, when economic optimism drives equity markets higher, capital tends to flow out of bonds. This flight-to-quality dynamic means that at most points in time, one of these two asset classes is performing well — and a strategy that holds whichever one is currently stronger captures value from both sides of this rotation.
The strategy uses a single signal: trailing three-month relative strength. Each month, SPY and TLT are compared by their recent returns, and the portfolio holds the winner. There is no protection mechanism, no trend filter, and no defensive cash position. The strategy is always fully invested in one of two assets. This radical simplicity produces a strategy with very specific characteristics: it captures the stock-bond rotation cycle effectively during normal markets but can suffer during the relatively rare periods when both stocks and bonds decline simultaneously — as occurred during the 2022 rate shock when aggressive monetary tightening crushed both equity and bond valuations.
Glenn's research demonstrated that this simple rotation produces surprisingly robust risk-adjusted returns over long historical periods, despite holding no more than two assets and using no sophisticated signal processing. The paired switching concept has since been extended to multi-pair strategies and incorporated as a building block within more complex portfolio systems.
How It Works
Relative Strength Comparison
Each month, SPY and TLT are compared by their trailing three-month total returns. The asset with the higher return — the one demonstrating stronger recent momentum — becomes the sole portfolio holding for the coming month. This three-month lookback balances responsiveness to trend changes against stability of the signal, producing moderate turnover as the stock-bond leadership alternates.
The relative comparison is pure and unconditional — there is no minimum return threshold, no trend filter, and no cash alternative. The strategy always holds whichever asset is "less bad" even during periods when both are declining. This design reflects a specific belief about the stock-bond relationship: that at virtually all points in time, one of these two assets offers acceptable risk-adjusted prospects, and that the momentum signal reliably identifies which one.
Continuous Rotation Without Defense
The absence of a defensive mechanism is a deliberate design choice. By eliminating cash as an option, the strategy avoids the whipsaw costs that defensive triggers introduce when they generate false signals. The portfolio is always earning the return of one of two major asset classes — never sitting in cash earning near-zero returns while waiting for a signal to clear.
This approach works well in the traditional stock-bond regime where equities and bonds are negatively correlated. During equity drawdowns, TLT typically appreciates due to flight-to-quality flows, and the momentum signal rotates the portfolio from declining equities into appreciating bonds. The strategy effectively captures the protective value of bonds without requiring a separate crash detection mechanism. The risk emerges during regime breaks — specifically, periods when both stocks and bonds decline together, leaving the strategy with no safe harbor in its two-asset universe.
Signal Characteristics and Use Cases
The three-month lookback produces a signal that changes direction more frequently than twelve-month momentum strategies but less frequently than single-month signals. In a typical year, the strategy makes between four and eight transitions between SPY and TLT, generating moderate turnover that is manageable in most account types.
Paired Switching serves multiple roles in portfolio construction. As a standalone strategy, it provides a simple, low-maintenance approach to capturing the equity-bond rotation with binary conviction. As a building block within multi-strategy portfolios, its specific signal profile — always invested, rotating between two negatively correlated assets — provides a return stream that is structurally different from cash-switching momentum strategies. When combined with strategies that use canary gates or breadth-based protection, the paired switcher contributes returns during the defensive periods when other strategies are in cash, improving the overall portfolio's capital efficiency.
Explore Paired Switching — SPY/TLT (PAIRED)
See the full backtest across 18 years of market data, or run your own what-if scenarios by adjusting all research parameters.