Inverse Volatility Weighting: Equalizing Risk Contribution Across Positions

Inverse volatility weighting allocates capital to each asset in proportion to the inverse of its recent realized volatility. Less volatile assets receive larger allocations; more volatile assets receive smaller allocations. The result is a portfolio where each position contributes approximately the same amount of risk — a property known as risk equalization that forms the foundation of risk parity investing.

How It Works

Each month, the trailing realized volatility of each asset is calculated from daily returns over a recent window (typically 60-126 trading days). Portfolio weights are then assigned as: w_i = (1/σ_i) / Σ(1/σ_j), where σ is the asset's volatility. This formula ensures weights sum to one while being inversely proportional to volatility.

In practice, this means bonds (volatility ~5%) might receive three to four times the weight of equities (volatility ~15-20%), and commodities (~20%) might receive a similar weight to equities. The heavy bond allocation is not a conviction bet on bonds — it is a mathematical consequence of risk equalization.

Why Equalize Risk

In an equal-weight portfolio of stocks and bonds, the equity position contributes approximately 85-90% of total portfolio risk because equities are roughly three to four times more volatile than bonds. This means the "diversified" portfolio's behavior is almost entirely determined by its equity component — the bond allocation provides income but negligible risk reduction.

Inverse volatility weighting addresses this imbalance. By allocating more to bonds and less to equities, each position contributes comparable risk, creating a portfolio whose overall behavior reflects all its constituents rather than being dominated by the most volatile one. During equity bear markets, the larger bond allocation provides meaningful offset rather than token diversification.

Strategies That Use Inverse Volatility

• Risk Parity Trend — US (RP-US) — inverse volatility with per-asset trend filters
• Risk Parity Trend — Global (RP-GLOBAL) — same approach with global equity exposure

The All Weather Portfolio uses a conceptually similar risk-based allocation, though its weights are set by Bridgewater's proprietary methodology rather than simple inverse volatility.

Limitations

Inverse volatility weighting assumes that recent realized volatility is a reasonable proxy for future volatility — an assumption that generally holds but can break down during regime changes when volatility structures shift rapidly. The heavy bond allocation also means the portfolio is sensitive to interest rate changes, potentially producing losses during sustained rate-hiking cycles when bonds decline.