Robust Risk-Aware Reinforcement Learning

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Robust Risk-Aware Reinforcement Learning

Sebastian Jaimungal
Silvana Pesenti
Ye Sheng Wang
Hariom Tatsat
Published on 8/23/2021
Equities
Derivatives
Reinforcement learning
Risk management
Machine learning

This paper presents a novel reinforcement learning (RL) approach for robust risk-aware optimization, where agents aim to minimize rank dependent expected utility (RDEU) under model uncertainty. The RDEU framework allows agents to incorporate both utility concavity and probability distortion, capturing a wide range of risk preferences. To robustify against distributional uncertainty, the policy's performance is evaluated under the worst-case distribution within a Wasserstein ball around the induced outcome distribution. This leads to a min-max formulation: the agent chooses a policy (outer problem) while an adversary selects a distribution (inner problem) to worsen performance. The authors derive explicit policy gradient formulae for both problems, enabling efficient gradient-based optimization. The inner gradient formula accounts for the RDEU risk measure and the Wasserstein constraint, while the outer gradient incorporates the dependence of the adversarial distribution on the agent's policy. Practical gradient estimation is achieved using kernel density estimators and mini-batch sampling. The framework is demonstrated on three financial applications: robust portfolio allocation, where the agent seeks a portfolio that minimizes worst-case RDEU; benchmark optimization, where a dynamic trading strategy aims to outperform a benchmark under uncertainty; and statistical arbitrage, where a robust trading strategy mitigates market impact and model risk. The results show that the proposed RL method effectively balances risk and robustness, providing a general tool for decision-making under uncertainty in finance and beyond.

Highlights

  • 1Develops a reinforcement learning framework for robust optimization of risk-aware performance criteria using rank dependent expected utility (RDEU).
  • 2Introduces distributional robustness by considering the worst-case distribution within a Wasserstein ball around the policy's outcome distribution.
  • 3Provides explicit policy gradient formulae for both the inner (adversarial) and outer (agent) optimization problems.
  • 4Demonstrates the framework's efficacy on three financial problems: robust portfolio allocation, benchmark optimization, and statistical arbitrage.

Methods

  • M
    Rank Dependent Expected Utility (RDEU) for risk assessment, incorporating utility and probability distortion.
  • M
    Wasserstein distance to define distributional uncertainty sets for robustification.
  • M
    Augmented Lagrangian method to handle constraints in the inner and outer optimization problems.
  • M
    Kernel density estimation (KDE) for gradient estimation of distribution functions in policy gradient updates.

Results

  • R
    Derived policy gradient formulae for the inner problem (adversarial) and outer problem (agent) under RDEU and Wasserstein constraints.
  • R
    The framework allows for both deterministic and randomized policies, with explicit gradient estimation using mini-batch samples.
  • R
    Illustrated on robust portfolio allocation, showing the agent can protect against model uncertainty while optimizing risk-reward profiles.
  • R
    Applied to benchmark optimization, enabling dynamic strategies that outperform a benchmark under worst-case distributional shifts.
  • R
    Demonstrated on statistical arbitrage, where the robust strategy mitigates adverse market impact and model misspecification.
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