Asymmetric REINFORCE (AsymRE)

• Asymmetric REINFORCE (AsymRE) comprises reinforcement learning algorithms that selectively weight positive and negative learning signals to improve stability and sample efficiency, particularly in off-policy settings.
• A key principle of AsymRE is setting the learning baseline below the mean reward to focus updates on successful outcomes and prevent instability from off-distribution negative samples.
• These methods are effectively applied in areas like large language model fine-tuning and molecular optimization, offering robust learning from diverse or sparse data.

Asymmetric REINFORCE (AsymRE) refers to a family of reinforcement learning algorithms and estimators that intentionally break the symmetry between how different samples or types of information contribute to the learning update—most commonly by giving special treatment to positive versus negative rewards, or by exploiting privileged information or off-policy data in an unbalanced (asymmetric) fashion. The rationale is to improve the stability, efficiency, and sample effectiveness of policy gradient methods in both classical RL and contemporary LLM fine-tuning, especially under off-policy, partially observed, or reward-sparse settings.

1. Foundations and Motivation

The origins of Asymmetric REINFORCE lie in limitations of classical policy gradient approaches, such as high variance, sensitivity to the form of rewards, or failures under off-policy sampling. In the standard (symmetric) REINFORCE setting, the expected policy gradient is computed as

$$\nabla_\theta J(\theta) = \mathbb{E}_{\tau \sim \pi_\theta}\left[\sum_t \nabla_\theta \log \pi_\theta(a_t|s_t) (r(\tau) - b)\right]$$

where $b$ is a baseline, traditionally used only for variance reduction and not affecting the expectation of the gradient on-policy. However, off-policy variants—where data are sampled from a behavioral policy $\mu \neq \pi$—introduce bias if $b$ is not chosen carefully. This effect becomes critical when negative rewards are due to errors outside the region of interest, such as samples never encountered under the intended or improved policy.

AsymRE stems from the insight that, in many practical scenarios (especially in RL for LLMs or molecular generation), overemphasizing negative samples generated from mismatched (off-policy) or privileged data can introduce catastrophic instability, premature collapse, or suboptimal local minima. The asymmetric method proposes to:

• Preferentially upweight positive examples.
• Downweight, truncate, or even exclude negative examples where their impact is not trustworthy under distributional shift.
• Asymmetrize the baseline, reward structure, or importance sampling such that learning is directed where it is most informative.

2. Algorithmic Forms and Theoretical Structure

The canonical formulation of AsymREINFORCE in the off-policy regime is given by

$$\nabla_\theta J(\pi) = \mathbb{E}_{y \sim \mu}\left[\nabla_\theta \log \pi_\theta(y) \, (r(y) - V)\right]$$

where $V$ is a scalar baseline. Off-policy, the limiting distribution and learning dynamics are critically dependent on $V$—unlike in the on-policy case.

Key theoretical findings include:

• Policy improvement guarantee: If the baseline $V$ satisfies $V < V^\mu = \mathbb{E}_{y \sim \mu} r(y)$, the expected reward under repeated AsymRE policy improvement is nondecreasing and converges to the optimal reward (Arnal et al., 25 Jun 2025).
• Phase transition: Setting $V \geq V^\mu$ causes support collapse, driving the policy to determinism on a small set of samples and potentially stalling further improvement.
• The algorithm is thus "asymmetric" in reward: lowering $V$ lessens the penalty on failures and focuses updates on successes (positive $A = r(y) - V$); raising $V$ shifts emphasis toward penalizing failures.

Variants of AsymRE include:

• Tapered off-policy REINFORCE (TOPR), which applies importsance sampling ratios asymmetrically: positive rewards are updated as in SFT (no downweighting), while negative rewards are downweighted (clipped IS) to avoid instability (Roux et al., 18 Mar 2025).
• Asymmetrized baseline methods where the baseline itself is anti-symmetric or optimally constructed to minimize variance or bias, as in the ARM estimator (Yin et al., 2018).

3. Balancing Positive and Negative Rewards

AsymRE algorithms implement the principle that in off-policy or mismatched distribution settings, positive and negative feedback do not carry equal information. Specifically:

• Data from negative reward samples, particularly those generated from an outdated policy $\mu$ far from $\pi$, may not generalize to the improved policy; over-penalizing failures from distant distributions leads to collapsed, over-deterministic solutions and prohibits further learning.
• The AsymRE update with $V < V^\mu$ emphasises positive samples (learn from successes); with $V > V^\mu$, learning becomes dominated by suppressing (possibly spurious) failures (Arnal et al., 25 Jun 2025).
• This finding is consistent with empirical results in LLM RLHF and molecular optimization, where data efficiency and robustness are maximized by focusing on positives and limiting the negative signal (Thomas et al., 27 Jan 2025, Roux et al., 18 Mar 2025).

A summary of the learning regime:

Setting	Baseline $V$ ($A = r - V$)	Learning Dynamics	Outcome
On-policy	Any	Only variance reduction; unbiased learning	Standard REINFORCE behavior
Off-policy	$V < V^\mu$	Positive-weighted updates, monotonic improvement	Efficient/robust learning
Off-policy	$V \geq V^\mu$	Support collapse, determinism, no further learning	Sudden collapse, instability

4. Theoretical Guarantees and Empirical Evidence

Theoretical results establish that, for AsymRE with tabular/softmax policies:

• Limiting distribution $\pi^*_{\mu,V}$ converges to a function of both $\mu$ and $V$, often shrinking support ("focus") as $V$ approaches $V^\mu$ from below.
• When $V < V^\mu$, repeated policy iteration with AsymRE leads to monotonic reward increase and convergence to optimal arms (for finite settings) (Arnal et al., 25 Jun 2025).
• For LLMs (Llama-3.1-8B, Qwen2.5-3B), off-policy AsymRE learns efficiently as long as $V$ is set slightly below the empirical mean reward (Arnal et al., 25 Jun 2025).

Empirically:

• In stochastic bandits, increasing $V$ toward $V^\mu$ increases focus on best arms, but exceeding $V^\mu$ collapses policy support (suboptimal behavior).
• In LLM RLHF, using a conservative baseline (slightly under mean reward) avoids entropy collapse, maintains diversity, and drives monotonic improvement in both training and test accuracy without the need for KL or additional regularization (Arnal et al., 25 Jun 2025, Roux et al., 18 Mar 2025).

5. Applications and Related Methodologies

AsymRE and related methods are widely adopted in both classic RL and LLM alignment:

• Policy gradient RL with experience replay, hill-climbing, or replay heuristics all introduce forms of off-policy asymmetry. These extensions benefit from setting the learning update to emphasize positive reward regions and regularize against negative or off-distribution samples (Thomas et al., 27 Jan 2025).
• Tapered REINFORCE (TOPR) formalizes this, using truncated importance sampling for negatives and standard weighting for positives, ensuring stable KL divergence and efficient use of both successful and unsuccessful samples (Roux et al., 18 Mar 2025).
• Minimalist approaches such as Reinforce-Rej and RAFT filter or exclude uninformative (all-negative or all-positive) prompts, focusing learning where reward variance is most informative (Xiong et al., 15 Apr 2025).
• The choice and scheduling of the baseline parameter not only reduces variance, but in off-policy or asymmetric regimes directly determines learning focus and regularization strength (Roux et al., 18 Mar 2025, Arnal et al., 25 Jun 2025).

Variants exploiting privileged information—where the critic (or baseline) uses state or information not available to the actor at deployment—are also captured in the AsymRE framework and enjoy both theoretical and empirical convergence guarantees under partial observability (Lambrechts et al., 31 Jan 2025, Baisero et al., 2021, Warrington et al., 2020).

6. Implications, Best Practices, and Limitations

AsymRE algorithms and their baselines enable efficient, robust, and scalable policy-gradient learning across RL domains:

• They offer a principled approach to avoid instabilities associated with naive off-policy gradient methods.
• By properly tuning the asymmetry (baseline $V$), practitioners can safely exploit off-policy data, avoid catastrophic collapse, and maximize the value of recorded successes without being misled by irrelevant failures.
• The main caveat is that excessive asymmetry (baseline too close to or above mean reward) results in collapsed diversity and irreversible support shrinkage.
• Setting the baseline is critical and data-dependent; context-corrected, rolling, or prompt-wise estimates are often used in LLM fine-tuning (Arnal et al., 25 Jun 2025).
• For environments with partial observability, discrete state/action, or limited replay, unbiased asymmetric formulations that use both state and history when available are necessary for theoretical validity (Baisero et al., 2021, Lambrechts et al., 31 Jan 2025).

7. Summary Table of AsymRE Algorithmic Variants

Variant	Reward Treatment	Baseline Role	Update Focus	Regime	Reference
Classic REINFORCE	Symmetric	Variance reduction only	All rewards	On-policy	[Williams, 1992]
Off-policy REINFORCE	Symmetric	Biased if $V\neq V^\mu$	All rewards	Off-policy	(Arnal et al., 25 Jun 2025)
AsymRE (editor term)	Asymmetric ($V<V^\mu$)	Learning focus, reg	Positives	Off-policy	(Arnal et al., 25 Jun 2025)
TOPR	Asymmetric IS (tapered)	Regulates negative weight	Pos > Negs	Off-policy	(Roux et al., 18 Mar 2025)
RAFT, Reinforce-Rej	Positive Only/Mixed	Data filtering	Informativeness	Mixed	(Xiong et al., 15 Apr 2025)
Privileged Critic	Asymmetric info	Leverages true state info	Reduces aliasing	POMDP/Offline	(Baisero et al., 2021)

• "Asymmetric REINFORCE for off-Policy Reinforcement Learning: Balancing positive and negative rewards" (Arnal et al., 25 Jun 2025)
• "Tapered Off-Policy REINFORCE: Stable and efficient reinforcement learning for LLMs" (Roux et al., 18 Mar 2025)
• "REINFORCE-ING Chemical LLMs in Drug Design" (Thomas et al., 27 Jan 2025)
• "A Theoretical Justification for Asymmetric Actor-Critic Algorithms" (Lambrechts et al., 31 Jan 2025)
• "Unbiased Asymmetric Reinforcement Learning under Partial Observability" (Baisero et al., 2021)
• "A Minimalist Approach to LLM Reasoning: from Rejection Sampling to Reinforce" (Xiong et al., 15 Apr 2025)

Asymmetric REINFORCE thus unifies a set of empirical practices and theoretical insights underpinning stable and effective RL, especially in off-policy, partially-observed, or reward-sparse settings, with immediate applications in RLHF for LLMs, sequence generation, goal-conditioned RL, and imitation learning. The central operative principle is the selective, principled weighting of learning signals to maximize policy improvement while preserving stability and robustness.