A Ridge Too Far: Correcting Over-Shrinkage via Negative Regularization

arXiv stat.ML / 4/21/2026

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Key Points

  • Conventional (positive) regularization is meant to reduce variance, but for small-data regression it can worsen underfitting when the useful predictive signal lies in weak directions of a restricted representation.
  • The paper analyzes a “negative-capable” ridge regression family that allows a feasible negative regularization region while keeping the estimator well-posed.
  • Within that negative region, negative regularization functions as controlled anti-shrinkage by increasing effective model complexity most strongly along weak eigen-directions.
  • The authors formalize weak-spectrum underfitting, prove a sign-switch phenomenon under conservative baseline shrinkage, and propose a criterion-based method to automatically select regularization across the full negative-capable family.
  • Experiments on synthetic and semi-synthetic data validate key theoretical claims, including feasibility of the negative region, spectral complexity growth, sign-switch behavior, and recovery of negative adjustments when appropriate.

Abstract

Conventional regularization is designed to control variance, but in small-data regression it can also aggravate underfitting when predictive signal is concentrated in weak directions of a restricted representation. We study a negative-capable ridge family that permits a feasible negative region whenever the estimator remains well posed, and show that negative regularization acts there as controlled anti-shrinkage by increasing effective complexity most strongly along weak eigendirections. Building on this mechanism, we formalize weak-spectrum underfitting, derive a sign-switch result under conservative baseline shrinkage, and study criterion-based automatic selection over the full negative-capable family. Synthetic and semi-synthetic experiments support the theory by verifying feasibility, spectral complexity increase, sign-switch behavior, and effective recovery of negative adjustments in the predicted regimes.