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Title: Adversarial Training Can Hurt Generalization

Paper content:
While adversarial training can improve robust accuracy (against an adversary), it sometimes hurts standard accuracy (when there is no adversary).
Previous work has studied this tradeoff between standard and robust accuracy, but only in the setting where no predictor performs well on both objectives in the infinite data limit.
In this paper, we show that even when the optimal predictor with infinite data performs well on both objectives, a tradeoff can still manifest itself with finite data.
Furthermore, since our construction is based on a convex learning problem, we rule out optimization concerns, thus laying bare a fundamental tension between robustness and generalization.
Finally, we show that robust self-training mostly eliminates this tradeoff by leveraging unlabeled data.
Neural networks trained using standard training have very low accuracies on perturbed inputs commonly referred to as adversarial examples BID11 .
Even though adversarial training BID3 BID5 can be effective at improving the accuracy on such examples (robust accuracy), these modified training methods decrease accuracy on natural unperturbed inputs (standard accuracy) BID5 BID18 .
Table 1 shows the discrepancy between standard and adversarial training on CIFAR-10.
While adversarial training improves robust accuracy from 3.5% to 45.8%, standard accuracy drops from 95.2% to 87.3%.One
explanation for a tradeoff is that the standard and robust objectives are fundamentally at conflict. Along
these lines, Tsipras et al. BID13 and Zhang et al. BID18 construct learning problems where the perturbations can change the output of the Bayes estimator. Thus
no predictor can achieve both optimal standard accuracy and robust accuracy even in the infinite data limit. However
, we typically consider perturbations (such as imperceptible ∞ perturbations) which do not change the output of the Bayes estimator, so that a predictor with both optimal standard and high robust accuracy exists.Another explanation could be that the hypothesis class is not rich enough to contain predictors that have optimal standard and high robust accuracy, even if they exist BID8 . However
, Table 1 shows that adversarial training achieves 100% standard and robust accuracy on the training set, suggesting that the hypothesis class is expressive enough in practice.Having ruled out a conflict in the objectives and expressivity issues, Table 1 suggests that the tradeoff stems from the worse generalization of adversarial training either due to (i) the
statistical properties of the robust objective or (ii) the
dynamics of optimizing the robust objective on neural networks.In an attempt to disentangle optimization and statistics, we ask does the tradeoff indeed disappear if we rule out optimization issues? After all
, from a statistical perspective, the robust objective adds information (constraints on the outputs of perturbations) which should intuitively aid generalization, similar to Lasso regression which enforces sparsity BID12 .Contributions
. We answer the
above question negatively by constructing a learning problem with a convex loss where adversarial training hurts generalization even when the optimal predictor has both optimal standard and robust accuracy. Convexity rules
out optimization issues, revealing a fundamental statistical explanation for why adversarial training requires more samples to obtain high standard accuracy. Furthermore, we
show that we can eliminate the tradeoff in our constructed problem using the recently-proposed robust self-training BID14 BID0 BID7 BID17 on additional unlabeled data.In an attempt to understand how predictive this example is of practice, we subsample CIFAR-10 and visualize trends in the performance of standard and adversarially trained models with varying training sample sizes. We observe that
the gap between the accuracies of standard and adversarial training decreases with larger sample size, mirroring the trends observed in our constructed problem. Recent results
from BID0 show that, similarly to our constructed setting, robust self-training also helps to mitigate the trade-off in CIFAR-10.Standard vs. robust generalization. Recent work BID10
BID15 BID4 BID6 has focused on the sample complexity of learning a predictor that has high robust accuracy (robust generalization), a different objective. In contrast, we study
the finite sample behavior of adversarially trained predictors on the standard learning objective (standard generalization), and show that adversarial training as a particular training procedure could require more samples to attain high standard accuracy.
In this work, we shed some light on the counter-intuitive phenomenon where enforcing invariance respected by the optimal function could actually degrade performance.
Being invariant could require complex predictors and consequently more samples to generalize well.
Our experiments support that the tradeoff between robustness and accuracy observed in practice is indeed due to insufficient samples and additional unlabeled data is sufficient to mitigate this tradeoff.

assistant: text: Even if there is no tradeoff in the infinite data limit, adversarial training can have worse standard accuracy even in a convex problem.