Can SSL reduce bias in machine learning models?

SSL (semi-supervised learning) can help reduce bias in machine learning models, but its effectiveness depends on how it’s applied and the quality of the data used. Bias often arises when models overfit to limited or unrepresentative labeled datasets. SSL addresses this by leveraging both labeled and unlabeled data, which can broaden the model’s exposure to diverse patterns. For example, if a labeled dataset for image classification contains mostly images of cats in indoor settings, adding unlabeled data with outdoor scenes or varied lighting conditions can help the model generalize better, reducing bias toward specific environments. However, SSL isn’t a guaranteed fix—if the unlabeled data itself is biased, the model might inherit those flaws.

One practical way SSL mitigates bias is through techniques like pseudo-labeling or consistency regularization. In pseudo-labeling, the model generates labels for unlabeled data, then retrains on those predictions. If the unlabeled data includes underrepresented groups (e.g., rare medical conditions in healthcare datasets), the model may learn to recognize them more accurately. Consistency regularization, which enforces stable predictions across slightly modified inputs (e.g., rotated images), can also reduce reliance on spurious correlations. For instance, a model trained to detect tumors might initially associate specific scanner artifacts with disease. By applying SSL with unlabeled data from diverse scanners, the model learns to focus on actual tumor features instead of equipment noise.

However, SSL’s success in reducing bias hinges on careful implementation. If the unlabeled data mirrors the biases in the labeled data—say, both primarily include young adults in a age-prediction task—SSL won’t resolve the issue. Developers must audit unlabeled data for diversity and actively seek representative samples. Additionally, SSL methods like self-training can amplify errors if initial biased predictions are treated as ground truth. To avoid this, techniques like confidence thresholding (only using high-confidence pseudo-labels) or combining SSL with fairness-aware loss functions can help. For example, in a hiring tool trained on biased historical data, supplementing with unlabeled resumes from underrepresented groups and penalizing demographic disparities during training could yield a fairer model. In short, SSL is a tool, not a solution—its impact depends on deliberate design choices.