REVIEW 3 major objections 4 minor 40 references
TowerDebias: A Novel Unfairness Removal Method Based on the Tower Property
T0 review · 3 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read The paper claims that averaging a black-box model's predictions over a sensitive attribute never increases that attribute's correlation with the predictions.
desk verdict The paper's central guarantee is false: the proof of Eq. (1) uses an incorrect projection inequality and a false identity, and a simple Gaussian example breaks the claimed result. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The Tower Property, $E(Y|X) = E[E(Y|X,S)|X]$, which says that the $S$-free regression function is the $S$-averaged $S$-conditional regression function. The argument works in the $L^2$ space of mean-zero random variables with inner product $\langle U,V \rangle = E(UV)$, so variance is squared norm and conditional expectation is orthogonal projection. For finite data, tDB approximates the inner expectation with $k$-nearest-neighbour averaging: average the black-box predictions over the $k$ training cases whose $X$ values are closest to the new $X$.
What would settle it
A direct check is to construct a synthetic Gaussian model with a known conditional mean (for instance $Y = X + S$ with $X$ and $S$ dependent), compute both sides of inequality (1), and see whether the left-hand correlation ever exceeds the right-hand one; if it does, the claimed guarantee fails.
Extended reading notes
Core claim
The paper's central claim is inequality (1), $\rho(E(Y|X),S) \le \rho(E(Y|X,S),S)$: replacing a black-box model's predictions with their $S$-averaged conditional expectation never increases the Pearson correlation between predictions and the sensitive attribute. The proof treats conditional expectations as vectors in an $L^2$ space of mean-zero random variables, applies the Tower Property to equate the two numerator terms, and compares denominators by the norms of the projections. The paper therefore claims a universal, model-independent fairness-improvement guarantee for tDB at the population level, with only the $k$-NN averaging step introducing finite-sample approximation error.
Load-bearing premise
The central guarantee depends on two identities about the original model's outputs: the $S$-free averaged predictions must have no smaller variance than the original predictions, and the average of $S$ times the averaged prediction must equal the average of $S$ times the original prediction.
Editorial extensions
If this is right
- Any black-box model — linear, tree, random forest, or neural network — can have $S$ influence reduced by averaging its outputs over $S$, with no retraining and no access to weights or training data.
- Correlation-based fairness metrics improve with tDB, so fairness gains can compound when tDB is applied after existing fair-training methods such as FRRM/FGRRM.
- Because tDB produces the best $S$-free predictor under squared loss, any accuracy loss beyond the gap between $E(Y|X,S)$ and $E(Y|X)$ is a bias-variance artifact of the $k$-NN approximation, controlled by $k$.
- In classification, applying tDB to predicted probabilities $P(Y=1|X,S)$ yields a debiased probability estimate with the claimed fairness guarantee, not just a relabeled decision.
Reading between the lines
- Editorial: the same averaging operation could be applied to any prediction that is a conditional expectation, such as quantile or risk estimates, because the Tower Property does not depend on how the black-box model was trained.
- Editorial: a direct synthetic-data test with non-Gaussian dependence would show whether the inequality holds beyond the Gaussian setting the paper uses for its closed-form correlation reduction.
- Editorial: if the variance and cross-moment identities in the proof hold only under special conditions, the universal guarantee would reduce to a conditional one, so checking those identities empirically on real model outputs is a natural next step.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes towerDebias (tDB), a post-processing fairness method for black-box models. The method estimates E(Y|X) by averaging a black-box model's predictions over the sensitive attribute S, using a k-nearest-neighbor approximation of the Tower Property. The authors claim a formal fairness-improvement theorem, Eq. (1) in Section 4.4, stating that the correlation between predictions and S never increases after applying tDB. They support this with a proof based on L2 projections and conditional expectations, and they report empirical results on five datasets for regression and classification tasks, comparing tDB against several machine learning baselines and fair-regression baselines.
Significance. If the formal theorem were correct, tDB would be an appealing and simple model-agnostic post-processing method: it requires no access to the model's internals, no retraining, and is easy to implement from a textbook identity. The empirical study is broad, covering multiple datasets, model classes, and both regression and classification, and the paper is commendable for making its central claim explicit and falsifiable. However, the central theoretical guarantee is invalid. Both premises of the proof in Section 4.4 are false, and the theorem is contradicted by a simple counterexample within the paper's own multivariate Gaussian assumption. The empirical results may indicate that tDB is a useful heuristic in some settings, but they cannot substitute for the broken formal claim, which is presented as a key contribution of the paper.
major comments (3)
- [Section 4.4] The proof's denominator claim is backwards. The paper states that ||E(Y|X)|| ≥ ||E(Y|X,S)||, but since σ(X) ⊆ σ(X,S), the conditional expectations E(Y|X) and E(Y|X,S) are projections onto nested subspaces. The Pythagorean relation ||E(Y|X,S)||² = ||E(Y|X)||² + ||E(Y|X,S) − E(Y|X)||² applies, so the norm of E(Y|X,S) is never smaller than the norm of E(Y|X). The variance comparison used to compare denominators in Eq. (1) therefore fails in the opposite direction.
- [Section 4.4] The numerator identity is false in general. The proof writes E[S E(Y|X)] = E{E[S E(Y|X)|S]}, and then identifies the inner term with S E(Y|X,S). But E[S E(Y|X)|S] = S E[E(Y|X)|S], and equality with S E(Y|X,S) would require E[E(Y|X)|S] = E(Y|X,S). This is not a consequence of the Tower Property and is not generally true; the Tower Property gives E[E(Y|X,S)|X] = E(Y|X), not the reverse relation used here.
- [Section 4.4] Eq. (1) is false even under the paper's basic assumption. Let X and S be centered Gaussian with Var(X)=Var(S)=1 and Cov(X,S)=0.9, and set Y = X − 0.9S. Then E(Y|X) = 0.19X and E(Y|X,S) = X − 0.9S. Direct computation gives ρ(E(Y|X),S) = 0.9 and ρ(E(Y|X,S),S) = 0, contradicting the claimed inequality ρ(E(Y|X),S) ≤ ρ(E(Y|X,S),S). This counterexample lies entirely within the multivariate Gaussian setting assumed in Section 4, so the formal fairness-improvement theorem is not merely unproven; it is incorrect.
minor comments (4)
- [Title and throughout] The method name is written inconsistently as both 'towerDebias' and 'TowerDebias'; the title uses 'TowerDebias', and the abstract and body should match.
- [Section 4.2.2] The property '||H1|| ≤ ||G||' stated for H1 = H − E(H|G) cannot be right in general: if H is independent of G with large variance, ||H1|| can exceed ||G||. This looks like a typo, likely intended to be an inequality involving ||H||, but as written it is false.
- [Section 4.4] The theorem and the fairness discussion use ρ without absolute values, while Section 2 defines the fairness measure as the absolute Pearson correlation. Eq. (1) should be stated with |ρ| or with an explicit clarification about signed correlation.
- [Section 5.6] There are several typographical issues, including 'Choose an appropriatek' missing a space, 'the initial correlation approach zero' (subject-verb agreement), and inconsistent capitalization of 'Law School Admission Results' versus 'Law School Admissions.'
Circularity Check
No significant circularity: the central fairness theorem is mathematically false, not self-referential or fitted.
full rationale
The claimed derivation chain is not circular. Section 4.4 attempts to prove Eq. (1) using L2 projection facts and the Tower Property. The proof is invalid because the asserted inequality ||E(Y|X)|| >= ||E(Y|X,S)|| is the reverse of the Pythagorean inequality for nested conditional-expectation subspaces, and the numerator identity E[S E(Y|X)] = E[S E(Y|X,S)] does not follow from the Tower Property. These are false mathematical premises, not a reduction of the conclusion to the assumptions by construction. The tDB output is a k-NN average of the black-box predictions and is not fitted to the reported fairness measure; no parameter is calibrated to rho(E(Y|X),S), and the empirical evaluation uses holdout data and external baselines. The only self-citation (Matloff and Zhang, 2022) appears in a related-work list and is not load-bearing. Therefore no step equates a 'prediction' with an input or imports a conclusion via a self-citation chain; the paper's issue, if any, is correctness of the theorem, not circularity.
Assumptions & free parameters
free parameters (1)
- k (number of nearest neighbors) =
not fixed; user-chosen
assumptions (4)
- domain assumption Basic Assumption (BA): (Y,X,S) follows a multivariate Gaussian distribution
- standard math Tower Property: E(Y|X) = E[E(Y|X,S)|X]
- ad hoc to paper Projection norm inequality ||E(Y|X)|| >= ||E(Y|X,S)||
- ad hoc to paper Numerator identity E[S E(Y|X)] = E[S E(Y|X,S)]
Cite this review
Pith. "Pith review of TowerDebias: A Novel Unfairness Removal Method Based on the Tower Property." pith.science (2026). https://pith.science/paper/GATIPWZ3
@misc{pith2026241108297,
author = {Pith},
title = {Pith review of: TowerDebias: A Novel Unfairness Removal Method Based on the Tower Property},
year = {2026},
howpublished = {\url{https://pith.science/paper/GATIPWZ3}},
note = {Machine review of arXiv:2411.08297}
}
read the original abstract
Decision-making processes have increasingly come to rely on sophisticated machine learning tools, raising critical concerns about the fairness of their predictions with respect to sensitive groups. The widespread adoption of commercial "black-box" models necessitates careful consideration of their legal and ethical implications for consumers. When users interact with such black-box models, a key challenge arises: how can the influence of sensitive attributes, such as race or gender, be mitigated or removed from its predictions? We propose towerDebias (tDB), a novel post-processing method designed to reduce the influence of sensitive attributes in predictions made by black-box models. Our tDB approach leverages the Tower Property from probability theory to improve prediction fairness without requiring retraining of the original model. This method is highly versatile, as it requires no prior knowledge of the original algorithm's internal structure and is adaptable to a diverse range of applications. We present a formal fairness improvement theorem for tDB and showcase its effectiveness in both regression and classification tasks using multiple real-world datasets.
Figures
Figures from the paper (7 more)
Reference graph
Works this paper leans on
-
[1]
Fair regression: Quantitative definitions and reduction-based algorithms
Alekh Agarwal, Miroslav Dud \' k, and Zhiwei Steven Wu. Fair regression: Quantitative definitions and reduction-based algorithms. In International Conference on Machine Learning, pages 120--129. PMLR, 2019
work page 2019
-
[2]
Machine bias: Technical response to northpointe, 2016
Julia Angwin and Jeff Larson. Machine bias: Technical response to northpointe, 2016. URL https://www.propublica.org/article/technical-response-to-northpointe
work page 2016
-
[3]
Julia Angwin, Jeff Larson, Surya Mattu, and Lauren Kirchner. Machine bias, 2016. URL https://www.propublica.org/article/machine-bias-risk-assessments-in-criminal-sentencing
work page 2016
-
[4]
Linear Algebra Done Right
Sheldon Axler. Linear Algebra Done Right. Springer, 2015
2015
-
[5]
Sina Baharlouei, Maher Nouiehed, Ahmad Beirami, and Meisam Razaviyayn. R 'enyi fair inference. arXiv preprint arXiv:1906.12005, 2019
work page Pith review arXiv 1906
-
[6]
Fairness and Machine Learning: Limitations and Opportunities
Solon Barocas, Moritz Hardt, and Arvind Narayanan. Fairness and Machine Learning: Limitations and Opportunities. MIT Press, 2023
2023
-
[7]
Lucas Bourtoule, Varun Chandrasekaran, Christopher A Choquette-Choo, Hengrui Jia, Adelin Travers, Baiwu Zhang, David Lie, and Nicolas Papernot. Machine unlearning. In 2021 IEEE Symposium on Security and Privacy (SP), pages 141--159. IEEE, 2021
work page 2021
-
[8]
Optimized pre-processing for discrimination prevention
Flavio Calmon, Dennis Wei, Bhanukiran Vinzamuri, Karthikeyan Natesan Ramamurthy, and Kush R Varshney. Optimized pre-processing for discrimination prevention. Advances in neural information processing systems, 30, 2017
work page 2017
Show all 40 references
-
[9]
Fairness-aware graph neural networks: A survey
April Chen, Ryan A Rossi, Namyong Park, Puja Trivedi, Yu Wang, Tong Yu, Sungchul Kim, Franck Dernoncourt, and Nesreen K Ahmed. Fairness-aware graph neural networks: A survey. ACM Transactions on Knowledge Discovery from Data, 18 0 (6): 0 1--23, 2024
2024
-
[10]
Ethics and discrimination in artificial intelligence-enabled recruitment practices
Zhisheng Chen. Ethics and discrimination in artificial intelligence-enabled recruitment practices. Humanities and Social Sciences Communications, 10, 09 2023. doi:10.1057/s41599-023-02079-x
2023 doi
-
[11]
Fair prediction with disparate impact: A study of bias in recidivism prediction instruments
Alexandra Chouldechova. Fair prediction with disparate impact: A study of bias in recidivism prediction instruments. Big data, 5 0 (2): 0 153--163, 2017
2017
-
[12]
Applied Multiple Regression/Correlation Analysis for the Behavioral Sciences
Jacob Cohen, Patricia Cohen, Stephen West, and Leona Aiken. Applied Multiple Regression/Correlation Analysis for the Behavioral Sciences. Lawrence Erlbaum Associates, Mahwah, NJ, 3rd edition, 2003. ISBN 978-0203774441
2003
-
[13]
Amazon scraps secret ai recruiting tool that showed bias against women
Jeffrey Dastin. Amazon scraps secret ai recruiting tool that showed bias against women. Reuters, October 2018
2018
-
[14]
How do the existing fairness metrics and unfairness mitigation algorithms contribute to ethical learning analytics? British Journal of Educational Technology, 53: 0 1--22, 04 2022
Oscar Deho, Chen Zhan, Jiuyong Li, Jixue Liu, Lin Liu, and Thuc le. How do the existing fairness metrics and unfairness mitigation algorithms contribute to ethical learning analytics? British Journal of Educational Technology, 53: 0 1--22, 04 2022. doi:10.1111/bjet.13217
2022 doi
-
[15]
Fairness guarantees in multi-class classification with demographic parity
Christophe Denis, Romuald Elie, Mohamed Hebiri, and Fran c ois Hu. Fairness guarantees in multi-class classification with demographic parity. Journal of Machine Learning Research, 25 0 (130): 0 1--46, 2024. URL http://jmlr.org/papers/v25/23-0322.html
2024
-
[16]
R. Durrett. Probability: Theory and Examples. Cambridge Series in Statistical and Probabilistic Mathematics. Cambridge University Press, 2019. ISBN 9781108473682. URL https://books.google.com/books?id=b22MDwAAQBAJ
2019
-
[17]
Fairness through awareness
Cynthia Dwork, Moritz Hardt, Toniann Pitassi, Omer Reingold, and Richard Zemel. Fairness through awareness. In Proceedings of the 3rd innovations in theoretical computer science conference, pages 214--226, 2012
2012
-
[18]
Equality of opportunity in supervised learning
Moritz Hardt, Eric Price, and Nati Srebro. Equality of opportunity in supervised learning. Advances in neural information processing systems, 2016. URL https://arxiv.org/abs/1610.02413
2016 arXiv
-
[19]
An algorithm for removing sensitive information
James E Johndrow and Kristian Lum. An algorithm for removing sensitive information. The Annals of Applied Statistics, 13 0 (1): 0 189--220, 2019
2019
-
[20]
Applied Multivariate Statistical Analysis
Richard Johnson and Dean Wichern. Applied Multivariate Statistical Analysis. Pearson, 1993
1993
-
[21]
Enhancing individual fairness through propensity score matching
Hamid Karimi, Muhammad Fawad Akbar Khan, Haochen Liu, Tyler Derr, and Hui Liu. Enhancing individual fairness through propensity score matching. In 2022 IEEE 9th International Conference on Data Science and Advanced Analytics (DSAA), pages 1--10, 2022. doi:10.1109/DSAA54385.202...
2022
-
[22]
Nonconvex optimization for regression with fairness constraints
Junpei Komiyama, Akiko Takeda, Junya Honda, and Hajime Shimao. Nonconvex optimization for regression with fairness constraints. In Jennifer Dy and Andreas Krause, editors, Proceedings of the 35th International Conference on Machine Learning, volume 80 of Proceedings of Machine...
2018
-
[23]
Fairness in credit scoring: Assessment, implementation and profit implications
Nikita Kozodoi, Johannes Jacob, and Stefan Lessmann. Fairness in credit scoring: Assessment, implementation and profit implications. European Journal of Operational Research, 2022
2022
-
[24]
A maximal correlation approach to imposing fairness in machine learning
Joshua Lee, Yuheng Bu, Prasanna Sattigeri, Rameswar Panda, Gregory Wornell, Leonid Karlinsky, and Rogerio Feris. A maximal correlation approach to imposing fairness in machine learning. In ICASSP 2022-2022 IEEE International Conference on Acoustics, Speech and Signal Processin...
2022
-
[25]
A statistical framework for fair predictive algorithms
Kristian Lum and James Johndrow. A statistical framework for fair predictive algorithms. arXiv preprint arXiv:1610.08077, 2016
2016 arXiv
-
[26]
Learning adversarially fair and transferable representations
David Madras, Elliot Creager, Toniann Pitassi, and Richard Zemel. Learning adversarially fair and transferable representations. In International Conference on Machine Learning, pages 3384--3393. PMLR, 2018
2018
-
[27]
Fairness-aware learning for continuous attributes and treatments
J \'e r \'e mie Mary, Cl \'e ment Calauz \`e nes, and Noureddine El Karoui. Fairness-aware learning for continuous attributes and treatments. In International Conference on Machine Learning, 2019. URL https://api.semanticscholar.org/CorpusID:174800179
2019
-
[28]
A novel regularization approach to fair ml
Norman Matloff and Wenxi Zhang. A novel regularization approach to fair ml. arXiv preprint arXiv:2208.06557, 2022
2022 arXiv
-
[29]
Improving fair training under correlation shifts
Yuji Roh, Kangwook Lee, Steven Euijong Whang, and Changho Suh. Improving fair training under correlation shifts. In International Conference on Machine Learning, pages 29179--29209. PMLR, 2023
2023
-
[30]
Machine learning: Algorithms, real-world applications and research directions
Iqbal Sarker. Machine learning: Algorithms, real-world applications and research directions. 2021. URL https://pubmed.ncbi.nlm.nih.gov/33778771/
2021
-
[31]
fairml: A statistician's take on fair machine learning modelling
Marco Scutari. fairml: A statistician's take on fair machine learning modelling. arXiv preprint arXiv:2305.02009, 2023. URL https://arxiv.org/abs/2305.02009
2023 arXiv
-
[32]
A general approach to fairness with optimal transport
Chiappa Silvia, Jiang Ray, Stepleton Tom, Pacchiano Aldo, Jiang Heinrich, and Aslanides John. A general approach to fairness with optimal transport. Proceedings of the AAAI Conference on Artificial Intelligence, 34: 0 3633--3640, 04 2020. doi:10.1609/aaai.v34i04.5771
2020 doi
-
[33]
Discriminated by an algorithm: A systematic review of discrimination and fairness by algorithmic decision-making in the context of hr recruitment and hr development
Marius Wehner and Alina Köchling. Discriminated by an algorithm: A systematic review of discrimination and fairness by algorithmic decision-making in the context of hr recruitment and hr development. BuR - Business Research, pages 1--54, 11 2020. doi:10.1007/s40685-020-00134-w
2020 doi
-
[34]
fairmodels: A flexible tool for bias detection, visualization, and mitigation
Jakub Wi \'s niewski and Przemys aw Biecek. fairmodels: A flexible tool for bias detection, visualization, and mitigation. arXiv preprint arXiv:2104.00507, 2021
2021 arXiv
-
[35]
Institute of statistics and decision sciences, 2009
Robert Wolpert. Institute of statistics and decision sciences, 2009. URL https://www2.stat.duke.edu/courses/Spring09/sta205/lec/topics/rn.pdf
2009
-
[36]
Muhammad Bilal Zafar, Isabel Valera, Manuel Gomez-Rodriguez, and Krishna P. Gummadi. Fairness constraints: A flexible approach for fair classification. Journal of Machine Learning Research, 20 0 (75): 0 1--42, 2019. URL http://jmlr.org/papers/v20/18-262.html
2019
-
[37]
Learning fair representations
Rich Zemel, Yu Wu, Kevin Swersky, Toni Pitassi, and Cynthia Dwork. Learning fair representations. In Sanjoy Dasgupta and David McAllester, editors, Proceedings of the 30th International Conference on Machine Learning, volume 28 of Proceedings of Machine Learning Research, page...
2013
-
[38]
Faht: an adaptive fairness-aware decision tree classifier
Wenbin Zhang and Eirini Ntoutsi. Faht: an adaptive fairness-aware decision tree classifier. arXiv preprint arXiv:1907.07237, 2019
1907 arXiv
-
[39]
Farf: A fair and adaptive random forests classifier
Wenbin Zhang, Albert Bifet, Xiangliang Zhang, Jeremy C Weiss, and Wolfgang Nejdl. Farf: A fair and adaptive random forests classifier. In Pacific-Asia conference on knowledge discovery and data mining, pages 245--256. Springer, 2021
2021
-
[40]
Towards fair classifiers without sensitive attributes: Exploring biases in related features
Tianxiang Zhao, Enyan Dai, Kai Shu, and Suhang Wang. Towards fair classifiers without sensitive attributes: Exploring biases in related features. In Proceedings of the Fifteenth ACM International Conference on Web Search and Data Mining, pages 1433--1442, 2022
2022
Reviewed August 12, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.