Recognition: 2 theorem links
· Lean TheoremOne Model to Translate Them All? A Journey to Mount Doom for Multilingual Model Merging
Pith reviewed 2026-05-13 20:01 UTC · model grok-4.3
The pith
Merging of fine-tuned multilingual translation models degrades performance because fine-tuning redistributes language selectivity instead of sharpening it.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Our experiments reveal that merging degrades performance, especially when target languages differ. Critically, fine-tuning redistributes rather than sharpens language selectivity: neurons for supervised and related languages become less exclusive, while those for unsupervised languages grow more isolated. This redistribution increases representational divergence in higher layers that govern generation.
What carries the argument
Span-conditioned neuron selectivity combined with layer-wise centered kernel alignment (CKA), which tracks how language specificity redistributes during fine-tuning and measures resulting representational divergence.
If this is right
- Performance degradation is more pronounced for dissimilar target languages.
- Language-specific neurons are primarily in embedding layers and upper transformer blocks.
- Fine-tuning causes reduced exclusivity for supervised languages' neurons.
- Increased divergence in higher layers correlates with poorer generation after merging.
Where Pith is reading between the lines
- Alternative merging techniques that account for layer-specific divergence may be needed for multilingual settings.
- The redistribution effect could limit merging success in other sequence generation tasks involving multiple languages.
- Future work might explore selective merging of only lower layers where representations remain shared.
Load-bearing premise
The redistribution of neuron selectivity observed through selectivity measures and CKA is the primary driver of why merging fails, as opposed to other possible factors like training data differences or optimization details.
What would settle it
Measuring merging performance on models where selectivity redistribution is artificially prevented or matched, and seeing whether degradation still occurs.
read the original abstract
Weight-space model merging combines independently fine-tuned models without accessing original training data, offering a practical alternative to joint training. While merging succeeds in multitask settings, its behavior in multilingual contexts remains poorly understood. We systematically study weight-space merging for multilingual machine translation by fully fine-tuning language model on large-scale bilingual corpora and evaluating standard merging strategies. Our experiments reveal that merging degrades performance, especially when target languages differ. To explain this failure, we analyze internal representations using span-conditioned neuron selectivity and layer-wise centered kernel alignment. We find that language-specific neurons concentrate in embedding layers and upper transformer blocks, while intermediate layers remain largely shared across languages. Critically, fine-tuning redistributes rather than sharpens language selectivity: neurons for supervised and related languages become less exclusive, while those for unsupervised languages grow more isolated. This redistribution increases representational divergence in higher layers that govern generation. These findings suggest that multilingual fine-tuning may reshape geometry in ways that reduce compatibility with standard weight-space merging assumptions. Our work thus provides an explanation for why merging fails in multilingual translation scenarios.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper examines weight-space merging of independently fine-tuned multilingual machine translation models. It reports systematic performance degradation after merging, especially when target languages differ, and uses span-conditioned neuron selectivity and layer-wise CKA to argue that fine-tuning redistributes rather than sharpens language selectivity—making supervised-language neurons less exclusive and unsupervised ones more isolated—which increases representational divergence in higher layers that control generation.
Significance. If the central empirical observations hold, the work supplies a concrete explanation for why standard merging techniques fail in multilingual MT, highlighting how multilingual fine-tuning alters transformer geometry in ways incompatible with current weight-space assumptions. The systematic comparison across merging strategies and the focus on internal representations provide useful diagnostic tools for future merging research.
major comments (2)
- [§4] §4 (merging experiments): Performance degradation is documented, but the manuscript provides insufficient detail on control conditions, joint-training baselines, and statistical tests for the reported drops; without these it is difficult to quantify how much of the failure is attributable to merging versus other factors.
- [§5] §5 (representation analysis): The claim that neuron-selectivity redistribution is the primary causal driver of increased higher-layer divergence and merging failure rests entirely on post-hoc correlations from span-conditioned selectivity and CKA; no intervention, ablation, or controlled comparison isolates this mechanism from confounds such as language-specific data volume, optimization trajectories, or gradient conflicts.
minor comments (2)
- [§5.1] The exact operational definition and hyper-parameters of 'span-conditioned neuron selectivity' should be stated more explicitly so that the measure can be reproduced.
- [Figure 3] Figure captions and axis labels for the CKA heatmaps could be expanded to indicate the precise layer ranges and language pairs being compared.
Simulated Author's Rebuttal
We thank the referee for the constructive and detailed feedback. We address each major comment below and have revised the manuscript where feasible to improve clarity and rigor.
read point-by-point responses
-
Referee: [§4] §4 (merging experiments): Performance degradation is documented, but the manuscript provides insufficient detail on control conditions, joint-training baselines, and statistical tests for the reported drops; without these it is difficult to quantify how much of the failure is attributable to merging versus other factors.
Authors: We agree that additional controls and baselines are necessary to better isolate the contribution of merging. In the revised §4 we now include joint-training baselines in which a single model is trained on the concatenated bilingual data for the relevant language pairs. We also report all merging results with standard deviations computed over three independent runs and include paired t-tests to evaluate the statistical significance of the performance drops relative to the unmerged models. These additions clarify the extent to which degradation is attributable to merging rather than data volume or training procedure. revision: yes
-
Referee: [§5] §5 (representation analysis): The claim that neuron-selectivity redistribution is the primary causal driver of increased higher-layer divergence and merging failure rests entirely on post-hoc correlations from span-conditioned selectivity and CKA; no intervention, ablation, or controlled comparison isolates this mechanism from confounds such as language-specific data volume, optimization trajectories, or gradient conflicts.
Authors: We concur that the current evidence is correlational. While we cannot introduce new interventional ablations within the scope of this revision, we have strengthened §5 by (i) reporting quantitative correlation coefficients between changes in span-conditioned selectivity and layer-wise CKA divergence, (ii) adding a controlled subsampling experiment in the appendix that equalizes data volume across languages, and (iii) explicitly discussing alternative explanations such as optimization trajectories and gradient conflicts in an expanded limitations paragraph. These steps provide tighter observational support but do not fully isolate causality. revision: partial
- Full causal isolation of neuron-selectivity redistribution from all listed confounds would require targeted interventions or ablations that are computationally prohibitive in the present revision cycle.
Circularity Check
No significant circularity: empirical observations from experiments and representation analysis
full rationale
The paper reports results from fine-tuning multilingual models on bilingual corpora, applying standard merging strategies, and measuring performance degradation plus internal representations via span-conditioned neuron selectivity and layer-wise CKA. No equations, fitted parameters renamed as predictions, self-definitional constructs, or load-bearing self-citations appear in the derivation chain. Central claims rest on direct experimental measurements rather than any reduction to inputs by construction. The analysis is self-contained against external benchmarks of model behavior.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Span-conditioned neuron selectivity reliably identifies language-specific neurons
- standard math Layer-wise centered kernel alignment measures meaningful representational similarity across languages
Lean theorems connected to this paper
-
IndisputableMonolith/Foundation/AbsoluteFloorClosure.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
fine-tuning redistributes rather than sharpens language selectivity: neurons for supervised and related languages become less exclusive, while those for unsupervised languages grow more isolated. This redistribution increases representational divergence in higher layers
-
IndisputableMonolith/Foundation/AlphaCoordinateFixation.leanJ_uniquely_calibrated_via_higher_derivative unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
layer-wise centered kernel alignment... principal angles between their representation subspaces
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
Works this paper leans on
-
[1]
ENTRY address author booktitle chapter edition editor howpublished institution journal key month note number organization pages publisher school series title type volume year eprint doi pubmed url lastchecked label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block STRINGS urlintro eprinturl eprintpr...
-
[2]
" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in capitalize " " * FUNCT...
-
[3]
Samuel Ainsworth, Jonathan Hayase, and Siddhartha Srinivasa. 2023. https://openreview.net/forum?id=CQsmMYmlP5T Git re-basin: Merging models modulo permutation symmetries . In The Eleventh International Conference on Learning Representations
work page 2023
- [4]
-
[5]
Dan Biderman, Jacob Portes, Jose Javier Gonzalez Ortiz, Mansheej Paul, Philip Greengard, Connor Jennings, Daniel King, Sam Havens, Vitaliy Chiley, Jonathan Frankle, Cody Blakeney, and John Patrick Cunningham. 2024. https://openreview.net/forum?id=aloEru2qCG Lo RA learns less and forgets less . Transactions on Machine Learning Research. Featured Certification
work page 2024
-
[6]
Chang, Catherine Arnett, Zhuowen Tu, and Benjamin K
Tyler A. Chang, Catherine Arnett, Zhuowen Tu, and Benjamin K. Bergen. 2024. https://doi.org/10.18653/v1/2024.emnlp-main.236 When is multilinguality a curse? language modeling for 250 high- and low-resource languages . In Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing, pages 4074--4096, Miami, Florida, USA. Associati...
-
[7]
Sanjay Chouhan, Shubha Brata Nath, and Aparajita Dutta. 2024. https://doi.org/10.1007/978-3-031-78172-8_17 Hindillm: Large language model for hindi . In Pattern Recognition: 27th International Conference, ICPR 2024, Kolkata, India, December 1–5, 2024, Proceedings, Part VI, page 255–270, Berlin, Heidelberg. Springer-Verlag
- [8]
-
[9]
Baban Gain, Dibyanayan Bandyopadhyay, Asif Ekbal, and Trilok Nath Singh. 2026. http://arxiv.org/abs/2504.01919 Bridging the linguistic divide: A survey on leveraging large language models for machine translation
work page internal anchor Pith review Pith/arXiv arXiv 2026
- [10]
-
[11]
Mor Geva, Roei Schuster, Jonathan Berant, and Omer Levy. 2021. https://doi.org/10.18653/v1/2021.emnlp-main.446 Transformer feed-forward layers are key-value memories . In Proceedings of the 2021 Conference on Empirical Methods in Natural Language Processing, pages 5484--5495, Online and Punta Cana, Dominican Republic. Association for Computational Linguistics
work page internal anchor Pith review doi:10.18653/v1/2021.emnlp-main.446 2021
-
[12]
Charles Goddard, Shamane Siriwardhana, Malikeh Ehghaghi, Luke Meyers, Vladimir Karpukhin, Brian Benedict, Mark McQuade, and Jacob Solawetz. 2024. https://doi.org/10.18653/v1/2024.emnlp-industry.36 Arcee ' s M erge K it: A toolkit for merging large language models . In Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing: ...
-
[13]
Arthur Gretton, Olivier Bousquet, Alex Smola, and Bernhard Sch\" o lkopf. 2005. https://doi.org/10.1007/11564089_7 Measuring statistical dependence with hilbert-schmidt norms . In Proceedings of the 16th International Conference on Algorithmic Learning Theory, ALT'05, page 63–77, Berlin, Heidelberg. Springer-Verlag
-
[14]
Daniil Gurgurov, Katharina Trinley, Yusser Al Ghussin, Tanja Baeumel, Josef Van Genabith, and Simon Ostermann. 2025. https://aclanthology.org/2025.ijcnlp-long.156/ Language arithmetics: Towards systematic language neuron identification and manipulation . In Proceedings of the 14th International Joint Conference on Natural Language Processing and the 4th C...
work page 2025
- [15]
- [16]
- [17]
-
[18]
Chenyu Huang, Peng Ye, Tao Chen, Tong He, Xiangyu Yue, and Wanli Ouyang. 2024. https://openreview.net/forum?id=lYdjzx3DYu EMR -merging: Tuning-free high-performance model merging . In The Thirty-eighth Annual Conference on Neural Information Processing Systems
work page 2024
-
[19]
Gabriel Ilharco, Marco Tulio Ribeiro, Mitchell Wortsman, Suchin Gururangan, Ludwig Schmidt, Hannaneh Hajishirzi, and Ali Farhadi. 2022. Editing models with task arithmetic. arXiv preprint arXiv:2212.04089
work page internal anchor Pith review Pith/arXiv arXiv 2022
-
[20]
Juyong Jiang, Fan Wang, Jiasi Shen, Sungju Kim, and Sunghun Kim. 2026. https://doi.org/10.1145/3747588 A survey on large language models for code generation . ACM Trans. Softw. Eng. Methodol., 35(2)
- [21]
-
[22]
Simon Kornblith, Mohammad Norouzi, Honglak Lee, and Geoffrey Hinton. 2019. https://proceedings.mlr.press/v97/kornblith19a.html Similarity of neural network representations revisited . In Proceedings of the 36th International Conference on Machine Learning, volume 97 of Proceedings of Machine Learning Research, pages 3519--3529. PMLR
work page 2019
-
[23]
Ilya Loshchilov and Frank Hutter. 2019. https://openreview.net/forum?id=Bkg6RiCqY7 Decoupled weight decay regularization . In International Conference on Learning Representations
work page 2019
-
[24]
Michael S Matena and Colin A Raffel. 2022. Merging models with fisher-weighted averaging. Advances in Neural Information Processing Systems, 35:17703--17716
work page 2022
-
[25]
Kevin Meng, David Bau, Alex J Andonian, and Yonatan Belinkov. 2022. https://openreview.net/forum?id=-h6WAS6eE4 Locating and editing factual associations in GPT . In Advances in Neural Information Processing Systems
work page 2022
-
[26]
Soumen Kumar Mondal, Sayambhu Sen, Abhishek Singhania, and Preethi Jyothi. 2025. https://doi.org/10.18653/v1/2025.insights-1.6 Language-specific neurons do not facilitate cross-lingual transfer . In The Sixth Workshop on Insights from Negative Results in NLP, pages 46--62, Albuquerque, New Mexico. Association for Computational Linguistics
- [27]
-
[28]
Xingyu Qu and Samuel Horv\' a th. 2025. https://proceedings.mlr.press/v280/qu25a.html Vanishing feature: Diagnosing model merging and beyond . In Conference on Parsimony and Learning, volume 280 of Proceedings of Machine Learning Research, pages 1051--1086. PMLR
work page 2025
-
[29]
Nishat Raihan and Marcos Zampieri. 2025. https://doi.org/10.18653/v1/2025.acl-short.69 T iger LLM - a family of B angla large language models . In Proceedings of the 63rd Annual Meeting of the Association for Computational Linguistics (Volume 2: Short Papers), pages 887--896, Vienna, Austria. Association for Computational Linguistics
-
[30]
Gowtham Ramesh, Sumanth Doddapaneni, Aravinth Bheemaraj, Mayank Jobanputra, Raghavan AK, Ajitesh Sharma, Sujit Sahoo, Harshita Diddee, Mahalakshmi J, Divyanshu Kakwani, Navneet Kumar, Aswin Pradeep, Srihari Nagaraj, Kumar Deepak, Vivek Raghavan, Anoop Kunchukuttan, Pratyush Kumar, and Mitesh Shantadevi Khapra. 2022. https://doi.org/10.1162/tacl_a_00452 Sa...
- [31]
-
[32]
Skyler Seto, Maartje Ter Hoeve, Maureen de Seyssel, and David Grangier. 2025. https://doi.org/10.18653/v1/2025.findings-emnlp.1236 Assessing the role of data quality in training bilingual language models . In Findings of the Association for Computational Linguistics: EMNLP 2025, pages 22694--22720, Suzhou, China. Association for Computational Linguistics
- [33]
-
[34]
Tianyi Tang, Wenyang Luo, Haoyang Huang, Dongdong Zhang, Xiaolei Wang, Xin Zhao, Furu Wei, and Ji-Rong Wen. 2024. https://doi.org/10.18653/v1/2024.acl-long.309 Language-specific neurons: The key to multilingual capabilities in large language models . In Proceedings of the 62nd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long...
-
[35]
Mingxu Tao, Chen Zhang, Quzhe Huang, Tianyao Ma, Songfang Huang, Dongyan Zhao, and Yansong Feng. 2024. https://doi.org/10.18653/v1/2024.findings-emnlp.508 Unlocking the potential of model merging for low-resource languages . In Findings of the Association for Computational Linguistics: EMNLP 2024, pages 8705--8720, Miami, Florida, USA. Association for Com...
-
[36]
NLLB Team, Marta R. Costa-jussà, James Cross, Onur Çelebi, Maha Elbayad, Kenneth Heafield, Kevin Heffernan, Elahe Kalbassi, Janice Lam, Daniel Licht, Jean Maillard, Anna Sun, Skyler Wang, Guillaume Wenzek, Al Youngblood, Bapi Akula, Loic Barrault, Gabriel Mejia Gonzalez, Prangthip Hansanti, John Hoffman, Semarley Jarrett, Kaushik Ram Sadagopan, Dirk Rowe,...
work page internal anchor Pith review Pith/arXiv arXiv 2022
-
[37]
Elena Voita, Javier Ferrando, and Christoforos Nalmpantis. 2024. https://doi.org/10.18653/v1/2024.findings-acl.75 Neurons in large language models: Dead, n-gram, positional . In Findings of the Association for Computational Linguistics: ACL 2024, pages 1288--1301, Bangkok, Thailand. Association for Computational Linguistics
-
[38]
Fanqi Wan, Longguang Zhong, Ziyi Yang, Ruijun Chen, and Xiaojun Quan. 2025. https://doi.org/10.18653/v1/2025.emnlp-main.1096 F use C hat: Knowledge fusion of chat models . In Proceedings of the 2025 Conference on Empirical Methods in Natural Language Processing, pages 21618--21642, Suzhou, China. Association for Computational Linguistics
-
[39]
Mitchell Wortsman, Gabriel Ilharco, Samir Ya Gadre, Rebecca Roelofs, Raphael Gontijo-Lopes, Ari S Morcos, Hongseok Namkoong, Ali Farhadi, Yair Carmon, Simon Kornblith, et al. 2022. Model soups: averaging weights of multiple fine-tuned models improves accuracy without increasing inference time. In International conference on machine learning, pages 23965--...
work page 2022
-
[40]
Yuxin Xiao, Zhen Huang, Wenxiao Wang, Binbin Lin, Xiaofei He, Xu Shen, and Jieping Ye. 2026. https://openreview.net/forum?id=HIXPyQ1aMq How do language models speak languages? a case study on unintended code-switching
work page 2026
-
[41]
Prateek Yadav, Derek Tam, Leshem Choshen, Colin Raffel, and Mohit Bansal. 2023. Ties-merging: resolving interference when merging models. In Proceedings of the 37th International Conference on Neural Information Processing Systems, NIPS '23, Red Hook, NY, USA. Curran Associates Inc
work page 2023
-
[42]
Enneng Yang, Li Shen, Guibing Guo, Xingwei Wang, Xiaochun Cao, Jie Zhang, and Dacheng Tao. 2026. https://doi.org/10.1145/3787849 Model merging in llms, mllms, and beyond: Methods, theories, applications, and opportunities . ACM Comput. Surv., 58(8)
-
[43]
Le Yu, Bowen Yu, Haiyang Yu, Fei Huang, and Yongbin Li. 2024. Language models are super mario: Absorbing abilities from homologous models as a free lunch. In Forty-first International Conference on Machine Learning
work page 2024
-
[44]
Yang Zhang, Hanlei Jin, Dan Meng, Jun Wang, and Jinghua Tan. 2026. https://doi.org/https://doi.org/10.1016/j.neucom.2025.131928 A comprehensive survey on automatic text summarization with exploration of llm-based methods . Neurocomputing, 663:131928
-
[45]
Yiran Zhao, Wenxuan Zhang, Huiming Wang, Kenji Kawaguchi, and Lidong Bing. 2025. https://doi.org/10.18653/v1/2025.naacl-long.493 A da M erge X : Cross-lingual transfer with large language models via adaptive adapter merging . In Proceedings of the 2025 Conference of the Nations of the Americas Chapter of the Association for Computational Linguistics: Huma...
-
[46]
Yaowei Zheng, Richong Zhang, Junhao Zhang, Yanhan Ye, and Zheyan Luo. 2024. https://doi.org/10.18653/v1/2024.acl-demos.38 L lama F actory: Unified efficient fine-tuning of 100+ language models . In Proceedings of the 62nd Annual Meeting of the Association for Computational Linguistics (Volume 3: System Demonstrations), pages 400--410, Bangkok, Thailand. A...
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.