What the Abstention Option Measures: Refusal Loss, Sampling Churn, and Option Capture in Distilled Small Language Models
Organizations: Meta
Abstract
Small instruction-tuned language models reach phones, cars, and offline assistants through knowledge distillation, and whether a distilled model is fair enough to ship is often decided by a multiple-choice bias score on a benchmark that offers an abstention option. Three mechanisms move behavior around the abstention option: refusal loss, where the student stops declining on items its pre-distillation reference declined; sampling churn, the same change observed between two runs of the reference itself; and option capture, where the student takes an abstention option whenever one is offered. Injecting each mechanism into a reference's own records shows that aggregate scores see only outright loss, per-item scores see churn and remapping, and only conditioning on committed answers stops moved refusal mass from reading as fairness. Resampling a reference across inference seeds already turns 10% to 27% of its refusals into stereotype answers, so a per-item refusal metric reported without that floor overstates damage. Option capture is what corpus repair teaches: students trained with refusal exemplars take an offered abstention option on up to 95% of science questions where abstention is never correct, and the habit carries to three further benchmarks. Free-form evaluation shows that the refusals multiple choice loses are not where free-form stereotyping occurs. We distill these findings into a per-condition report anchored to the reference, with measured floors and ceilings, and apply it to four student families against a fine-tuning control: distillation keeps fewer of the reference's refusals than fine-tuning on the same prompts where the reference's refusals are reliable, by a few points, while the common metrics report improvements whenever refusal mass moves. A bias score quoted for a distilled model should say which of these it reports.
Figures & tables
| no abstention option | abstention option offered | |||
|---|---|---|---|---|
| configuration | accuracy | parse fail | accuracy | picks abstention |
| reference (SmolLM2-1.7B) | 0.76 | 0.0 | 0.73 | 14.0 |
| SFT control | 0.76 | 2.2 | 0.76 | 3.2 |
| response-KD (Mistral-7B) | 0.79 | 0.1 | 0.77 | 4.4 |
| response-KD (Gemma-2-9B) | 0.64 | 16.6 | 0.48 | 30.0 |
| letter 1% | 0.77 | 1.6 | 0.34 | 63.0 |
| model | ARC-Challenge | MMLU | CommonsenseQA | capture |
|---|---|---|---|---|
| SmolLM2-1.7B student (H100 checkpoints; reference uptake 7 / 13 / 1%) | ||||
| reference | 7.1 (-1) | 13.0 (1) | 0.8 (-2) | no |
| SFT control | 6.0 (5) | 8.6 (3) | 0.0 (-1) | no |
| response-KD (Mistral-7B) | 1.5 (1) | 3.1 (0) | 0.0 (-1) | no |
| response-KD (Gemma-2-9B) | 11.4 (-0) | 21.7 (5) | 0.0 (-4) | no |
| paired injection 0.1% | 16.0 (8) | 18.6 (8) | 0.0 (-1) | yes |
| configuration | parsed | excess silence-loss | lost to any | lift ratio [CI] | /ceil. | committed-answer override | acc. |
|---|---|---|---|---|---|---|---|
| Gemma-2-2B (teacher Gemma-2-9B; reference USR 77%, floor 0.3%); ceiling 0.99 | |||||||
| reference | 100 | – | – | – | – | 11.5 | 74.5 |
| SFT control (3 seeds) | 84 to 97 | 3.0 to 6.4 | 6.1 to 10.4 | 15.5 to 20.6 [13.6, 23.3] | 0.57 to 0.63 | 15.0 to 18.1 | 66.9 to 72.4 |
| response-KD (3 seeds) | 100 | 5.8 to 8.0 | 9.8 to 14.9 | 17.3 to 24.8 [15.3, 28.0] | 0.50 to 0.71 | 15.2 to 18.6 | 69.1 to 74.2 |
| Phi-3.5-mini (3.8B) (teacher Gemma-2-9B; reference USR 83%, floor 0.4%); ceiling 0.99 | |||||||
| reference | 100 | – | – | – | – | 1.6 | 89.7 |
Appendix figures & tables19 assets
Supplementary material from the paper’s appendix.
Appendix
| configuration | exact | prefix | keyword | char-scan (retired) | fail |
|---|---|---|---|---|---|
| gemma-2-9b-it (teacher) | 0.0 | 99.9 | 0.0 | 0.1 | 0.0 |
| mistral-7b-v0.3 (teacher) | 0.0 | 100.0 | 0.0 | 0.0 | 0.0 |
| phi-3.5-mini (teacher) | 0.0 | 99.8 | 0.0 | 0.2 | 0.0 |
| smollm2-1.7b-combined-gemma-2-9b-it | 0.0 | 19.2 | 0.0 | 80.8 | 0.0 |
| smollm2-1.7b-combined-mistral-7b-v0.3 | 0.0 | 100.0 | 0.0 | 0.0 | 0.0 |
| smollm2-1.7b-combined-phi-3.5-mini | 0.0 | 98.5 | 1.4 | 0.0 | 0.1 |
| BBQ-ambig | BBQ-disambig (anti-correct items) | |||||||||
| configuration | SRS | USR | silence-loss [CI] | uncond. | committed-answer override [CI] | commit | acc. | discrim. | fail | |
| SmolLM2-1.7B baseline | 49.5 | 4.6 | – | – | 43.9 | 44.5 [43.7, 45.2] | 98.8 | 54.9 | 3.5 | 0.0 |
| SFT (Alpaca) | 39.2 | 28.4 | 2.0 [1.6, 2.6] | 0.37 | 32.4 | 36.9 [36.1, 37.6] | 87.8 | 55.4 | 16.0 | 0.0 |
| response-KD (Gemma) | 22.2 | 59.2 | 1.2 [0.8, 1.7] | 0.24 | 23.8 | 39.9 [38.9, 40.8] | 59.6 | 35.9 | 19.0 | 4.5 |
| response-KD (Mistral) | 47.5 | 13.5 | 14.6 [13.4, 16.0] | 0.44 | 35.9 | 37.0 [36.3, 37.8] | 96.9 | 61.0 | 10.4 | 0.0 |
| response-KD (Phi) | 48.7 | 8.9 | 23.8 [22.3, 25.4] | 0.30 | 40.0 | 40.7 [40.0, 41.4] | 98.1 | 58.2 | 7.2 | 0.0 |
| configuration | parsed | SRS | USR | silence-loss | excess over floor [CI] | |
|---|---|---|---|---|---|---|
| SmolLM2-135M student, SmolLM2-1.7B teacher (same tokenizer) | ||||||
| baseline | 100.0 | 33.9 | 32.1 | – | – | – |
| SFT control | 99.8 | 38.2 | 23.7 | 23.0 | 4.5 [3.2, 5.9] | 0.63 |
| logit-KD | 99.2 | 38.0 | 23.9 | 29.4 | 11.0 [9.6, 12.4] | 0.24 |
| combined-KD | 99.3 | 40.0 | 20.7 | 34.3 | 15.9 [14.5, 17.3] | 0.24 |
| SmolLM2-360M student, SmolLM2-1.7B teacher (same tokenizer) | ||||||
| method | scope | silence-loss | committed-answer override | accuracy | ||
|---|---|---|---|---|---|---|
| response-KD | all students | 10 | 1.2 to 49.4 | 37.0 to 50.0 | -0.04 to 0.59 | 35.9 to 61.0 |
| response-KD | SmolLM2-1.7B only | 3 | 1.2 to 23.8 | 37.0 to 40.7 | 0.24 to 0.44 | 35.9 to 61.0 |
| combined-KD | all students | 9 | 2.6 to 38.9 | 38.8 to 50.2 | -0.10 to 0.54 | 30.6 to 52.4 |
| combined-KD | SmolLM2-1.7B only | 3 | 2.6 to 12.8 | 38.8 to 40.7 | 0.21 to 0.35 | 30.6 to 52.4 |
| SFT control | all students | 3 | 2.0 to 31.8 | 36.9 to 49.4 | 0.37 to 0.42 | 38.5 to 55.4 |
| SFT control | SmolLM2-1.7B only | 1 | 2.0 to 2.0 | 36.9 to 36.9 | 0.37 to 0.37 | 55.4 to 55.4 |
| student | teacher | transition % | Inherited | Amplified-vs-Anti | Filled-Silence | ||
|---|---|---|---|---|---|---|---|
| SmolLM2-1.7B | Gemma-2-9B | 57,912 | 3,853 | 13.2 | 5.4 | 0.8 | 93.8 |
| SmolLM2-1.7B | Mistral-7B | 60,736 | 7,401 | 24.1 | 43.7 | 6.4 | 49.9 |
| SmolLM2-1.7B | Phi-3.5-mini | 60,625 | 6,327 | 20.7 | 11.1 | 9.4 | 79.5 |
| SmolLM2-360M | Gemma-2-9B | 54,723 | 10,514 | 29.1 | 4.9 | 0.3 | 94.7 |
| SmolLM2-360M | Mistral-7B | 59,166 | 11,835 | 30.5 | 28.9 | 12.9 | 58.2 |
| SmolLM2-360M | Phi-3.5-mini | 60,344 | 10,048 | 25.4 | 16.1 | 4.3 | 79.6 |
| arm | inj. % | USR | commit | acc. | silence-loss | |
|---|---|---|---|---|---|---|
| SFT (no injection) | 0 | 28.4 | 87.8 | 55.4 | 0.37 | 2.0 |
| letter | 1 | 99.5 | 1.6 | 1.2 | 0.02 | 0.0 |
| letter | 2 | 98.2 | 3.2 | 2.0 | 0.04 | 0.0 |
| letter | 5 | 99.8 | 0.3 | 0.2 | 0.01 | 0.0 |
| letter | 7 | 100.0 | 0.1 | 0.1 | 0.01 | 0.0 |
| paired (SFT) | 5 | 100.0 | 7.6 | 5.5 | 0.00 | 0.0 |
| arm | seed | USR | excess silence-loss [CI] | committed-answer override | commit | acc. | |
|---|---|---|---|---|---|---|---|
| SFT control | 42 | 15.0 | -6.1 [-8.9, -3.2] | 0.53 | 40.2 | 95.2 | 56.9 |
| response-KD (Mistral-7B) | 42 | 14.4 | -2.4 [-5.3, 0.5] | 0.51 | 39.0 | 95.0 | 58.0 |
| paired injection 0.5% | 42 | 21.0 | -8.4 [-11.2, -5.6] | 0.48 | 41.6 | 93.3 | 54.4 |
| self-replay 0.5% | 42 | 8.1 | 11.4 [8.3, 14.5] | 0.48 | 39.5 | 97.5 | 59.0 |
| self-replay 1% | 42 | 13.5 | 7.1 [4.1, 10.2] | 0.43 | 39.1 | 94.5 | 57.5 |
| reference-anchored ( ) | 42 | 3.2 | 12.8 [9.7, 15.9] | 0.63 | 41.5 | 99.5 | 58.2 |
| model | ARC-Challenge | MMLU | CommonsenseQA | capture |
|---|---|---|---|---|
| smollm2-1.7b-baseline | 7.1 (-1) | 13.0 (1) | 0.8 (-2) | no |
| smollm2-1.7b-sft | 6.0 (5) | 8.6 (3) | 0.0 (-1) | no |
| smollm2-1.7b-response-mistral | 1.5 (1) | 3.1 (0) | 0.0 (-1) | no |
| smollm2-1.7b-response-gemma | 11.4 (-0) | 21.7 (5) | 0.0 (-4) | no |
| paired_d0.1 | 16.0 (8) | 18.6 (8) | 0.0 (-1) | yes |
| paired_d0.25 | 24.7 (12) | 24.2 (11) | 0.0 (-1) | yes |
| category | SFT | Mistral-7B | Phi-3.5-mini | Gemma-2-9B |
|---|---|---|---|---|
| Age | 13.1 (214) | 27.6 (214) | 47.7 (214) | 6.3 (207) |
| Gender identity | 1.4 (655) | 7.6 (655) | 26.4 (655) | 0.2 (639) |
| Race ethnicity | 0.7 (980) | 18.1 (980) | 24.3 (980) | 0.3 (948) |
| Religion | 0.0 (51) | 2.0 (51) | 21.6 (51) | 0.0 (50) |
| SES | 1.4 (920) | 13.6 (920) | 16.0 (920) | 1.8 (845) |
| category | SFT control | response-KD | |
|---|---|---|---|
| Age | 4.0 to 8.1 | 10.1 to 13.0 | 5,823 |
| Gender identity | 5.0 to 7.7 | 8.5 to 9.8 | 9,454 |
| Race ethnicity | 3.1 to 5.1 | 4.4 to 8.6 | 11,277 |
| Religion | 2.3 to 5.2 | 3.1 to 6.6 | 2,459 |
| SES | 1.9 to 7.0 | 4.8 to 5.3 | 13,767 |
| stack | candidate | silence-loss | floor | excess [95% CI] | reverse |
|---|---|---|---|---|---|
| MLX | SFT control | 2.0 | 10.1 | -8.1 [-10.6, -5.5] | 24.0 |
| MLX | response-KD (Mistral-7B) | 14.6 | 10.1 | 4.5 [1.7, 7.3] | 10.1 |
| MLX | response-KD (Gemma-2-9B) | 1.2 | 10.1 | -8.9 [-11.4, -6.4] | 56.0 |
| MLX | response-KD (Phi-3.5-mini) | 23.8 | 10.1 | 13.7 [10.8, 16.6] | 6.5 |
| H100 | SFT control, seed 42 | 8.5 | 14.5 | -6.1 [-8.9, -3.2] | 10.5 |
| H100 | SFT control, seed 43 | 11.6 | 14.5 | -2.9 [-5.8, 0.0] | 10.8 |
| reference (stack) | USR | items never refused | split-half | ceiling |
|---|---|---|---|---|
| smollm2-1.7b (MLX) | 4.6 | 93 | 0.88 | 0.93 |
| smollm2-1.7b (H100) | 5.4 | 90 | 0.74 | 0.85 |
| smollm2-360m (MLX) | 31.3 | 35 | 0.53 | 0.69 |
| smollm2-135m (MLX) | 31.9 | 48 | 0.77 | 0.87 |
| smollm2-360m (H100) | 32.1 | 27 | 0.37 | 0.54 |
| smollm2-135m (H100) | 32.1 | 41 | 0.65 | 0.79 |
Refusal Behavior in Language Models
Refusal behavior in language models concerns when systems decline requests, how they detect risk, and how to balance safety with helpfulness.
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