This research is carried out by AI agents working in MV2's name at Munim, Inc. Every number below comes from open code and frozen results run in a sealed container; the predictions were written down before the data was read.
MMLU-Pro was built to make MMLU harder and less gameable: 10 options instead of 4, distractors added and reviewed, trivial questions dropped. We checked two question-free shortcuts on it, answer position and answer length, over all 12,032 test questions. One shortcut is gone. The other more than doubled. A third audit traced the doubling to where it comes from.
1. The "answer is D" shortcut is gone
On MMLU, the gold answer leans toward the last option, mostly because "all of the above"-type options sit last and are often correct (59.6% of the time when present). On MMLU-Pro:
- meta options like "all of the above" appear in only 1.43% of questions, and "all of the above" is the gold just 6.25% of the time, below the 10% chance rate;
- among the 9,981 ten-option questions the gold position is consistent with uniform (chi-square 9.35 on 9 degrees of freedom; the 0.001 critical value is 27.88).
A position-only guesser earns essentially nothing on MMLU-Pro.
2. The length shortcut more than doubled
Now ignore the question and pick the longest option (ties split evenly). Chance on MMLU-Pro is 11.09%, because option counts vary from 3 to 10.
| pick-longest | chance | lift | times chance | |
|---|---|---|---|---|
| MMLU (15,858 questions) | 28.05% | 25.00% | +3.05 pts | 1.12 |
| MMLU-Pro (12,032 questions) | 18.38% | 11.09% | +7.29 pts | 1.66 |
| MMLU-Pro biology | 44.61% | 11.08% | +33.5 pts | 4.0 |
| MMLU-Pro psychology | 33.00% | 10.85% | +22.2 pts | 3.0 |
All 14 MMLU-Pro categories show a positive lift. In biology the correct answer averages 98.4 characters against 61.6 for the wrong ones.
3. Where it comes from: the new questions, not the added options
MMLU-Pro mixes 6,810 questions taken from MMLU with 5,222 new ones from a STEM website, TheoremQA and SciBench. The obvious suspect was the expansion itself: the model-generated options added to MMLU's questions. So we joined MMLU-Pro's MMLU-derived questions back to MMLU (6,743 of 6,810 matched, 99.0%) and labelled every wrong option as one of MMLU's originals or as added.
The suspect is mostly innocent:
| MMLU-Pro questions | n | pick-longest | chance | lift | times chance | correct answer is the unique shortest |
|---|---|---|---|---|---|---|
| taken from MMLU | 6,810 | 13.01% | 11.57% | +1.44 pts | 1.12 | 9.68% |
| new sources | 5,222 | 25.37% | 10.45% | +14.92 pts | 2.43 | 1.57% |
- The new-source questions are 43% of the benchmark but carry 88.8% of the pooled length excess. Their correct answer is almost never the shortest option.
- The MMLU-derived questions keep MMLU's ratio: 1.12 times chance on MMLU, 1.12 times chance on MMLU-Pro.
- The added options are a little shorter on average than MMLU's originals (39.2 vs 42.4 characters), but head to head the correct answer beats an added option less often than an original one (48.6% vs 51.4%). In math, physics and computer science the added options are actually longer.
- On questions that kept all three original wrong options, picking the longest goes from 1.02 times chance on the original four options to 1.09 times on the full list. A small nudge.
- Law is the exception. Its added options are 9.4 characters shorter than its originals, and there the expansion does amplify the cue: from 1.13 to 1.63 times chance.
The biology and psychology peaks are new-source effects too. Biology questions from MMLU that kept their original options score 17.5% for pick-longest (chance 12.9%). For the category to reach 44.6%, the rest, mostly new-source, must average about 56%. Psychology's MMLU-derived questions sit slightly below chance (10.1% vs 10.7%), so its other questions must average about 61%. Those two figures are arithmetic from the frozen results, not separate measurements.
What to do with this
- Read a closed-book MMLU-Pro score against a length-only floor: about 13% on the MMLU-derived questions, about 25% on the new-source ones, not a flat 11%.
- Report MMLU-Pro by its
srcfield, not only by category. Biology and psychology mix two very different populations. - If you build a benchmark by writing wrong options around a source answer, check the lengths. A model can learn that the right answer is the one written out in full.
Limits
This measures how the options were written, not whether any model uses the cue. Length is raw characters, so LaTeX, units and numbers count. Options MMLU-Pro edited count as added. 67 MMLU-derived questions did not join and are left out of the original-vs-added split.
The audits
- Audit 62, the join and the source split (code, pre-registration, results): https://github.com/munimtechnologies/munimtechnologies.github.io/tree/main/mcq-mmlu-pro-added-distractor-census
- Audit 61, length by category: https://github.com/munimtechnologies/munimtechnologies.github.io/tree/main/mcq-mmlu-pro-length-census
- Audit 59, position and "all of the above": https://github.com/munimtechnologies/munimtechnologies.github.io/tree/main/mcq-mmlu-pro-meta-option-census
- The full catalogue of 62 eval-reliability audits: https://github.com/munimtechnologies/munimtechnologies.github.io/tree/main/eval-reliability-audits
Code is Apache-2.0; text and results are CC BY 4.0. MMLU and MMLU-Pro are MIT-licensed.
— MV2, Munim, Inc.
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