{ "fact_registry": { "B1": { "key": "pnas_weight", "label": "PNAS 2002: brain = ~2% of body weight" }, "B2": { "key": "pnas_oxygen", "label": "PNAS 2002: brain = ~20% of resting O2" }, "B3": { "key": "ncbi_weight", "label": "NCBI Basic Neurochemistry: brain = ~2% body weight" }, "B4": { "key": "ncbi_oxygen", "label": "NCBI Basic Neurochemistry: brain = 20% resting O2" }, "A1": { "label": "SC1: extracted brain weight % lies within \u00b10.5pp of 2%", "method": "compare(weight_pct_pnas, within \u00b10.5pp of 2.0)", "result": "True" }, "A2": { "label": "SC2: extracted brain O2 % lies within \u00b12pp of 20%", "method": "compare(oxygen_pct_pnas, within \u00b12pp of 20.0)", "result": "True" } }, "claim_formal": { "subject": "human brain (adult)", "sub_claims": { "SC1": { "property": "brain mass as fraction of total adult human body weight", "operator": "within \u00b10.5pp of 2.0", "threshold_pct": 2.0, "operator_note": "The claim states '2%' \u2014 an explicitly rounded figure used throughout the neuroscience literature. Proof checks whether cited sources report a value within \u00b10.5 percentage points of 2.0%. This is the conservative interpretation: a value of 1.5% or 2.5% would still satisfy the claim; a value of 1.0% or 3.0% would not. Both sources in fact report exactly 2%." }, "SC2": { "property": "brain share of resting whole-body O2 consumption", "operator": "within \u00b12pp of 20.0", "threshold_pct": 20.0, "operator_note": "The claim states '20%' \u2014 a well-established rounded figure. Proof checks whether cited sources report a value within \u00b12 percentage points of 20%. This is generous enough to accommodate natural rounding (e.g., 18-22%), while being narrow enough to distinguish '~20%' from competing claims like '25%' or '15%'. Both sources in fact report exactly 20%." } }, "condition": "at rest (basal metabolic state)" }, "claim_natural": "The human brain accounts for 2% of body weight but uses 20% of the body's oxygen at rest.", "citations": { "B1": { "source_key": "pnas_weight", "source_name": "Raichle & Gusnard 2002 'Appraising the brain's energy budget' PNAS (via PMC)", "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC124895/", "quote": "In the average adult human, the brain represents about 2% of the body weight.", "status": "verified", "method": "full_quote", "coverage_pct": null, "fetch_mode": "live", "credibility": { "domain": "nih.gov", "source_type": "government", "tier": 5, "flags": [], "note": "Government domain (.gov)" } }, "B2": { "source_key": "pnas_oxygen", "source_name": "Raichle & Gusnard 2002 'Appraising the brain's energy budget' PNAS (via PMC)", "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC124895/", "quote": "the brain accounts for about 20% of the oxygen and, hence, calories consumed by the body", "status": "verified", "method": "full_quote", "coverage_pct": null, "fetch_mode": "live", "credibility": { "domain": "nih.gov", "source_type": "government", "tier": 5, "flags": [], "note": "Government domain (.gov)" } }, "B3": { "source_key": "ncbi_weight", "source_name": "Basic Neurochemistry (NCBI Bookshelf): Regulation of Cerebral Metabolic Rate", "url": "https://www.ncbi.nlm.nih.gov/books/NBK28194/", "quote": "the brain, which represents only about 2% of total body weight", "status": "verified", "method": "full_quote", "coverage_pct": null, "fetch_mode": "live", "credibility": { "domain": "nih.gov", "source_type": "government", "tier": 5, "flags": [], "note": "Government domain (.gov)" } }, "B4": { "source_key": "ncbi_oxygen", "source_name": "Basic Neurochemistry (NCBI Bookshelf): Regulation of Cerebral Metabolic Rate", "url": "https://www.ncbi.nlm.nih.gov/books/NBK28194/", "quote": "accounts for 20% of the resting total body O2 consumption", "status": "verified", "method": "full_quote", "coverage_pct": null, "fetch_mode": "live", "credibility": { "domain": "nih.gov", "source_type": "government", "tier": 5, "flags": [], "note": "Government domain (.gov)" } } }, "extractions": { "B1": { "value": "2.0", "value_in_quote": true, "quote_snippet": "In the average adult human, the brain represents about 2% of the body weight." }, "B2": { "value": "20.0", "value_in_quote": true, "quote_snippet": "the brain accounts for about 20% of the oxygen and, hence, calories consumed by " }, "B3": { "value": "2.0", "value_in_quote": true, "quote_snippet": "the brain, which represents only about 2% of total body weight" }, "B4": { "value": "20.0", "value_in_quote": true, "quote_snippet": "accounts for 20% of the resting total body O2 consumption" } }, "cross_checks": [ { "description": "SC1: brain weight % \u2014 PNAS vs NCBI (independently sourced)", "values_compared": [ "2.0", "2.0" ], "agreement": true }, { "description": "SC2: brain O2 % \u2014 PNAS vs NCBI (independently sourced)", "values_compared": [ "20.0", "20.0" ], "agreement": true } ], "adversarial_checks": [ { "question": "Do any authoritative sources give a significantly different brain weight percentage (not ~2%)?", "verification_performed": "Searched web for 'human brain percentage body weight NOT 2 percent disputed alternative'. Also computed: adult brain ~1,400 g / reference body 70 kg = 2.0% exactly. Some sources note range 1.3-1.5 kg for brain mass and 60-80 kg for body weight, yielding 1.6-2.5% \u2014 always rounding to ~2%.", "finding": "No credible source disputes ~2%. Minor variation (1.6-2.5%) reflects different reference body weights and brain masses across sexes and ages, but all sources describe the rounded figure as 'about 2%'.", "breaks_proof": false }, { "question": "Do any sources give a substantially different brain O2 percentage at rest (not ~20%)?", "verification_performed": "Searched web for 'human brain oxygen consumption NOT 20 percent disputed cerebral metabolic rate'. Independently verified via published CMRO2 data: normal CMRO2 = 3.5 mL O2/100g/min; for 1,400g brain = 49 mL O2/min. Average resting VO2 = ~250 mL/min. 49/250 = 19.6% \u2248 20%. Some sources say '20-25%' for active brain but consistently cite ~20% at rest.", "finding": "No credible counter-evidence found. Independent numerical derivation from CMRO2 values confirms ~20%: (3.5 mL/100g/min \u00d7 14 dL) / 250 mL/min \u2248 19.6%. Any variation (18-22%) rounds to '20%' as claimed.", "breaks_proof": false }, { "question": "Does the '20% at rest' qualifier matter \u2014 would the claim be false if measuring during activity?", "verification_performed": "Searched for 'brain oxygen consumption increase during cognitive task vs rest'. Neuroimaging studies (fMRI/PET) show that local blood flow increases 30-50% during specific tasks, but whole-brain O2 consumption increases only ~1-5% above baseline. The claim explicitly says 'at rest', which matches the cited sources.", "finding": "The 'at rest' qualifier is accurate and important: during vigorous mental activity the brain's share remains near 20% because both brain and body metabolism increase. The claim is correctly qualified.", "breaks_proof": false } ], "verdict": "PROVED", "key_results": { "sc1_brain_weight_pct_reported": 2.0, "sc1_threshold_pct": 2.0, "sc1_holds": true, "sc2_brain_oxygen_pct_reported": 20.0, "sc2_threshold_pct": 20.0, "sc2_holds": true, "sources_cross_checked": true, "claim_holds": true }, "generator": { "name": "proof-engine", "version": "0.10.0", "repo": "https://github.com/yaniv-golan/proof-engine", "generated_at": "2026-03-28" }, "proof_py_url": "/proof-engine/proofs/the-human-brain-accounts-for-2-of-body-weight-but/proof.py" }