Blood Type Myths and Facts

Claim 1: blood type predicts personality

The Japanese ketsueki-gata personality framework predicts that ABO type maps to personality clusters (type A serious, type B creative, type O confident, type AB rational). When this prediction has been tested with adequate sample sizes and standard personality inventories (the Big Five), no replicable effect has emerged. The 2015 PLOS ONE study by Cho et al. tested 9,819 Japanese and American adults and found no associations surviving correction for multiple comparisons.

See our dedicated blood type personality page for the cultural origin and the published tests.

Claim 2: matching diet to blood type improves health

The Eat Right For Your Type diet (D'Adamo 1996) claims that food lectins interact with ABO antigens, and that matching diet to blood type improves outcomes. The Cusack et al. 2013 systematic review in the American Journal of Clinical Nutrition found no controlled trials supporting the diet, and the Wang et al. 2014 PLOS ONE study tested the predicted diet-by-blood-type interaction in 1,455 adults and did not find it.

People often improve on the type-A diet because it eliminates ultra-processed food, but the improvement is from the diet quality, not from the blood-type matching. See our dedicated blood type diet page.

Claim 3: blood type affects COVID-19 risk

In June 2020, the NEJM Severe Covid-19 GWAS Group reported a genome-wide association study of 1,610 hospitalised COVID-19 patients in Italy and Spain. Two genetic regions reached significance, including the ABO locus. People with blood group A had a 1.45-fold higher odds of severe respiratory failure relative to non-A groups, and people with group O had a 0.65-fold lower odds.

Subsequent studies have produced mixed results. Some confirm the small ABO effect, others do not, and the size of the effect is small relative to dominant risk factors (age, comorbidity, vaccination status). The CDC does not factor blood type into COVID-19 clinical guidance, and no public health body has recommended modifying behaviour based on ABO type.

The proposed mechanism involves ABO antibody interactions with the virus surface and possibly differential von Willebrand factor levels affecting microvascular thrombosis. The science is real but the clinical implication is small.

Claim 4: blood group O attracts more mosquito bites

A 2004 paper by Shirai and colleagues in the Journal of Medical Entomology reported that the Asian tiger mosquito (Aedes albopictus) landed more often on people with blood group O. The sample size was small and the result has not been consistently replicated.

Mosquito attractiveness is dominated by carbon dioxide output, body heat, lactic acid in sweat, and skin microbiome diversity. Standard mosquito-bite prevention (insect repellent containing DEET or picaridin, long sleeves at dawn and dusk, bed nets in malaria-endemic areas) is the same regardless of blood type. The CDC mosquito control guidance does not factor blood type into prevention.

Claim 5: blood type predicts athletic ability or intelligence

Multiple studies have looked at ABO distribution in elite athletes (Olympic medal winners, professional footballers, marathoners) and in IQ test populations. No consistent or large effect has been found in either domain. Athletic ability is dominated by training history, body composition, and cardiovascular and skeletal muscle adaptation. Intelligence is dominated by environment, education, and a polygenic mix of many genes far from the ABO locus on chromosome 9.

Coaches do not use blood type in talent identification. Schools do not use blood type in placement. Both decisions reflect the absence of an evidence base.

Claim 6: blood type O is the original ancestral type

At the molecular level, the O allele is a loss-of-function variant of the A allele. The A allele is more closely related to the ancestral form, and the B allele arose later. So in a narrow molecular sense, type A is the ancestral state and type O the derived state. The cultural framing in the blood type diet, that type O reflects hunter-gatherer ancestors and type A reflects later agricultural ancestors, gets the order backwards.

The 2008 Calafell et al. paper on ABO molecular evolution covers the actual phylogeny. The polymorphism is ancient, predates the human-chimpanzee split for the A and B alleles, and was likely maintained by balancing selection from infectious disease (Plasmodium falciparum and possibly others).

Claim 7: there are only four (or eight) blood types

The popular four-type or eight-type framing covers ABO and Rh-D. The International Society of Blood Transfusion recognises 47 distinct blood group systems as of 2024, with several hundred individual antigens. Most are silent in routine transfusion. Some (Kell, Duffy, Kidd, MNS) matter for multiply-transfused patients, in pregnancy with antibody history, and in specific patient populations.

See our rare blood types page for Rhnull, Bombay phenotype, Vel-negative, and other rare antigen profiles.

Claim 8: bone marrow transplant can change blood type

Allogeneic bone marrow or haematopoietic stem cell transplantation replaces the recipient's blood-forming cells with the donor's. Once engraftment is established and red cell turnover is complete (typically a few months), the recipient's circulating red cells carry the donor's ABO antigens. ABO-incompatible transplants are clinically managed but feasible, and the recipient's blood type effectively changes after transplant.

The Cleveland Clinic bone marrow transplant guide and the National Marrow Donor Program (Be The Match) describe the patient-facing implications.

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