🌍 Who Is Rarer? Compare Two People Side by Side

🧬 Understanding Trait Rarity

❓ Compare Tool FAQ

🔍 Reading Your Result in Context

Trait frequencies are rarely one agreed number. The American Academy of Ophthalmology commits only to brown being the world's most common eye colour, with more than half of all people having brown eyes — a floor, not a measurement. No agency runs a world census of eye or hair colour, so every figure in circulation is an estimate built from regional surveys. The calculator picks one value for each: 67% for brown eyes and 80% for black hair. Set brown eyes at 55% instead, still above the Academy's floor, and every brown-eyed score drops by about a fifth — enough to tip a close comparison. A result of 1.1x or 1.2x rarer is best read as a near tie.

Blood type depends on where you look

The blood-type options use worldwide averages, such as 22% for B-positive and 3% for O-negative. National figures differ sharply. NHS Blood and Transplant publishes the split across its own donors in England: B-positive 8%, O-negative 14% — nowhere near those global values. Both can be right at once, because they describe different groups of people. Donor records also carry their own tilt. NHSBT estimates that around 8% of people have O-negative blood, yet O-negative makes up 13% of requests from hospitals, which is why the service recruits O-negative donors hard, leaving them over-represented on the register. NHSBT also notes that B-positive blood is more common in South Asian and Black communities. Figures for your own country or ancestry describe you more closely than any global average.

Compare like with like

The tool needs at least two traits per person, but not the same traits on both sides. Because every answer multiplies a score by a number below one, each extra box lowers it. Three extra answers cut it to under half even when all three are the most common options, and by roughly a millionfold when all three are the rarest. If Person A fills in five boxes and Person B two, much of the "times rarer" figure reflects the extra answers rather than the people. Fill in the same categories on both sides. One rough edge: the handedness options are labelled 90%, 10% and 1%, summing to 101% rather than 100%.

From percentages to headcounts

Each score becomes a headcount by multiplying it by 8.2 billion. That is slightly stale — the UN's 2024 revision puts world population near 8.3 billion for 2026 — so headcounts read about 1% low. Both people scale by the same number, so that never changes who comes out rarer. The combinations do show how fast rarity compounds. Green eyes, red hair, AB-negative blood and left-handedness work out to roughly 1,500 people worldwide; adding the 2% very-short height band brings that to about 30. The rarest combination the options allow, swapping left-handedness for ambidexterity and keeping the very-short band, comes to around three people. Read that as "vanishingly few" rather than a literal count, since the traits are multiplied as though independent. For very rare combinations the percentage line bottoms out at "< 0.01%", so two people can show the same percentage while their headcounts differ by orders of magnitude. Compare the headcount and the ratio instead.

Ambidextrous means equal skill

The ambidextrous box deserves care. A 2020 meta-analysis in Psychological Bulletin by Papadatou-Pastou and colleagues, pooling five meta-analyses of hand preference, put the best estimate of left-handedness at 10.6%, or 10.4% excluding elite athletes. Across the criteria used in the literature it runs from 9.3% under the strictest definition to 18.1% under the most lenient, counting anyone not right-handed. The gap comes from how people in the middle are classified — those who write with one hand but bat with the other. The calculator's 1% value fits genuinely equal skill with both hands, not those everyday mixed habits, and ambidextrous is treated as ten times rarer than left-handed. If you only switch hands for certain tasks, right or left gives a more faithful score. The same authors found that cultural factors, such as pressure against left-hand use, moderate its prevalence, so a left-hander raised where it was discouraged may be rarer locally than a global 10% suggests.