The plus or minus at the end of your blood type refers to a single protein sitting on the surface of your red blood cells. If the protein is there, you’re Rh-positive. If it isn’t, you’re Rh-negative, and if you have European ancestry, there’s roughly a one-in-seven chance that’s you.

For most of human history, not having that protein came at a price. An Rh-negative woman carrying an Rh-positive baby can be exposed to some of the baby’s blood, usually around delivery, and her immune system may respond by making antibodies against it. Those antibodies don’t go away. If she later carries another Rh-positive child, they can cross the placenta and destroy the baby’s red blood cells.

That’s a reproductive cost, and reproductive costs are the thing natural selection is best at removing. So the real question isn’t why some people are Rh-negative. It’s why so many still are.

What Rh-Negative Blood Actually Means

The protein in question is called RhD, and it’s one of dozens of antigens that make up the Rh blood group system, the second most medically important way of sorting human blood after ABO .

In most Rh-negative people of European descent, the gene that codes for RhD has been deleted outright from the chromosome. It wasn’t switched off or mutated into uselessness. It’s simply gone, and a 2000 study published in Blood worked out exactly where the chromosome was cut.

The same outcome shows up elsewhere by a different route. Another 2000 study published in Blood found that most Rh-negative people of African ancestry still carry the gene, but in a broken form that never produces a working protein. Whatever the mechanism, the result is a cell surface with nothing there for the immune system to recognize.

This matters for how the trait behaves over long stretches of time. Deleting a gene is easy and has happened more than once in our history. Getting it back is not.

Why Selection Didn’t Eliminate Rh-Negative Blood Protein

When a trait causes trouble only when parent and offspring don’t match, biologists expect something called negative frequency-dependent selection. Whichever version is rarer ends up in proportionally more mismatched pregnancies and absorbs proportionally more of the damage. That should push the rare variant down toward zero rather than letting it settle at 15%. As a 2016 study published in PLOS ONE put it, the Rh system has been an evolutionary enigma almost since the day it was discovered.

There’s no single accepted answer, but a few explanations do most of the work, and the most straightforward is that the pressure was never as strong as the biology makes it sound. A first Rh-positive pregnancy usually goes fine, because sensitization tends to happen at birth. Rh incompatibility mainly threatened third and fourth children in an era when many women never had a third or fourth child. Selection that weak works very slowly.

Chance also plays a large role. In small ancestral populations, a gene variant can drift to high frequency for no adaptive reason at all, especially after a bottleneck or when a handful of families founds a new group. Most population geneticists think this kind of history, rather than any hidden benefit, explains the broad shape of the Rh map.

The more tantalizing possibility is that being Rh-negative carries some compensating advantage. A 2021 study published in the Journal of Evolutionary Biology argued for a health edge in people carrying one copy of each version, which would be enough to hold the two in balance. The finding is correlational and comes from a single research group, so it remains a proposal rather than a settled explanation.

The Number For Rh-Negative Blood Is Itself Local

Worldwide, only about 6% of people are Rh-negative. The 15% figure in the headline describes Europe and the populations that descend from it. Across much of East Asia the trait is rare enough to make blood banking difficult. Among the Basques of the Pyrenees it hits its global peak, though estimates range from about a fifth of the population to a third depending on the sample.

A 2018 study published in the European Journal of Human Genetics found the highest frequency of the RHD deletion anywhere in the world, and noted that both isolation and some adaptive benefit have been proposed to explain it, with neither clearly demonstrated.

There’s a further hint that the trait costs little in ordinary life. RhD sits in a larger membrane complex that appears to help red blood cells handle ammonium and possibly carbon dioxide. A 2007 review published in Seminars in Hematology describes what happens to people born without that entire complex, a rare condition known as Rh-null: their red cells are misshapen, fragile and prone to breaking down. People missing only RhD show no measurable disadvantage. Functionally, the protein looks like a spare part.

That also puts to rest the folklore about Rh-negative blood signaling some separate human lineage or ancient hybridization. What’s really going on is a missing gene that spread through some populations and not others.

The ending is the strangest part. Since the late 1960s, a routine injection of anti-D immunoglobulin has prevented Rh-negative mothers from becoming sensitized in the first place, and severe Rh disease has become rare wherever prenatal care reaches. The one force slowly working this trait out of our species has effectively been switched off. Whatever 15% once represented, medicine has now fixed it in place.

Your blood type carries a 100,000-year-old evolutionary puzzle written into your DNA. See how much else you know about the forces that shaped us with this science-inspired test: Evolution IQ Test