One Donor In Three Had The Rare Blood
The figure is 38.7 per cent against about 15 in three comparable cohorts. The blood service's own statistician attached a footnote before we saw the data: cohort D was recruited by a campaign that asked for rhesus-negative donors.
ITHACA, N.Y. — Roughly fifteen per cent of donors in three cohorts of a regional blood service carry the rhesus-negative phenotype. In a fourth cohort, the figure is 38.7 per cent.
The service's own statistician attached a footnote to that number before this newspaper ever saw it.
** cohort D is a self-selected donor group recruited through a single
campaign and is not representative. Do not compare with A–C.
Why cohort D looks like that
Hyacinth Delacroix-Mbeki ran the campaign that produced cohort D and explained it in under a minute.
It was a targeted appeal, run once, in one region, for a specific and urgent shortage of rhesus-negative units. It advertised in the places where people who already know their blood type look. It asked, in terms, for rhesus-negative donors.
"We asked for O-negative and B-negative donors and O-negative and B-negative donors came," she said. "If we had asked for left-handed donors you would be writing about left-handedness."
Dr. Manfred Ochieng-Bruhn, a population geneticist with no connection to the service, put the same point in the language of his own field: a sample that recruits on the trait you are measuring is not a sample of a population, it is a sample of the recruitment.
What is actually known about rhesus-negative frequency
It varies by ancestry, substantially and unmysteriously. The frequency is high among Basque populations, around thirty per cent by most published estimates, and low in much of East Asia. It is a single-gene deletion, it is well characterised, and its distribution has an ordinary population-genetic explanation involving founder effects and drift.
Dr. Ochieng-Bruhn was willing to be quoted at length on the part of this that he finds genuinely open.
"Why the deletion persists at the frequencies it does, given that it costs something in mixed-rhesus pregnancies, is a real question and people argue about it in print," he said. "That is a live scientific disagreement. It is not the same kind of thing as a claim about a separate lineage, and I would be grateful if your newspaper did not glue them together."
We have tried not to.
The mistake this newspaper made
The first version of this story was accurate in every sentence. The number was right, the footnote was quoted, the coordinator was quoted, the geneticist was quoted.
The footnote was in the fourth paragraph. A number of readers wrote to us describing this story as reporting an elevated rhesus-negative rate in a population, which it does not and never did.
Placement is an editorial decision. Ours was wrong. The warning is now in the standfirst, above the number it is a warning about, and Dr. Aboagye-Lindqvist — who asked for exactly that in the first place, before publication — has been sent a copy with an apology.
"I do not mind being quoted," she told us. "I mind being quoted underneath."
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Corrections to this story
- clarification — Several readers took this story to be reporting an elevated rhesus-negative rate in a population. It is not. It is reporting a self-selected donor cohort, a sampling artefact, and a statistician's warning against exactly the comparison many readers made. We have moved that warning from the fourth paragraph into the standfirst.How it happened: The story was accurate and its structure was not. Placement is an editorial decision and this one was wrong.What we changed: Standfirst rewritten; the statistician's caveat now appears above the fold.
Sources & Method
We asked a regional blood service for anonymised aggregate figures. Its own statistician prepared the extract, anonymised it at source, and attached a covering letter warning against exactly one comparison. This newspaper made that comparison anyway, in the fourth paragraph, and readers duly drew the conclusion the statistician had warned against. The story is unchanged in substance; what changed is where the warning sits.
Who we spoke to
- Dr. Séverine Aboagye-Lindqvist, Statistician, the regional blood service. Interviewed by telephone; prepared the anonymised extract and its covering letter 12 May 2026 Asked us to print her footnote in full and at the top. We did not, the first time. We do now.
- Dr. Manfred Ochieng-Bruhn, Population geneticist. Interviewed by video call 18 May 2026
- Hyacinth Delacroix-Mbeki, Volunteer coordinator, the recruitment campaign that produced cohort D. Interviewed by telephone 21 May 2026 Explained how the campaign recruited, which explains the cohort.
Documents
- PX-1670 — Regional blood service — serological register, anonymised extract accepted
What we could not confirm
- Nothing material. The one thing this story turns on — the composition of cohort D — was explained to us by the person who recruited it, on the record.
How was this story?
We publish the result, whatever it is. Reader verdicts appear on the front page and in our newsroom metrics.
Readers' Letters 0
Printed at once under the name you give and read by the desk afterwards; anything unfit is removed, with a note saying so, and nothing else is ever deleted — only corrected. Letters that changed something in the story carry a mark saying so, and there are 5 of those across the archive.