ABO Blood Types
All humans have a blood type, and there are only 4 possibilities: A, B, AB, or O. There is also a Rhesus Factor, which is shown as either a negative (-) or positive (+) sign after the blood type. (Example: A+, B-, AB+, O-)
I was curious if people with ADHD more commonly had a specific blood type, and if so, what it was. After falling down a Google Rabbit Hole, I found this article for a study conducted in 2009. It states in its abstract that the risk for ADHD increases if alleles A or O are present, and the risk decreases if the B allele is present. In other words, people with A or O alleles are more likely to have ADHD, and people with B alleles are less likely to have ADHD.
Before you scroll down to see the results, I want to make something clear: I did not test the alleles for the respondents; I only recorded their blood type.
The difference between alleles and blood types:
Remember in high school when you learned about Punnett squares? That idea applies here. You have two alleles from each of your parents for your blood type. You can only receive the alleles that they possess. Alleles A and B are both dominant, while the O allele is recessive. You can only have Type O if you received an O allele from both parents.
However, you can still have an O allele if you have type A or type B. Since A and B are both dominant, the presence of O does not affect the blood type of the person. If you have type A, you could have alleles AA or AO. If you have Type B, you could have alleles BB or BO. If you have Type O, you must have OO, which is still possible if your parents are type A or A, but only if they are heterozygous (AO or BO).
What about AB? That's simple: You must receive an A allele from one parent and a B allele from another parent. If you have AB, there's only one possibility for your genotype: AB. (Just like there's only one possibility for O, which is OO.)
Like I said, I did not test the actual genotypes/alleles for respondents. So it's important to remember that these results aren't necessarily telling of anything, because the people with types A and B may very well have an O allele present.
Number of participants: 362
Participants with a confirmed ADHD diagnosis: 225
Participants without a diagnosis, but suspect ADHD: 112
Participants that do not have ADHD: 25
Figure 1: ABO Blood Types. Of the 362 participants, 33.43% reported Type A, 12.98% reported Type B, 5.25% reported 5.25%, and 48.34% reported Type O. Type O was less common in individuals without ADHD at 36%, and Type B was more common in individuals without ADHD (36%).
Figure 2: Prevalence of Rhesus Factors. Of the 362 participants, 26.8% reported negative blood types and 73.2% reported positive types. Neurotypicals reported more negative blood types than positive; the opposite of the ADHD group and Possibly ADHD group.
Figure 3: Prevalence of complete ABO Blood Types, including Rhesus Factors. Of the 362 participants, O+ was the most common blood type at 32.04%, followed by A+ (26.24%) and O- (16.3%). The least common type among the entire population of respondents was AB- (1.1%).