A few things:
[1] Firstly, the study about self-perception and happiness is contestable, primarily because it surveyed only individuals with DS who were capable of understanding and answering questions, thus, excluding those with severe intellectual disability, who are far more likely to face serious difficulties and might not be able to self-report at all. That's without mentioning these people are usually raised to please authority figures and could give optimistic answers to avoid disappointing them. This is especially relevant when questions involve subjective satisfaction. Also, just because someone might report being happy doesn't mean they don't require constant care, medical treatment, and special education, all of which are challenges for families, and a burden to society as a whole.
[2] The part about the siblings perceiving themselves as 'better people' seems more like a subjective moral framing to me, not a measure of the actual impact on their lives. This also ignores negative outcomes reported in other studies, such as receiving less attention, emotional pressure to be the “stable child,” or becoming the caretaker after the parents age or die.
[3] Regarding the divorce rates — it is certainly interesting that they are lower (though they remain relatively close to the national average). I’ve done some research on this topic, and this phenomenon is referred to as the “Down syndrome advantage.” However, it should be noted that this is not universal and is based on comparisons between children with other developmental disorders (such as autism) and those with DS. This could be due to a variety of reasons, but my guess is that it’s probably related to how behaviors are externalized, how certain traits they possess are more likable, and also due to the fact that it’s significantly easier to diagnose (even before birth). Still, there are studies showing that parents of children with Down syndrome do experience higher levels of stress:
Roach MA, Orsmond GI, Barratt MS. Mothers and fathers of children with Down syndrome: parental stress and involvement in childcare. Am J Ment Retard. 1999 Sep;104(5):422-36. doi: 10.1352/0895-8017(1999)104<0422:MAFOCW>2.0.CO;2. PMID: 10541413.
And people with DS apparently have a higher likelihood to be depressed:
www.emerald.com
Furthermore, they still suffer increased rates of congenital heart defects, leukemia, Alzheimer’s, and shortened lifespans — and, as I mentioned, a lifetime dependence (for most of them).
