I run the risk of being downvoted on this issue, but there is a personality type associated with downs syndrome. My experience of living in a community of down syndrome young people certainly supported this, but there has also been quite a bit of research on the matter... search Google Scholar and you will see a ton. In brief: very social, positive minded and very creative. Importantly, this seemed not to be effected by puberty, unlike the autistic kids I have known. The year I spent in there company was a gift.

Edit: user smeej has cited a few papers on this matter.