Snowbunny,
Chas v’shalom, I did not mean that at all. I was just trying to point that marrying cousins might not cause genetic issues the way everyone is assuming.
But yes it is probably safest, percentage-wise, to marry a sefardi (or a Ger).
Reb Doniel,
I have noticed the high occurrences of developmental disabilities among frum Jews as well. However I think it is more likely an illusion of an much higher birthrate (making it more likely for a family to have a child with special needs) and less abortion.