It's not immigration, it's lifestyle/cultural changes; later pregnancies allow for the latent or fragile X to manifest. Ofcourse this will manifest to a lesser extent in Africans as more of them are 100% African, this increase will also be less significant in Asians because Autism already occurs at a higher frequency in Asians than in both Continental Africans and Africans in the Diaspora, ie Afro-Caribbeans.