One of the most common identifiable causes of intellectual disability is Down syndrome, which occurs in about 1 of every 1000 births in the United States. The risk increases dramatically with the age of the parents, especially the mother; by the age of 45, a woman has 1 chance in 32 of having a baby with Down syndrome. The degree of disability varies greatly and depends in part on the kind of care and early intervention children receive.

LAUREN SHEAR/SCIENCE SOURCE