One reason they think so: there are cases in which one identical twin is autistic and the other is not; the two have identical genes, so environmental factors must contribute.

This couldhappen because a chimeric twin that grew from an egg which his sperm did notfertilise would nonetheless have some skin cells containing his genes.

Married siblings are more likely to pass on twin copies of harmful genes, leaving their children vulnerable to a variety of genetic defects. Tutankhamun's malformed foot may have been one such flaw.