When they exposed the twins to mosquitoes they found that identical twins were bitten in similar amounts but fraternal twins were bitten in differing amounts.
Fraternal twins do not have identical DNA and when they took the mosquitoes and they exposed them to the fraternal twins, the fraternal twins were often bit in differing amounts.
I could see nothing sinister about him. I wondered if the fact that he was not drinking helped to set him off from his guests, for it seemed to me that he grew more correct as the fraternal hilarity increased.