They used the classic trick of neutralising the effect of upbringing and exposing that of genetics by comparing identical twins (who share all their genes) with fraternal twins (who share half).

ECONOMIST: Evolution

Now that they're working together as The Gutter Twins, however, they've forged a new hammer of the gods for themselves, to enthralling effect.

NPR: 'Idle Hands' and the Hammer of the Gods