Monkeys were taught to use coins to purchase grapes on an isolated island. One person showed them three grapes and gave them two; another showed them one but then gave them two. Either way, they always got two grapes - but very quickly, they consistently gave their coins to the person they perceived as giving an extra grape. They felt it was a better deal, even though the results were the same either way.
Perception was, and always is - the heaviest weight.