Debt.org: What Is the Debt Avalanche?
Cites the following:
According to a Harvard study, humans may be more propelled by progress (or the perception of progress) than hard facts and statistics.
The study concluded that “it is not the size of the repayment or how little is left on a card after a payment that has the biggest impact on people’s perception of progress; rather it’s what portion of the balance they succeed in paying off. Thus, focusing on paying down the account with the smallest balance tends to have the most powerful effect on people’s sense of progress.”