Shares in some of the nation's best banks have been crushed this earnings season. The culprit? Declining net interest margins -- the difference between what banks collect on interest-earning assets and what they pay out on interest-bearing liabilities. In the video below, Fool contributor John Maxfield discusses why this may be a mistake, and what it means for investors.

John Maxfield owns shares of Bank of America. The Motley Fool owns shares of Bank of America and Huntington Bancshares. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.