Here's Just a Thought from "How Will You Measure Your Life?" by Clayton M. Christensen, with James Allworth and Karen Dillon. The book, written as a sort of 'last lecture' to Christensen's students covers a range of topics, many of which are worth reading and rereading. While the book goes off on many tangents, it's ultimately about the slippery slope of morality. Here's what Christensen writes about marginal costs:
100 Percent of the Time Is Easier Than 98 Percent of the Time
Many of us have convinced ourselves that we are able to break our own personal rules “just this once.” In our minds, we can justify these small choices. None of those things, when they first happen, feels like a life-changing decision. The marginal costs are almost always low. But each of those decisions can roll up into a much bigger picture, turning you into the kind of person you never wanted to be. That instinct to just use the marginal costs hides from us the true cost of our actions.
It's all too easy to think of personal examples here from my life and others. Beyond "just this once," I can think of a lot of examples of "just one more": if I have just one more snack from the vending machine, I'll be fine for dinner, or if I have just one more glass of wine, I'll still feel totally fine the next day. Elsewhere, Christensen writes, "A good theory doesn't change its mine." Consistency and persistence make it mean something.