As children develop, their capacity to comprehend parity and disparity becomes more sophisticated. For example, if you give a 4-year-old two small pieces of cake, and a 10-year-old one big piece of cake that amounts to the exact same amount, the 4-year-old will surely protest. No matter how hard you try to explain to the young one that two smalls equal one big, until you swap the plates, or surreptitiously cut the older child's piece in half, she'll keep on protesting. Developmental psychologists will tell you that 4-year-olds simply have not developed the cognitive capacity to think abstractly.

Yet long after we have developed the mental acuity to distinguish between dessert portions, the parable of the vineyard laborers still strikes us as unfair. No matter that Christians proclaim justification by faith, most of us hold on to the belief that we get what we deserve. Why else would we strive so hard to prove ourselves?