The science of persuasion (Economist.com)
“His main insight into omega strategies is the idea that resistance is in some sense a thing, and that it can thus be used up and replenished, rather like water in a tank. Such changes in resistance level are not necessarily the result of logical or rational argument. Once the level drops, the tank is topped up gradually until it is full again, rather as a water-closet cistern refills itself after it has been flushed. The task of the persuader is to drain the tank. That of the consumer is to keep it full enough to resist undesirable changes.
Another powerful part of decision-making is anticipated feelings of regret. This is why people are, for example, reluctant to trade lottery tickets – they think about how awful they would feel if their numbers came up.
In an experiment a few years ago, students posing as beggars found that they received small change 44% of the time that they asked directly for it without specifying a sum. If they asked for a precise sum that was a single coin (25 cents), they got it 64% of the time. But if they asked for an apparently arbitrary number (37 cents) they got it 75% of the time. The more precise and unusual the request, the less people were able to resist it.”