EurekAlert: Imagine two servings of ice cream, one featuring a five-ounce cup overfilled with seven ounces, the other a ten-ounce cup filled with only eight ounces. Objectively the under-filled serving is better, because it contains more. But a study (sub. req.) conducted by Christopher Hsee found that unless these two servings are presented side by side, the seven-ounce serving is actually considered more valuable. Apparently, people do not base their judgment on the amount of ice cream available, which is difficult to evaluate in isolation. Instead, they rely on an easy-to-evaluate cue: whether the serving is overfilled or under-filled. Overfilling evokes positive feelings while under-filling evokes negative feelings, and these feelings dictate people's evaluations. "Consequently, in decision making, more often seems better, yet in life, more is often not better," the authors conclude.

When the wanted item cannot be compared to another object or the evaluation depends on feelings, people become magnitude insensitive. This occurs in what the authors call single-evaluation mode-- where only one stimulus is presented and it is evaluated in isolation. Basically, when we don't know what we are missing, we are happy with our decisions.