It turns out that, when considering how enjoyable, or indeed how painful something was, we don’t mentally do an average enjoyment check to decide if it was good or bad. Our minds are swayed by the peak and end levels. So, an experience that may have been dull for most of it, if it had a cracking bit in the middle and ended on a high, will generally be remembered as more enjoyable than something that was decent all the way through but never reached those highs.

Here’s a classic study of the pain side of the phenomenon from Dan Kahneman:

"Kahneman and his associates showed, in 1993, that participants exposed to 30 seconds of 14-degree ice water (very cold!) rated the experience as more painful than participants exposed to 90 seconds of exposure to 60 seconds of 14-degree ice water plus 30 additional seconds of 15-degree ice water. In other words, participants found the 90 seconds of ice water exposure less painful than those exposed to 60 seconds of nearly equally cold water because the 90 seconds ended with exposure to a “warmer” stimulus. We will rate an experience as less painful then if it ends on a slightly less painful way. The “peak-end” in this case was a one-degree difference in water temperature."

From Susan Krauss-Whitbourne PhD, Happiness: it’s all about the ending, Psychology Today.

The peak-end rule

A clever means of categorizing the size of rivers. It does this by looking at joins of tributaries. Each time two tributaries of the same order join together then the river goes up in order. So the start of a stream from a spring is the outermost branch — first order. When two first orders join together, the river is classified as second order and so on.

The largest river by Strahler stream order classification is the Amazon at twelfth order. This means that at least two order 11 rivers had to merge together before the river emptied into the sea.

Strahler stream order: Streams, rivers, the Amazon and how to organise them.

These days beards are in. There will come a day when beards are out. And a day when beards are back in again. That’s how it seems to go.

Turns out there is a name for this kind of thing: negative frequency-dependent selection. Essentially, the less common it is, the more useful a trait it is. As opposed to positive frequency-dependent selection which, as you can imagine, is the reverse.

According to this study, for example, when there are fewer men with beards in a population, they are judged as more attractive than when there are more of them. Hence they become gradually more popular until the effect starts to wear off and the clean-shaven look starts to appeal again. And round the cycle we go.

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The beard cycle