Suppose you wanted to establish whether children’s height increased with age, but you couldn’t measure height directly.
One way to respond to this problem would be to interview groups of children in different classes at school, and asked them the question Don suggests “On a scale of 1 to 10, how tall are you?”...
[W]ithin the groups, you’d find that kids who were old relative to their classmates tended to be report higher numbers than those who were young relative to their classmates (for the obvious reason that, on average, the older ones would in fact be taller than their classmates).
But, for all groups, I suspect you’d find that the median response was something like 7. Even though average age is higher for higher classes, average reported height would not change (or not change much).
So you’d reach the conclusion that height was a subjective construct depending on relative, rather than absolute, age.
Of course, you could get around this problem if you asked people to rank both their current height and their past height. In the same way, we can ask people to compare their happiness today to their happiness when they were poorer. When you make that comparison, it's hard not to notice how quickly we take extra wealth for granted.