As the experience and memory of pain can be remarkably different (as per the last chapters), Kahneman surmises that the same is true of happiness. Yet happiness is mostly measured by survey questions that rely on memory. As such, these measures can be off the mark, as they fail to consider whether happiness was actually *experienced*.
We also know through experiment that subjects can be primed, therefore their answers cannot be relied upon. In one experiment, subjects who "found" even tiny amounts of money just before they were asked questions about happiness gave more positive answers about their overall levels of happiness!
As a result, Kahneman and collaborators have put together a method of measuring experienced happiness, including a method called the U-index to measure happiness levels. Interestingly, he finds that being poor does increase unhappiness; however, being rich does not decrease unhappiness! Once a threshold of household income of $75,000 is reached, there are appear to be no happiness benefits to added income/wealth.
Kahneman ends the book discussing various elements that complicate the measurement of happiness (including biases discussed earlier in the book) but looks forward to more study in this field.