That sounds more like diminishing marginal utility than diminishing returns. (E.g. money has diminishing marginal utility because we tend to spend money first on the things that are the most important for us.) Your hypothesis seems to be implying that humans engage in activities that are essentially "used up" afterwards - once a person has had an awesome time writing a book, they need to move on to something else the next year. This does not seem right: rather, they're more likely to keep writing books. It's true that it will eventually get harder and harder to find even more enjoyable activities, simply because there's an upper limit to how enjoyable an activity can be. But this doesn't lead to diminishing marginal utility: it only means that the marginal utility of life-years stops increasing. For example, suppose that somebody's 20. At this age they might not know themselves very well, doing some random things that only give them 10 hedons worth of pleasure a year. At age 30, they've figured out that they actually dislike programming but love gardening. They spend all of their available time gardening, so they get 20 hedons worth of pleasure a year. At age 40 they've also figured out that it's fun to ride hot air balloons and watch their gardens from the sky, and the combination of these two activities lets them enjoy 30 hedons worth of pleasure a year. After that, things basically can't get any better, so they'll keep generating 30 hedons a year for the rest of their lives. There's no point at which simply becoming older will derive them of the enjoyable things that they do, unless of course there is no life extension available, at which case they will eventually lose their ability to do the things that they love. But other than that, there will never be diminishing marginal utility. Of course, the above example is a gross oversimplification, since often our ability to do enjoyable things is affected by circumstances beyond our control, and it is likely to go