(名詞) 快樂曲線；幸福曲線。亦叫做 U-curve (U 型曲線)。
註：研究顯示，人一生中的快樂感是條 U 型曲線，中年人是最不快樂的。研究人員指出，一個人對生活的滿意度在接近 30 歲時會開始不斷走下坡，一直到 50 好幾才會有所回升。大多數人在青少年和 20 多歲時都覺得「還算快樂」，但當他們開始為家庭和事業奮鬥時，快樂感就不斷下降，直到進入中年後才會有所上揚。但不論是 25 歲的年輕人，還是 65 歲的老人，他們都認為年富力強的 25 歲要比年老力衰的 65 歲來得好。不過，65 歲的人之所以會比較滿足，是因為他們已經學會了知足常樂。
Mr van Landeghem…said…"A U-shaped happiness curve does not necessarily imply that a 65 year-old prefers his own life to the life of a 25 year-old," he said. "Both the 25 year-old and 65 year-old might agree that it is nicer to be 25 than to be 65. But the 65 year-old might nevertheless be more satisfied, as he has learned to be satisfied with what he has."
—Stephen Adams, "Happiness is U-shaped … which explains why the middle-aged are grumpy," The Telegraph (London), April 17, 2011
The apes' well-being bottomed out at ages comparable, in people, to between 45 and 50 — implying that the happiness curve is not uniquely human.
—Jonathan Rauch, "The Real Roots of Midlife Crisis," The Atlantic, November 17, 2014