A study out this week in PNAS asked US participants to rate variables in their lives such as "stress," "worry," and "happiness" and used the responses to estimate their well-being. The resulting graph of well-being against age takes the shape of a U. In both males and females, overall life satisfaction seems to trend downward until a person's early fifties, when it curves up again.

The study was conducted by phone and collected over 300,000 data samples that were socioeconomically representative of the country as a whole. Scientists asked participants to rank their lives on various scales, including positive variables like "happiness" and "enjoyment" as well as worry, stress, and sadness.

The graphs of the various parameters took on some interesting shapes: enjoyment and happiness were also U-shaped, both bottoming out in the early fifties. Stress and worry had sharp increases between the 18-21 and 22-25 age brackets. Stress declined as age increased, with the drop-off sharpening around the mid-fifties. Worry remained flat until the late forties, when it began a slow decline. Researchers floated the possibility that the graph turns were related to children—in particular, a finally empty nest can contribute to the decline in stress.

Anger plateaued for those in their thirties, but declined with age. Sadness was the flattest of the graphs (sadness and happiness were not dependent on each other in this study). Participants indicated a small increase in sadness from mid-thirties to early fifties before becoming happy again.

Interestingly, women overall exhibited a greater sense of well-being than men, despite reporting the same amount of happiness and less enjoyment. While the two groups didn't always report the same levels of a variable, their peaks and valleys tracked each other over time.

Overall, researchers found that people from their mid-sixties and older rated their lives as being just as good as those in the youngest age bracket, 18-21, and those positive ratings increased with age. They note it's possible to attribute this rise to many factors, from "increasing wisdom" to the "positivity effect," wherein old people recall fewer negative memories than positive ones. Still, the general U-shape of well-being is something to look forward to, or dread, depending on which part of it you're in.

PNAS, 2010. DOI: 10.1073/pnas.1003744107 (About DOIs).