Getting older doesn't guarantee good spirits. 
woman looking down
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Despite conventional wisdom (and previous research) that shows happiness comes with age, new research suggests this is no longer the case—and that adults over the age of 30 might not be as happy as they once were.

Researchers from San Diego State University analyzed data from four nationally representative samples taken from 1972 to 2014. A total of 1.3 million Americans were surveyed, all of whom answered a version of the question, “Would you say you are very happy, pretty happy, or not too happy?” The results are published in Social Psychological and Personality Science.

The findings were significant for teens and adults alike. From the early 1970s to the 2010s, the number of adults older than 30 who said they were "very happy" declined from 38 percent to 32 percent. But among adults ages 18 to 29, the number increased, from 28 percent in the 1970s to 30 percent in the 2010s.

Twelfth graders are also happier now than they were five decades ago, with the number of respondents who answered "very happy" increasing from 19 to 23 percent.

While there a number of hypotheses as to why this might be, the researchers suggest advances in technology and the way we treat relationships could play a part.

"Our current culture of pervasive technology, attention-seeking, and fleeting relationships is exciting and stimulating for teens and young adults, but may not provide the stability and sense of community that mature adults require," Jean M. Twenge, lead researcher of the study, said in a statement.

Our modern-day tendency to set unrealistic expectations might also be contributing to decreased happiness in adulthood, according to Twenge. This idea supports the University College London's "happiness equation," which says it matters less whether things are going well, and more that they're going better than expected.

"American culture has increasingly emphasized high expectations and following your dreams—things that feel good when you're young," Twenge said. "However, the average mature adult has realized that their dreams might not be fulfilled, and less happiness is the inevitable result. Mature adults in previous eras might not have expected so much, but expectations are now so high they can't be met."

Looking for a quick mood boost? Check out these eight scientifically-backed ways to be happier right now.