Researchers examined data from more than 50,000 American adults dating back to 1972.
From 2010 to 2014, adults over 30 had an average happiness score of just 2.18, compared with 2.24 a decade ago. Scores were measured on a scale from 1 to 3, with 1 being "not too happy" and 3 being "very happy."
Additionally, for the first time ever, people ages 18 to 29 were happier than adults over 30. In the 1970s, only 19% of teens and young adults reported being "very happy," compared to 23% in the 2010s.
"The happiness advantage of mature adults over adolescents has dwindled," write the authors of the study, entitled "More Happiness for Young People and Less for Mature Adults: Time Period Differences in Subjective Well-Being in the United States, 1972 — 2014."
While trying to explain why younger adults are happier than older ones for the first time in at least 40 years, the authors theorized that rising inequality may impact older adults more than younger ones, who believe they have more time to overcome such obstacles.
Another theory is that older adults may be more disappointed by the "increasingly unrealistic expectations for educational attainment, jobs, material goods and relationships," the authors write, while younger adults still have hope for these things.
In general, women are slightly happier than men, the authors found – a finding that is supported by other research, including a worldwide survey by Gallup, which found that 40% of women were very happy, compared with only 34% of men.
Researchers are unsure why this is, but the differences between the genders in terms of happiness tend to be fairly small.
