It all goes downhill after 30 — at least when it comes to happiness.

“Adults over 30 are less happy than their predecessors,” concludes a study published in 2015 in the journal Social Psychology and Personality Science, which examined happiness data from more than 50,000 adults, gleaned from the General Social Survey, carried out by NORC at the University of Chicago, a nonpartisan, independent research organization, which has collected information about American adults since 1972.

From 2010 to 2014, adults over 30 had an average happiness score of just 2.18, compared with 2.24 a decade ago. That’s significant considering happiness scores were measured on a tiny scale from just 1 to 3, with 1 being “not too happy” and 3 being “very happy.” (The data used five-year cohort periods so that single year fluctuations were smoothed out.)

What’s perhaps even more interesting is that, for the first time ever, adults ages 18 to 29 were happier than adults over 30. “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 the authors don’t know for sure why younger adults are happier than older ones for the first time in at least 40 years, they do have some theories. First, rising inequality may have more of an impact on the well-being of older adults than on younger ones, who are more apt to think they can overcome such things given that they have more time. And 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.

That said, there are plenty of studies that show we get happier as we get older (including a study published in 2011 in the journal Psychology and Aging, which revealed that “emotional experience improves with age”).

In general, women are slightly happier than men, the authors found — a finding that’s backed up 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 aren’t quite sure why this is, but the differences between the genders in terms of happiness tend to be fairly small.

This story was updated on Nov. 30, 2016.