Here’s a question with a less-than-straightforward answer: Are Americans happier, or less happy, than they used to be?

Based on the usual causes of well-being, a case could be made for either. Increases in median family income, education, and leisure time suggest that happiness should rise. However, social relationships are now weaker and income inequality has risen, suggesting happiness might decline.

To examine long-term trends in happiness in the U.S., my colleagues and I merged reams of survey data from 1972 to 2014, from four nationally representative samples totaling 1.3 million Americans. All were between the ages of 13 and 96, and all were asked the same standard question on general happiness: “Would you say that you are very happy, pretty happy, or not too happy?”

Our findings, published on Thursday in the journal Social Psychological and Personality Science, were startling, revealing a pattern that doesn’t fit with conventional wisdom on age and happiness. Past research has found that people grow steadily happier as they age from adolescence to older adulthood, with happiness peaking when people reach their 60s and 70s; the moodiness of youth subsides, and maturity brings more contentment. But our analysis found that this was no longer true: In the last five years, the once-reliable correlation between age and happiness among adults has vanished. Adults 30 and over are less happy than they used to be, while, teens and young adults are happier; in fact, adults over 30 are no longer happier than their younger counterparts. It seems that mature adults’ happiness has waned, while young people’s happiness has flourished.

One reason for this shift may be a collective rise in how well Americans expect their lives to go. Happiness is sometimes defined as reality divided by expectations: One study, for example, found that the amount of a monetary payoff after a game didn’t matter for players’ happiness—what mattered was whether the dollar amount was more or less than they study instructions had led them to expect. According to the large national surveys we analyzed, 64 percent of today’s high-school students expect to be a manager or a professional by age 30, up from 48 percent in 1976—but the number of people who actually go on to attain these jobs has held steady at about 18 percent since the 1970s.

With expectations so high, less happiness in adulthood may be the inevitable result. Big dreams feel great when you’re an adolescent or a young adult just starting out. But somewhere around their late 20s, most people begin to realize reality isn’t going to match up. When those dreams are more widespread than they used to be, the inevitable crash will be, too. Rising income inequality may exacerbate this effect: Past research has shown that adults’ happiness is lower when income inequality is higher, and older adults, compared to young adults and teens, may feel its effects more acutely.

Another possible factor is the rise of individualism, a cultural system that places more emphasis on the self and less on social rules and relationships with others. Indicators of individualism—including more positive self-views, more unique names, less religion, and more equality and tolerance—are all higher now in the U.S. than in previous decades. Stable relationships are on the decline, with fewer marriages and less community involvement. Individualism works well for young people, who are often unattached and working on finding themselves—adolescence and young adulthood are inherently self-focused life stages. But for those moving into later stages of life, individualism fails to provide the ingredients for happiness in the same way.

Some theories of aging suggest that young people are more likely to take risks and seek novelty and information that will benefit them down the road, while older people, who have more limited futures, are more likely to cultivate relationships that they can enjoy in the present. The prevalence of digital technology and the greater freedom of individualism both encourage more information-seeking, novelty, and risk-taking, but provided fewer opportunities for emotionally close, long-term relationships. For Americans over 30, in other words, modern culture may supply less of what they need to be happy.

It’s impossible to know for sure why adults are less happy, while adolescents and young adults are happier—there’s no way to do a true experiment, which would have to randomly assign people to live in different eras. But we can look for correlations, and they have told us this much: As cultural values change, the happiest stage of life seems to change along with them.