The new Pew survey of millennials presents a portrait of young people that is both depressing and optimistic. Millennials, young adults between 18 and 33, are low on social trust: Just 19 percent believe that “most people can be trusted,” compared to 40 percent of Baby Boomers and 31 percent of Generation X.

Why are young people less trusting than their elders? The Pew study offers several hypotheses:

Racial diversity. Minorities have historically been less trusting than whites. The millennial generation is far more diverse than their elders — 43 percent are minorities. Internet usage: Millennials are strongly connected to online communities and friends. The average millennial has 250 Facebook friends. Strong Internet connections may lead to closed social networks and a wariness to put too much trust in people who are not part of your network. Pessimism for the future. However, the Pew study finds that millennials are upbeat about the country’s future and their own prospects for financial success. But we should reconsider these conclusions. My research strongly suggests that economic pessimism, not racial diversity or the Internet, is the key to explaining lower trust.

The first two explanations seem plausible, but they gather little support from the data. The Pew data are not publicly available, but Pew’s surveys use the same trust question as the General Social Survey (GSS): “Generally speaking, do you believe that most people can be trusted or can’t you be too careful in dealing with people?” In the 2012 GSS, 19 percent of millennials expressed trust in others, the same as the Pew survey.

The main reason to discount racial diversity is that it cannot account for the generational decline in trust among whites. Minorities have always been less trusting than whites — not just in the United States, but virtually everywhere the question has been asked. But unlike among whites, the trend among minorities has been fairly stable over time: minority young people are slightly more mistrusting than their minority elders, but the difference is small (5 percent). Although the increasing share of young people who are racial minorities may lower trust overall, it does not explain the key trend: why each successive generation of whites has become less trusting — with white millennials almost as mistrusting as minorities of any age. Roughly 22 percent of white millennials trust others, compared to 16 percent of black/non-white millennials.

There is also little reason to blame the Internet for lowering trust. I have shown (gated) that there is no relationship between Internet use and trust. And the logic behind this linkage is suspect. Most of our interactions on the Internet are with people we know. Generalized trust is all about having faith in strangers — and there is no clear way to go from one form of trust to the other.

This leaves economic pessimism. Although Pew noted that young people are fairly optimistic about their future, the key question is not whether you believe that you will succeed in life. Through thick and thin, Americans have been optimistic about their own abilities to overcome obstacles to success. What matters most are relative expectations.

Here, the pessimism of millennials stands out: relative to Baby Boomers, millennials are more likely to say that their generation is worse off than their parents. Millennials also worry that “today’s young adults face greater economic challenges than their parents’ generation faced when they were starting out,” as stated in the Pew report. And fewer millennials now consider themselves to be middle class (42 percent to 53 percent in 2008). More millennials now see themselves as lower or lower-middle class now than in 2008 (46 percent compared to 25 percent).