Millennials say their finances cause them more stress than politics, work or health, according to a new study.

"Politics is a huge stress for people and we know work is a big stress," says Michael Katchen, CEO and co-founder of Wealthsimple, the online investment management service behind the study. "But we found that money is twice as stressful as work for people, and several times more stressful than politics."

But, the data showed, a lot of the anxiety comes from knowing you should invest, but aren't yet.

The study, conducted by The Harris Poll, found that while 92% of young people (ages 21 to 37) save something, only about a third invest outside of an employee sponsored retirement plan. By comparison, nearly half of Gen Xers and half of Boomers have outside investments.

Young people say they don't know how to start investing. Around 30% of the Millennials who do not currently invest beyond an employee-sponsored retirement plan, say their barrier to investing is that they don't know how.

Women have fewer investments and are more stressed than men, with only 26% of Millennial women investing outside their employer, while 40% of their male peers are. Nearly half of Millennial women find money the most stressful thing in their lives, compared with 34% of Millennial men.

Sitting out of investing makes the earnings gap between men and women even worse.

The compound interest that comes from investing is so powerful for women, says Katchen, "that if they aren't taking advantage of investments, that pay gap will grow over their earning life."

Wealthsimple, like other online investment platforms like Ellevest, M1Finance and Betterment, are working to find more seamless ways for people to get over their fears of investing. And they're also rethinking the approach.

"Are we going to retire at 65 as a generation?" asks Katchen. "Probably not."

As a result, the returns model — investing more now so that you'll have a larger reserve in retirement (an approach that motivates Boomers and, to some extent, Gen Xers) — doesn't have as much resonance with Millennials.

Instead, the idea of putting your money toward something good is motivating.

"Millennials think about money differently," says Katchen. "It isn't just about making money, it is about putting their money into causes they believe in and globally investing and putting money into socially responsible investments."

Also, they're committed to financial freedom, he says.

"Young people want to live the lifestyle they want," Katchen says. "Maybe I want that at 40 or 50 or at 80. We've got to figure out a way to engage them on the future, because they aren't set up for it. They aren't set up to have the financial freedom they want."