The average American holds on to just 7.5% of their disposable income. Forty percent of people in the U.S. don't have $400 set aside for an emergency, according to the Federal Reserve.

A measly 2% said they'd buy something they couldn't otherwise afford.

Good intentions abound: More than a third of respondents said they'd pay off debt. Many others claimed they'd save for short-term expenses (22%) or invest for long-term goals (15%).

People were recently asked in the CNBC and Acorns Invest In You Savings Survey how they would spend an unexpected $5,000 bonus.

You want to do the financially responsible things, so why is it so hard to actually do them?

"Spending is much easier than saving for most people," said Stephen Brobeck, a senior fellow at the Consumer Federation of America.

Companies, of course, set out to separate us from our cash.

"We're bombarded by ads for products and shop in stores designed to encourage spending," he said. At the same time, he added, "those without substantial assets receive much less encouragement from the marketplace to save regularly."

A $5,000 bonus can go far if used well. If you invested that amount at an annual return of 8%, you'd have $50,313 in 30 years, $108,623 in 40 years and $234,508 in 50 years. If you directed that bonus toward debt, you could save years of bills. That's because if you made only the minimum payments on a credit card with a $5,000 balance and an interest rate of nearly 18%, you'd be stuck in the red for more than 18 years and have forked over $6,400 in interest, according to Ted Rossman at CreditCards.com.

Add to capitalism another force against our bank balances — evolution, said Teresa Ghilarducci, an economics professor at the New School for Social Research.

"Saving is not what neuropsychologists call a 'selective trait,'" Ghilarducci said. "It's not a trait that helps propagate the species."

While interpersonal skills and trust are qualities that lead us to reproduction, she said, "a young person saving for retirement is a bit weird."

The blame shouldn't be put on consumers for not managing their money well enough, she said, but on the student lenders, banks and auto dealers that offer them more debt than they can realistically handle.

When it comes to saving for retirement, Ghilarducci believes there should be a mandatory system in place that's not based on "unrealistic discipline."