Investors worldwide, and especially millennials, have unrealistic expectations about what their investments can achieve, says The Schroders Global Investor Study 2016.

Globally, the average level of desired income from investments is 9.1 percent, but with many countries’ interest rates at historic lows, plenty of investors are going to be disappointed, says the study by Schroders, a global asset manager with $466 billion in assets based in London and with offices in the United States.

According to the study of 20,000 investors in 28 countries, millennials' expectations were even more unrealistic. The 18- to 35-year-olds expect minimum returns of 10.2 percent a year, compared with 8.4 percent for investors over the age of 35.

U.S. investors want even more, according to the study, which says they want to generate 11.1 percent annually in investment income versus their in peers in Europe and Asia, who seek 7.9 percent and 9.7 percent, respectively.

On the other hand, U.S advisors target annual investment income of 5 percent on average.

The study also found a bias toward short-term investing. On average, investors expect to hold their investments for an average of 3.2 years.

“While this may be fine for cash and certain types of bonds, it will often prove too short a time period to counteract the volatility associated with equities,” says the study. Only 18 percent of investors say they hold investments for at least five years, the minimum realistic holding period for equity investments, Schroders says.