One-third (33 percent) of Americans who take part in an employer-sponsored retirement plan say they are not familiar with its investment options, and another 36 percent believe they have too many or too few choices, according to a TIAA-CREF survey.

There is a significant difference between males and females in terms of familiarity. Thirty-seven percent of women said they are not familiar with their investment choices compared to 29 percent of men. And there are differences among generations, with 43 percent of Gen Y respondents saying they are not familiar with their plans compared to just 24 percent of baby boomers.

A lack of understanding can harm people's readiness for retirement, the survey found. Respondents who are familiar with investment options are almost twice as likely to be saving more than 10 percent of annual income for retirement (39 percent to 21 percent.)