The National Retirement Risk Index (NRRI) is calculated by comparing retirement income as a percent of pre-retirement income with target rates that would allow retirees to maintain their pre-retirement standard of living.
The NRRI shows that in 2013, 50 percent of households were at risk of lowering their standard of living once they retire.
That is up from 44 percent in 2007, and down only modestly from 53 percent in 2010, despite huge gains in the stock market and a rebound in housing values since 2010, according to a report from the Center for Retirement Research at Boston College.
The reason the improvement has been modest is that most of the gains have occurred in the stock market rather than the housing market. Since the third quarter of 2010, equity prices have increased by 45 percent after adjusting for inflation, while house prices have increased only about 6 percent.
"The house is a much more significant asset than stock holdings for most households, making trends in house prices a major influence on the NRRI results," the report states.
For low-income households, equities comprise only 2 percent of total wealth. For middle-income households, the figure is 6 percent. But high-income households have 17 percent of their total wealth in stocks.
As a result, 60 percent of low income households are considered to be at risk, as are 52 percent of middle-income households. But just 40 percent of high-income households are at risk.
"The fundamental message [is] that half of today's working-age households are unlikely to have enough resources to maintain their standard of living once they retire," according to the report. "The only way out of this box is for people to save more and/or work longer."