Nearly 45% of all working Americans are at risk of being unable to maintain their standard of living at retirement, a study by the Center for Retirement Research at Boston College estimates. Moreover, the number of Americans between the ages of 51 and 62 that are at risk has increased significantly in the past decade and a half, the study found. About 32% of Americans in that age range were at risk in 2004, up from 20% in 1992. The study did not indicate the percentage of all working Americans at risk in 1992. The increase is partly the result of declining Social Security replacement rates, increased life expectancy, lower real interest rates and the continued shift to 401(k) plans from defined benefit, according to the study.