Americans of all ages and income brackets continue to grow increasingly unhappy at work, a long-term trend that should be a red flag to employers, according to a report released by The Conference Board.

The report, based on a survey of 5,000 U.S. households conducted for The Conference Board by TNS, finds only 45 percent of those surveyed say they are satisfied with their jobs, down from 61.1 percent in 1987, the first year in which the survey was conducted.

"While one in 10 Americans is now unemployed, their working compatriots of all ages and incomes continue to grow increasingly unhappy," says Lynn Franco, director of the Consumer Research Center of The Conference Board. "Through both economic boom and bust during the past two decades, our job satisfaction numbers have shown a consistent downward trend."

Fewer Americans are satisfied with all aspects of their employment, and no age or income group is immune. In fact, the youngest cohort of employees (those currently under age 25) expresses the highest level of dissatisfaction ever recorded by the survey for that age group.

"The downward trend in job satisfaction could spell trouble for the overall engagement of U.S. employees and ultimately employee productivity," adds Franco.

"These numbers do not bode well given the multi-generational dynamics of the labor force," says Linda Barrington, managing director, Human Capital, The Conference Board. "The newest federal statistics show that baby boomers will compose a quarter of the U.S. workforce in eight years, and since 1987 we’ve watched them increasingly losing faith in the workplace." Twenty years ago, some 60 percent of that generation was satisfied with their jobs. Today, that figure is roughly 46 percent.

Barrington adds: "The growing dissatisfaction across and between generations is important to address because it can directly impact the quality of multi-generational knowledge transfer-which is increasingly critical to effective workplace functioning." The drop in job satisfaction between 1987 and 2009 covers all categories in the survey, from interest in work (down 18.9 percentage points) to job security (down 17.5 percentage points) and crosses all four of the key drivers of employee engagement: job design, organizational health, managerial quality, and extrinsic rewards.

"Challenging and meaningful work is vitally important to engaging American workers," adds John Gibbons, program director of employee engagement research and services at The Conference Board. "Widespread job dissatisfaction negatively affects employee behavior and retention, which can impact enterprise-level success."

In fact, 22 percent of respondents said they don’t expect to be in their current job in a year. “This data throws up a big, red flag because the increasing dissatisfaction is not just a ‘survivor syndrome’ artifact of having co-workers and neighbors laid off in the recession," says Gibbons.