For all the discussion about the ROI of wellness programs, there’s no debate that a healthier workforce is more productive and less costly than a less-healthy one. Many factors may hinder employers’ efforts to nudge workers toward healthy behaviors, one being that the workers seem to be delusional about how healthy they are, new research suggests.

Among 2,800 employees at a wide range of companies with more than 1,000 workers, 87% said their health was at least good. Only 23% said they were overweight or obese. Yet almost three times that many (66%) reported height and weight suggesting a body mass index (BMI) that would classify them as overweight or obese according to standard definitions.

That means more than half (53%) of those who reported being in good health actually had a higher BMI than is thought to be consistent with good health.

“That is a dollars-and-cents finding,” says Joann Hall Swenson, health-engagement leader at consulting firm Aon Hewitt, which sponsored the study along with the National Business Group on Health (NBGH) and The Futures Co. “When we have unrealistic views of our health, we don’t necessarily take the right actions to be healthier. In the long run, that will affect companies’ finances.”

“It’s a disturbing message,” agrees Helen Darling, CEO of the NBGH, which represents the interests of 350 large-employer members on health-care matters. Still, companies should be realistic about what actions will motivate employees. “A healthy skepticism about what will work over the long term is good,” she says.

For example, the NBGH advises its members to be careful about offering to pay for employees’ gym memberships. “That’s not necessarily going to get you very much,” Darling says. “It’s expensive. You will immediately be paying for something that healthy, fit people were already paying for.” More to the point, meaningful health improvements will come only with a long-term commitment to work out, “and the number of other people who will take that up, and stick with it even if they do, is pretty light.”

Why are employees so clueless about their health status? “It’s probably just the overall behavioral economics principle of overconfidence,” says Swenson. “People don’t have a realistic picture of themselves.” While that could cost companies at the bottom line, finance executives, who tend to evaluate things in black-and-white terms, may not be aware of the problem.

Employees also are amiss in their estimates of how much of the health-care-cost pie is paid for by their employers.

While companies’ average cost for employee-only health care in 2012 was $8,318 per worker, according to a separate Aon Hewitt survey of 1,800 employers, participants in the employee survey guessed the figure was only $4,648. Further, while a company’s costs for employee plus spouse or partner coverage are roughly double those for employee-only coverage, the surveyed employees estimated an increase of only 48%.

Employees themselves incurred annual total costs of just $4,404, about evenly split between health-care premiums deducted from their paychecks and out-of-pocket expenses. A lack of clarity on how much of the burden employers are bearing could have some adverse effect on companies’ efforts to retain employees. Also likely is some degree of disappointment for newly arriving employees. “With the economy improving, people are switching jobs more, often without paying much attention to how much value they’d been getting from their health plans,” says Swenson. “They might be surprised, if they go to a company with a weaker benefits program and they hadn’t understood that value.”

The NBGH recommends that employers advise health-plan participants during the annual open-enrollment period of how much the company will be paying for health benefits. In fact, Darling says, companies also should inform employees of situations where, for example, the company paid $1 million in the past year in claims for a small handful of employees who suffered what should have been preventable heart attacks.

In another interesting survey finding, workers in management positions were 20% more likely to report high stress than those without direct reports. However, fewer managers (22%) said stress has a negative impact on their work performance than nonmanagers (28%). That suggests managers are less likely to make lifestyle or other changes to reduce stress, which is known to be among the leading health risks.