Workplace wellness plans appear to have little effect on employees health or absenteeism or on employer healthcare costs, according to preliminary results of a new study.

The findings run counter to some previous analyses, which the researchers say were biased because participation in such programs is voluntary, and therefore those employees who choose to join tend to have better health and lower medical costs before they enroll than those employees who choose not to participate.

For the new study, researchers from the University of Chicago and the University of Illinois divided about 6,300 employees of the University of Illinois at Urbana-Champaign into two groups, people who were given a year of access to a workplace wellness program similar to what many companies offer workers, and a control group of people who weren’t given access to the program.

“Ultimately, in the first year of the study, we found virtually no difference in health spending between the control group and the treatment group,” said University of Chicago Harris School of Public Policy Associate Professor Damon Jones, Ph.D. “There was no difference in sick leave. There was no difference in salary or job separation. In fact, gym visits were nearly identical between the two groups, and there was no real difference in running participation.”

In the study, called the Illinois Workplace Wellness Study, people given access to the wellness plan were offered varying incentives to participate. The researchers found that offering incentives can increase program participation, but there is a point of diminishing returns.

“The first portion of our study was an initial screening, and we offered monetary incentives to encourage participation. We found that upping the completion award from $0 to $100 increased completion by 12 percent,” Jones said. “But we also found that raising the incentive even further did not increase participation, and we just ended up giving more money to people who were already participating.”

Employees who already exercised were among the most likely people to join the wellness program, researchers found, whereas smokers were among the least likely to join.

The study will continue for two more years.

In a 2016 study of senior living organizations conducted by Willis Towers Watson for Argentum, 57% of respondents said their organizations offered wellness plans, and 32% said their companies planned to do so in the future. Where wellness programs exist, Willis Towers Watson said at the time, only about one-fourth of employees participate, on average.

Senior living organizations and other employers also will be paying attention when the Equal Employment Opportunity Commission issues a new notice of proposed rulemaking on workplace wellness plans later this year. The EEOC had changed its definition of “voluntary” participation in a wellness plan to mean that an employer did not offer a health insurance premium discount of more than 30% to employees who answered questions related to disabilities or underwent medical exams as part of the program. The AARP had sued, arguing that employees who could not afford to pay a 30% increase in premiums for non-participation would be forced to disclose their protected information when they otherwise would choose not to do so. A judge in the case ordered the EEOC to revise its rule.