An increasing number of employers are offering consumer-driven health plans, in which workers take more responsibility for covering their medical costs. But a majority of companies are still skeptical about the programs’ ability to lower costs, according to a study released Tuesday.

Twenty-eight percent of companies surveyed this year said they are offering consumer-directed plans, up from 22 percent in 2005, according to a study of 434 employers by Aon Consulting and the International Society of Certified Employee Benefit Specialists, a nonprofit education group.