According to a recent study of opt-in versus opt-out wellness program signups by employees of self-insured employers

...only 35% of employees in a voluntary sign-up group opted into the wellness program, while 91 % of the employees in an opt-out group were enrolled, simply by choosing not to take action to drop out.

Source: "Automatic enrollment significantly boosts employer-based health plan sign-ups," UCLA Newsroom Press Release, October 8, 2014, http://newsroom.ucla.edu/releases/automatic-enrollment-significantly-boosts-employer-based-health-plan-sign-ups