Any tax on labor income reduces the labor supply depresses the labor supply, this is no secret. Theory also tells us that whether the employer or the employee pays any withheld tax does not matter. Contributions to pension plans, which are typically paid by both employers and employees, look like a tax on the pay stub and should thus obey the same principle. Well not quite.
Iñigo Iturbe-Ormaetxe argues that the size of the pension fund contribution says something about the future benefits. If the employer contribution remains hidden, the employee is not aware what great benefit he is getting. Were he to pay the whole contribution, after a corresponding pay rise that is revenue neutral to the employer, the employer would be happier about his pay and would increase his labor supply. It would work similarly if the employer would simply indicate on the pay stub her contribution. This assertion is backed with a crude cross-country regression of employment rates in the OECD which shows that at least male employment rates a negatively affected by employer contributions, but not by employee contributions.