More evidence of the effectiveness of default options in retirement plans come from an NBER working paper by Gopi Shah Goda and Colleen Flaherty Manchester who studied the impact of a particular firm's transition from a defined-benefit (DB) to defined-contribution (DC) retirement plans.
The transition offered existing employees a one-time opportunity to make an irrevocable choice between plans, and employees who did not make a choice were defaulted to switch to the DC plan if under age 45 or remain in the DB plan if age 45 or older. They then used a regression discontinuity framework to estimate the causal effect of the default rule and found
"Using a regression discontinuity framework, we estimate that the default increased the probability of enrolling in one plan over the other by 60 percentage points... Our simulations show that for a broad range of levels of risk aversion, allowing the default for the choice between pension plans to vary by age can substantially improve outcomes relative to a uniform default policy. Our results suggest that considerable welfare gains are possible in our model by varying defaults by observable characteristics."