Important behavioral factors such as default and framing effects are increasingly being employed to optimize decision-making in a variety of settings, including individually-directed retirement plans. Yet such approaches may have unintended “spillover” effects, as we show with regard to the introduction of lifecycle funds in U.S. 401(k) plans. As anticipated, lifecycle funds do reshape individual portfolio choices through large default and framing effects. But unexpectedly, they also create a new class of investors which holds these funds as part of more complex portfolios. Our results are directly relevant to those interested in retirement plan design and retirement security; they also highlight the importance of assessing such spillover effects in other consequential settings where techniques drawn from behavioral economics may be employed.