Being "allowed" and being "prudent" are two different things. I have no problem with the FIDUCIARY decision being made by a plan fiduciary (just as having a plan fiduciary making all investment decisions and not allowing participant direction is "allowed.") The question still remains as to WHY a fiduciary would want to make that decision? The way I look at it is, if you take into consideration ONE fact or factor, explain to me why you chose THAT particular fact or factor in making your decision, but chose NOT to take into consideration x, y & z facts or factors in making that decision? It's the distinguishing between two groups on the basis of ONE factor that causes me concern - and will be the crux of the fiduciary's defense when sued. Now one could argue that TDF's do the same thing - BUT 1) the industry has recognized that as "more" prudent than virtually all other investment options; 2) the options glide path ensure that the investments change when the one factor considered changes; and 3) there is safety in numbers - prudence is determined by what OTHER experts would do, and many, many other experts use TDF's. I am aware of none that actually make a "tax" decision for employees.