For the sheer simplicity they bring to 401(k) investment decisions, retirement experts have been big fans of target date funds for years. Now, their popularity is soaring with the people who really count: employees. Last year, 401(k) participants poured a record $70 billion into target date funds (TDFs), an investment option that automatically shifts the asset allocation in the portfolio to reduce risk as employees approach a designated retirement date. TDFs have become the first choice for people who, rather than go it alone and pick their own mutual funds, like having their employer’s mutual fund manager do it. According to a new report by Morningstar, the Chicago research firm, the new money flowing in has averaged $66 billion annually…