The S Fund, which invests in the stocks of small- and mid-sized American companies, grew the most, at 4.40 percent last month. The fund tracks the Dow Jones Wilshire 4500 Index, which invests in the 4,500 next-largest domestic companies after the 500 tracked by the common stock, or C Fund. May's growth brought the S Fund's 12-month gains to 21.89 percent.
The C Fund, which tracks Standard & Poor's 500 Index, was the second highest earner, with gains of 3.52 percent last month. Its 12-month gain was 22.81 percent.
The international, or I Fund, which invests in stocks in Europe, Australia and some countries in Asia, grew 2.54 percent in May. The fund had the highest 12-month gains, however, coming in at 26.92 percent.
The government securities (G) fund, which is the most reliable, earned 0.34 percent last month for a yearlong 4.92 percent increase.
The F Fund was the lone loser among the five basic options for May, dropping 0.70 percent. The fund, made up of fixed-income bonds, still had positive 12-month earnings, at 6.71 percent.
All of the TSP's life cycle fund options, which automatically adjust to more conservative allocations as investors near their target retirement date, saw gains last month. The options designed for younger employees grew the most, because they are the most heavily invested in funds that are riskier but also have greater earnings potential.
The L 2040 fund, designed for TSP participants anticipating retirement around the year 2040, grew 2.79 percent in May. The L 2030 Fund gained 2.52 percent; the L 2020 rose 2.15 percent; the L 2010 increased 1.53 percent; and the L Income, designed for employees with planned retirements in the very near future, gained 0.92 percent.
The L funds with riskier allocations also earned more over 12 months. L 2040 gained 20.92 percent, L 2030 grew 18.87 percent, L 2020 gained 16.85 percent, L 2010 earned 12.79 percent and L Income made 8.64 percent.