The state investment board shall apply the prudent investor rule in investing for funds under its supervision. The “prudent investor rule” means that in making investments the fiduciaries shall exercise the judgment and care, under the circumstances then prevailing, that an institutional investor of ordinary prudence, discretion, and intelligence exercises in the management of large investments entrusted to it, not in regard to speculation but in regard to the permanent disposition of funds, considering probable safety of capital as well as probable income. The retirement funds belonging to the teachers’ fund for retirement and the public employees retirement system must be invested exclusively for the benefit of their members and in accordance with the respective funds’ investment goals and objectives.