A determination of whether the board of trustees has exercised prudence in an investment decision must be made by considering the investment of all of the assets of the trust over which the board has management and control, rather than by considering the prudence of a single investment. In making investments for the retirement system, the board of trustees shall exercise the judgment and care, under the circumstances, that persons of prudence, discretion, and intelligence exercise in the management of their own affairs, considering the probable income from the securities and probable safety of their capital.