Public pension funds are governed by trustees who owe fiduciary duties of loyalty and prudence under state constitutions and statutes, not under the federal ERISA law that covers private plans. The core obligation is the 'exclusive benefit' or 'sole interest' rule: trustees must invest solely in the interest of members and beneficiaries, for the exclusive purpose of paying their benefits.
Public pension funds manage the retirement savings of teachers, firefighters, police officers and other government workers — often hundreds of billions of dollars per fund. The people who run them are fiduciaries, bound by the highest duties the law recognises. But the legal framework that governs them is different from the one most people assume, and that difference sits at the centre of the loudest investment-policy fights of the 2020s.
Public funds are not governed by ERISA
The most common misconception is that all US retirement plans fall under ERISA — the Employee Retirement Income Security Act of 1974. They do not. ERISA governs private-sector plans; governmental plans are expressly exempt. A public pension fund such as CalPERS, CalSTRS, the New York State Common Retirement Fund or a state teachers' system is instead governed by the constitution and statutes of its state.
In practice, many states borrowed ERISA's language when they wrote their own pension fiduciary rules, so the duties look familiar. The crucial difference is enforcement: there is no single federal regulator equivalent to the Department of Labor overseeing these plans. Accountability runs through state law — beneficiaries, state attorneys general, legislatures and courts — which makes the design and independence of the pension board far more consequential than for a private plan.
The exclusive-benefit rule
The bedrock obligation of a public pension trustee is the exclusive benefit or sole interest rule. California's Government Code requires CalPERS trustees to discharge their duties "solely in the interest of the participants and beneficiaries" and "for the exclusive purpose of providing benefits to members, retired members, and their survivors and beneficiaries." Versions of this language appear in state constitutions and statutes across the country.
The rule does two things. It establishes loyalty — the trustee serves the members, not the government, not a political agenda, not the trustee's own interest. And it establishes purpose — the assets exist to pay pensions, full stop. Across America the sole-interest rule is codified in state constitutions, statutory law and case law, and it charges public pension trustees with the highest legal duties of loyalty and prudence.
The core duties
Public pension fiduciary duty resolves into a familiar set of obligations:
Duty of loyalty. Act solely in the interest of members and beneficiaries, free of conflicts of interest. A trustee may not use the fund to pursue personal, political or third-party objectives.
Duty of prudence. Invest with the care, skill, prudence and diligence of a prudent expert — the modern descendant of the prudent-investor rule. This is judged on process, not hindsight: a prudent decision that turns out badly is not a breach.
Duty to diversify. Spread investments to minimise the risk of large losses, unless it is clearly prudent not to.
Duty of impartiality. Balance the interests of current retirees drawing benefits today against those of younger workers and future beneficiaries who will draw them in decades — a duty that bears directly on how much investment risk and how long a horizon the fund should adopt.
Where the fights are: ESG and the meaning of "sole interest"
The exclusive-benefit rule has become the battleground for the dispute over whether public funds may consider environmental, social and governance factors. The mainstream fiduciary view is that ESG factors may be weighed where they are financially material — where they represent economic risks or opportunities that a qualified investment professional would treat as relevant under generally accepted investment theory. On that view, analysing climate risk to a fund's energy and real-asset holdings is simply prudence, not politics.
The contested view, advanced by several states, is stricter: that trustees may consider only pecuniary factors and may not promote "non-pecuniary" goals. Under this framing, an ESG consideration counts only if it presents a material economic risk or return; pursuing social or environmental outcomes for their own sake is treated as a breach of loyalty. Some state attorneys general have issued opinions concluding that ESG practices introducing "mixed motivations" violate fiduciary duty, while other states have moved in the opposite direction and required climate-risk assessment. The result is a genuinely fragmented landscape that varies state by state.
What unites both camps is the principle, not the application: the investment purpose must remain the beneficiaries' financial interest. The disagreement is about whether a given ESG factor is financial or political — a question of fact and judgement, decided differently in different jurisdictions.
What this means for a universal owner
The largest public funds are universal owners: so big and diversified that they effectively hold a slice of the entire economy. That status sharpens the fiduciary question. A universal owner cannot diversify away systemic risks such as climate change or financial instability, because those risks hit the whole portfolio. For such a fund, analysing and engaging on systemic risk is arguably a direct expression of the prudence and impartiality it owes future beneficiaries — provided the analysis is genuinely about long-term financial outcomes and the fund can document that reasoning.
The ownable insight: public pension fiduciary duty is not weaker than ERISA's — in many states the constitutional sole-interest rule is more demanding. The hard questions are no longer about the standard of loyalty and prudence, which is settled and severe, but about which long-horizon risks a prudent expert must now treat as financial. That is where the law is still being written.
This explainer is general information about how fiduciary duty applies to public pension funds and is not legal advice; the precise duties depend on the governing state constitution and statutes.