investment return assumption

Every pension calculation and many insurance ones rest on a guess about the future: how much will the invested money earn each year between now and when the promises come due? That guess is the investment return assumption, and it is one of the most consequential numbers an actuary chooses. Set it high and contributions or premiums can be lower today, because investment earnings are expected to do more of the heavy lifting; set it low and you must put more money in up front to be safe.

It shows up in two related jobs. In pricing and funding, the assumed return determines how small the money paid in can be: a pension that assumes 7 percent needs far smaller contributions than one assuming 4 percent, because it is counting on the markets to make up the difference. In valuation, a closely related rate is used to discount the liabilities (see discounting liabilities). The numbers compound dramatically over long horizons — over 30 years, money growing at 7 percent ends up more than double what it would at 4 percent — so a one-point change in the assumption swings the required funding enormously. That leverage is exactly why the choice is scrutinized, disclosed, and stress-tested.

Why it matters, and the central caveat: the investment return assumption is a forecast of an uncertain future dressed up as a single firm number, and there is a constant temptation to set it too high because doing so makes promises look cheaper today and pushes the bill to tomorrow. Underfunded public pensions around the world are in large part a monument to optimistic return assumptions that markets did not deliver. A sound assumption is grounded in the actual asset mix and realistic long-term market returns, is reviewed regularly, and is paired with honest disclosure of how wrong it could be. A return you assume is not a return you will earn; the gap between the two, when it goes the wrong way, is paid out of capital.