An actuarial assumption is a model-based estimate about an uncertain future outcome, such as mortality, inflation, lapse behavior, or investment return, used to price and value long-dated financial promises.
Common categories
Actuaries usually work with four broad families of assumptions:
- Demographic: mortality, longevity improvement, disability, recovery.
- Behavioral: lapses, withdrawals, early retirement, option exercise.
- Economic: discount rates, inflation, wage growth, asset returns.
- Expense-related: acquisition, administration, and claims costs.
Where assumptions enter the math
A simplified present value of expected benefits is:
$$ PV = \sum_{t=1}^{T} \frac{\mathbb{E}[B_t]}{(1+r)^t} $$
Every part of that expression depends on assumptions. Change the expected benefit path or the discount rate and the measured liability changes immediately.
Why assumptions are difficult
Good assumptions require more than historical averages. Trends can shift because of medical innovation, regulation, market structure, or climate risk. That is why actuaries distinguish between best-estimate assumptions, conservative reserving assumptions, and stress scenarios used for capital planning.
Practical implication
If an insurer assumes policyholders will live one year longer on average, life-annuity liabilities rise because the firm now expects to make more payments. One modeling change can alter pricing, reserves, and solvency at the same time.