Definition
Path dependency is the property of a system in which the outcome at any point depends on the specific sequence of events that produced it, not merely on the total amount or distribution of those events.
Why it matters
Path dependency is a feature of non-ergodic systems that makes individual outcomes irrecoverable from population averages — two individuals with the same expected outcomes can experience structurally different realized outcomes because the order of events differs. For sequential decisions under uncertainty and for compounding processes, path dependency is the structural reason why population averages are not, by default, predictions of any individual path.
How it works
A path-dependent system is one where the next step's dynamics depend on the history that produced the current state, not just on the current state itself. Multiplicative wealth dynamics are typically path-dependent because the order of returns matters: a sequence of large positive returns followed by a small negative one produces a different end-of-period wealth than the same returns experienced in reverse order, because the intermediate values affect how subsequent compounding plays out. Sequence-of-returns risk in retirement portfolios is a specific instance of path dependency — withdrawals taken from a portfolio early in the drawdown phase impose larger long-run costs than equivalent withdrawals taken later, because the early withdrawals reduce the base on which subsequent compounding operates. More broadly, any system with absorbing barriers is path-dependent in the sense that paths that cross the barrier early have categorically different futures from paths that do not.
In practice
For an individual making decisions whose outcomes depend on sequential events — investment timing, drawdown scheduling, single-realization commitments — path dependency is the structural reason why the realized outcome can differ from the expected outcome computed across the distribution of possible paths. Concretely, in retirement planning, the same expected long-run portfolio return can produce a depleted portfolio if poor returns arrive early in the drawdown phase, and a comfortable one if poor returns arrive late — the average obscures the sequence-of-returns dimension that path dependency makes structural.
In the Longevity Standard Framework
Path dependency is the foundational concept in ergodicity economics on which the structural distinction between cross-sectional and longitudinal outcomes rests. The framework's treatment of solo drawdown, pooled, and transferred-risk arrangements operates on the specific wealth trajectory each arrangement generates under the participant's parameters, with realized value measured on the actually realized path rather than on the ensemble of possible paths. Mortality outcomes are themselves path-dependent at the individual level — death is an absorbing barrier from the individual's perspective — which is part of why cooperative pooling can produce time-average benefits that the underlying mortality distribution alone does not reveal.
Related terms
- Non-ergodic system
- Wealth trajectory
- Multiplicative dynamics
- Absorbing barrier
- Time average
- Sequence of returns risk
- Ergodicity
- Ruin probability