Time decay
Time decay is the situation where something the user needed in order to take the next step was there a moment ago and isn't anymore, because it faded with elapsed time. The thing that faded might be motivational, like attention, energy, or a spike of resolve, or material, like a session that timed out, a token that expired, or a skill that's gone stale without recent practice. The chance of the next step landing is highest right after the prior step and drops the longer the user goes without taking it.
Definition
A reachability failure mode where a precondition for the next step is present at one moment and gone at the next because it fades, expires, or elapses.
The decay window
The user, at the moment of completing a step, is in their highest-probability state for continuing to the next. As time passes without the next step landing, that probability decays. The precondition — whatever the next step requires from the user's side — drains away on its own.
This produces a reachability window rather than a binary presence condition. Steps that depend on time-decaying preconditions can be initiated successfully if the offering surfaces the next affordance before the window closes, and fail if it doesn't, even though nothing went wrong with the step itself.
Where time decay applies
Time decay shows up across both groups of factor-types the framework distinguishes:
- Motivation-shaped: attention windows, energy reserves, decisiveness spikes following a commitment, the residual lift from a satisfying prior step.
- Material-shaped: loading windows the user has time to act within, skill recency from recent practice, token validity, an open integration session.
What makes a precondition vulnerable to time decay isn't its category but its half-life. Preconditions that hold steady within the user's current state (an asset they own, a skill they've internalized) don't decay. Preconditions with a half-life from the moment they appear (attention, decisiveness, token validity, recent-practice skill) do.
Window-of-opportunity diagnosis
Time decay is a window-of-opportunity diagnosis: it names both the decay-axis (time) and the offering's job within it (land the next step before the window closes). This is structurally different from symptom-shaped diagnoses like "user lost interest" or "user dropped off," which describe outcomes without naming what the offering had a chance to do and didn't.
A window-of-opportunity diagnosis surfaces the moment at which continuation was highly likely and the specific work the offering needed to do at that moment. The remediation follows directly: compress the temporal gap, or land the affordance closer to the moment the precondition is at its peak.
Time decay vs. scent-breaking
Scent-breaking is the framework's other named reachability failure mode and the one most likely to be confused with time decay. Both are window-of-opportunity diagnoses, but the decay axis differs. Time decay tracks elapsed time; scent-breaking tracks wrong turns — each piece of contradicting evidence drops probability regardless of how fast it arrives.
Misdiagnosis produces wrong interventions. Treating scent-breaking as time decay leads to accelerating delivery of a contradicting affordance, which accelerates the scent-break rather than preventing it. Treating time decay as scent-breaking leads to maintaining signal continuity across a gap that's already drained the motivation the signal was meant to carry. Different mechanisms, different fixes.
Example
A SaaS product runs a clean signup flow that ends on a "Welcome! Check your email for next steps" confirmation page. The product's own next-step affordance is the welcome email, which arrives eight hours later. The signup itself was a decisiveness spike — the user committed, expended energy, and was in their highest-continuation state at that moment. By the time the email lands, that state has decayed. The user opens the email, or doesn't, in a different motivational frame than the one they were in when the offering had its best window.
The symptom-shaped framing is "users aren't engaging with our onboarding emails." The window-of-opportunity diagnosis is that the offering deferred the next step out of the window in which continuation was at peak probability. The fix is structural: move the next step inside the post-signup success state, where the precondition is still present, rather than trying to win the email back later.