Case · All × Return / Stay ("why we measure")
The gym megastudy: most effects didn't last
Thirty scientists designed 54 four-week programs to get 61,293 members of a US gym chain to work out more. About 45% raised weekly visits while they ran, but only about 8% still showed an effect after they ended, and impartial forecasters couldn't predict the winners (Nature, 2021).
The outcome · 2021
54 interventions, 61,293 members of 24 Hour Fitness: 45% significantly raised weekly gym visits (by 9-27%) during the four-week programs; only 8% produced measurable change after they ended; impartial forecasters failed to pick the winners (Milkman et al., Nature, Dec 8, 2021).
Stage cell: All × Return / Stay ("why we measure").
What they did. A megastudy: 30 scientists each designed four-week digital programs (reminders, planning prompts, points, rewards, framing) and all 54 were tested at once on 61,293 members of 24 Hour Fitness.
The mechanism. Measurement first F26. Planning prompts and fresh starts F21 (Implementation intentions and goal achievement: a meta-analysis, Gollwitzer & Sheeran, Advances in Experimental Social Psychology, 2006) were among the tested ideas; the best performer rewarded coming back after a missed workout, a form of forgiving lapse recovery HK-RWD34 (Hooked Ch.4).
The outcome. 45% raised visits; 8% lasted (2021) Case grade A. From a regulator, a public filing or a peer-reviewed study.. Forecasters, including experts, could not tell the winners in advance.
What it means at your stage. Don't trust intuition about which nudge will work, ours included. Test variants in parallel when you have the traffic, pre-register a follow-up window, and judge habits on what remains after the incentive stops.
Caveats. Gym members in a four-week window; effects on gym visits, not on a product. Mechanism link Evidence grade A. Robust evidence: replicated, peer-reviewed research..
Would you have called it?
This case is also a round of Predict the Outcome: read the situation, pick what you think happened, then see the evidence.
Play this roundTry this
The techniques behind it
Each one is a starting point, not a recipe. Gamify It turns a technique into an experiment card for your product: the change, the metric, the guardrail and how long the test needs at your traffic.
- TechniqueEvidence grade B. Moderate evidence: solid studies with some limits.
Streaks and completion chains with a safety valve
a visible chain you can repair
Mark each finished unit with a clear 'done' moment and a visible chain of days, and ship forgiveness with it: freezes, repair windows, rest days, partial credit and a running total that survives a break. Never let one miss erase everything.
- TechniqueEvidence grade B. Moderate evidence: solid studies with some limits.
A reminder tied to the plan the user chose
if-then reminder at a chosen time
Let the user pick a goal and a time during setup, then send one reminder at that time that names their own plan and opens today's unit directly.
Limits
What we don’t claim
- The Hook Model is a practical design tool, not a validated or complete theory of behavior.Hooked, Introduction
- Habit metrics need the longer habit horizon (weeks to months) before they can be read.
- No success is attributed to a single technique: real outcomes have many causes.
- Company names are used editorially; no endorsement is implied.
Sources
Hooked · Nir Eyal
Ch.4
Megastudies improve the impact of applied behavioural science · Nature (Milkman et al.), 2021
Case grade A. From a regulator, a public filing or a peer-reviewed study.Evidence grade A. Robust evidence: replicated, peer-reviewed research.Implementation intentions and goal achievement: a meta-analysis · Gollwitzer & Sheeran, Advances in Experimental Social Psychology, 2006
Evidence grade B. Moderate evidence: solid studies with some limits.
Book content is paraphrased, never quoted at length. Last verified October 24, 2026.
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