Repeat-Purchase and Evangelism Over Growth Rate
Judge a product by its repeat rate and evangelists, not its signup curve
- Difficulty
- Easy
- Time to result
- ~months to results
- Steps
- 5
- Confidence
- 76%
Sutherland warns that rate of growth is a treacherous metric: you can post exponential signups that consist entirely of people trying something and deciding it's useless. The data that actually reassures you are the repeat purchase rate and the subsequent conversion to evangelism. Genuinely good products often start slowly on a sigmoid curve, with few early adopters who then become passionate advocates who recruit others. He contrasts Threads, whose signup numbers were astronomical because joining was trivial and leaving meant deleting your Instagram account, with the air fryer, which spread slowly but turned every owner into an evangelist. The lesson for any business: look past how many people arrived and measure how many came back and told their friends.
Origin
Rory Sutherland illustrates this with his early evangelism for Philips air fryers and a critique of Threads' adoption metrics, on Modern Wisdom.
Core principles
- 01Signup and growth rate can be a vanity metric masking mass rejection
- 02The signal of a genuinely good product is repeat purchase and conversion to evangelism
- 03Great products often follow a slow sigmoid curve, not an early spike
- 04Easy-to-join, hard-to-leave products inflate adoption numbers dishonestly
How to run it
- 1
Distrust the growth headline
Treat raw signup or growth-rate numbers as suspect; they can be inflated by curiosity and easy onboarding.
Watch out Exponential signups can mask that most triers quit immediately.
- 2
Measure repeat purchase
Track how many users buy or return a second and third time; this is the first real signal of value.
- 3
Measure conversion to evangelism
Count how many users actively recommend the product unprompted; evangelists drive the sigmoid's steep middle.
Pro tip The air fryer had few early users but each became an evangelist, which is what compounds.
- 4
Expect a slow start
Accept that genuinely good products often begin flat on a sigmoid curve rather than spiking; don't kill them prematurely.
- 5
Check for false lock-in
Ask whether your retention comes from love or from friction that traps users, which flatters your numbers dishonestly.
Watch out Threads looked huge partly because leaving required deleting your Instagram account.
In the wild
Threads showed insane exponential adoption because signing up was trivial, but users looked, disliked it, and couldn't easily leave because that meant deleting Instagram. The impressive growth curve masked a near-wasteland of genuine engagement.
→ A record signup rate that meant almost nothing about real demand.
Sutherland urged Philips, then the sole air-fryer maker, to promote a product few people owned. It spread slowly, but every owner became an evangelist, and the category eventually became a mass phenomenon led by brands like Ninja.
→ Slow initial adoption that compounded into a durable mainstream market.
Common mistakes
Chasing viral signup numbers
Optimising for how many people sign up rewards easy onboarding over real value and can hide catastrophic churn.
Mistaking lock-in for love
Retention driven by exit friction, not enthusiasm, inflates your metrics while the product is quietly being abandoned in spirit.
Is it for you?
Best for
Founders, product leaders, and investors assessing whether early traction reflects real demand.
Not ideal for
One-off or genuinely single-use purchases where repeat rate is not a meaningful signal.
From the transcript
“the data you need to look at to give yourself reassurance... is the repeat purchase rate or the subsequent conversion to evangelism”
“you can have extraordinary rates of growth which consist of lots of people trying something and deciding it's rubbish”
“you can't actually leave threads because if you want to leave threads you got to delete your Instagram account”
From the episode
Hidden Psychology Of The World’s Best Advertising - Rory Sutherland - #735
Rory Sutherland