You send two versions of the same email. One promises "20% off." The other promises "a surprise inside." Which gets opened more — and does the answer hold up the third time you send it?

That question sits at the crossroads of behavioral psychology and email marketing. And the research suggests the surprising answer depends less on the reward itself and more on how many times your subscriber has seen the envelope.

The Pull of the Unknown

Behavioral economists have known for decades that uncertainty can be more motivating than a certain outcome. In a now-famous 2005 study, Colin Camerer and his colleagues found that people chose uncertain rewards over guaranteed ones even when the uncertain option had a lower expected value. They weren't being irrational in the way a spreadsheet would define it. The anticipation of a mystery carried its own value.

In email terms, a subject line that reads "Your reward is waiting" activates something a flat "15% off" doesn't. The brain fills the gap. That gap-filling is the engagement.

This maps onto what B.F. Skinner called variable-ratio reinforcement — the principle that unpredictable rewards drive more persistent behavior than predictable ones. It's the same mechanism that makes people check a slot machine, a social feed, or a mystery box. The reward isn't the point. The not knowing is the point.

Where the Third Open Breaks the Spell

Here's the part most email marketers miss.

The mystery advantage is real — but it has a shelf life. Research on curiosity and habituation suggests that once a subscriber has opened two mystery emails and found something underwhelming both times, the pattern flips. The unknown stops feeling like a gift and starts feeling like a bait-and-switch.

A concrete example: a mid-size e-commerce brand tested mystery subject lines against transparent ones over a six-week sequence. Opens for the mystery version beat the transparent version by roughly 34% on the first send and 28% on the second. By the third send, the gap had collapsed to under 5% — and click-throughs on the mystery version were actually lower, because subscribers who opened expecting something special found a standard offer.

That's the "third open" problem. Curiosity is a loan. You pay it back with the actual reward.

Loss Aversion Cuts Both Ways

Kahneman and Tversky's work on loss aversion adds another layer. People feel losses roughly twice as intensely as equivalent gains. A mystery email that promises a reward creates a small anticipatory gain. But if the reward disappoints, the subscriber experiences it as a loss — not just a missed opportunity, but a violation of trust.

That asymmetry explains why mystery emails can produce spectacular first-send metrics and then quietly poison your list. The subscriber isn't just less interested. They feel slightly cheated.

What to Do With This

The practical takeaway isn't "stop using mystery." It's to treat mystery as a finite resource with a real payoff obligation.

Pair it with genuine surprise — something the subscriber couldn't have predicted, not just a discount they could have gotten anyway. Rotate it. Don't let it become your default voice. And track not just opens but downstream behavior: reply rates, unsubscribes, repeat purchases. The third send is where the truth lives.

Forward-looking email teams are already building this into their testing calendars — not as a one-off A/B test, but as a longitudinal question. How many times can you ask someone to be curious before they stop trusting you?

That's the number worth knowing.