How comparing a curiosity-driven subject line to an exclusivity-based subject line to determine impact on opens and engagement
Experiment ID: #195757
Luther Seminary
Experiment Summary
Timeframe: 01/17/2025 - 01/17/2025
Luther Seminary develops useful ebooks and other digital resources that help their audience deepen their faith and knowledge of the Bible. They recently created a new ebook about the book of Exodus and wanted to send it to their housefile email audience.
We wanted to know what kind of subject line would drive more opens and clicks for this content offer email, so we decided to run an A/B test.
The Control subject line used exclusivity messaging: “Be the first to access our new Exodus Study Guide.” The Treatment subject line was shorter and designed to spark curiosity: “Here’s your first look.”
We hypothesized that the curiosity-driven subject line would generate more opens by enticing recipients to learn more.
This was a 50/50 A/B test, with half of the audience receiving the Control subject line and the other half receiving the Treatment.
Research Question
We believe that using a vague, curiosity-driven subject line (“Here’s your first look”) instead of an exclusivity-based subject line (“Be the first to access our new Exodus Study Guide”) for people viewing a content offer email in their inbox will achieve an increase in email opens because the curiosity gap encourages recipients to open the email to learn more.
Design
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Results
| Treatment Name | Open Rate | Relative Difference | Confidence | |
|---|---|---|---|---|
| C: | Control | 19.2% | ||
| T1: | Treatment 1 | 21.0% | 9.4% | 100.0% |
This experiment has a required sample size of 3,798 in order to be valid. Since the experiment had a total sample size of 44,176, and the level of confidence is above 95% the experiment results are valid.
Flux Metrics Affected
The Flux Metrics analyze the three primary metrics that affect revenue (traffic, conversion rate, and average gift). This experiment produced the following results:
9.4% increase in traffic
× 0% increase in conversion rate
× 0% increase in average gift
Key Learnings
This experiment confirmed that a curiosity-driven subject line can significantly increase email opens. The Treatment subject line (“Here’s your first look”) outperformed the Control (“Be the first to access our new Exodus Study Guide”) by increasing opens 9.4%, with a 100% statistical level of confidence.
The success of the curiosity-based subject line aligns with the Curiosity Gap Theory, which suggests that people feel compelled to seek out missing information when presented with an incomplete or intriguing statement. Unlike the Control, which relied on exclusivity messaging, the Treatment sparked interest without revealing too much—prompting recipients to open the email to learn more.
However, while opens increased, click rates remained the same (~2.2% for both versions). This suggests that while curiosity successfully drove more people to open the email, it did not necessarily increase engagement with the content inside. A possible explanation is that the Treatment subject line created intrigue but did not set clear expectations about the offer, leading some recipients to open the email without a strong intention to engage further.
To build on this learning, future tests should explore:
- Refining curiosity-driven subject lines – Testing variations that maintain intrigue while hinting more explicitly at the content’s value.
- Combining curiosity with faith-based language – Testing curiosity-driven phrasing with biblical references to see if it enhances both opens and clicks. Update: We ran a follow-up experiment to test curiosity-based language against a subject line featuring a specific biblical reference to see which approach is more effective at engaging faith-driven audiences.
- Aligning email content with subject line expectations – Ensuring that the email itself delivers on the curiosity generated in the inbox, potentially increasing clicks.