03/19/25 | The Rehm Copy Newsletter

I’ve spent years testing email subject lines across dozens of ecom/DTC brands…

And most of what you've been taught about subject lines is wrong.

Those “ultimate guides” with their “proven templates” miss what actually matters in today’s competitive inbox environment.

Let’s break down what genuinely makes people WANT to open email content.

First, the psychology behind high-performing subject lines...

The best subject lines create what psychologists call a “curiosity gap” - they hint at valuable information but don’t reveal everything.

This works because humans are naturally driven to resolve uncertainty. When we encounter a knowledge gap, we feel compelled to fill it.

Believe it or not - we can actually reference a point from Mr. Beast, who's the MOST popular Youtuber on the planet in case you're not aware of him...

He talks about this principle regarding his Youtube videos.

He says that you need to make the title and thumbnail so intriguing that if they don't watch, they should think at night, "I wonder what actually went down in that video."

We want this same effect in our emails as often as we can. That's what you should be aiming for.

Now, let's dive into the core subject line approaches...

There are four psychological approaches that consistently outperform all others...

1 / The Curiosity Gap

These subject lines create an information gap that can only be filled by opening the email:

Quick examples:

“This ingredient changed [XYZ] forever…”

“What we found when we tested [X experiment]…”

“The biggest mistake most brands make with [X mechanism]…”

Why it works: Creates an “itch” that needs to be scratched. The human brain is designed to seek closure.

When to use it: Best for educational content, case studies, or when revealing unexpected information.

2 / Specificity Over Generality

Compare these two subject lines:

"New products just arrived!” (generic)

“The 30-second routine that fixed Amy's skin” (specific)

Why it works: Specificity creates a mental image and implies real, tested value rather than marketing hype.

When to use it: Use for product launches, educational content, and when sharing specific benefits.

3 / Personal Story Hook

Personal narratives consistently outperform promotional angles:

Quick examples:

“I was shocked when I saw this…”

“What happened when I switched to…”

“My biggest mistake with [product]…”

Why it works: Humans are wired for stories. We’re more receptive to information in narrative form.

When to use it: Best for testimonials, founder stories, and authentic brand moments.

4 / Numbers and Data Points

Subject lines that include specific numbers signal concrete value:

Quick examples:

“How we increased conversion by 43%”

“3 steps to better [benefit]”

“The 5-minute routine that…”

Why it works: Numbers cut through vague promises and suggest proven, measurable results.

When to use it: Perfect for tutorials, case studies, and clear benefit statements.

Now, I wanted to include a section about what DOESN'T work...

But there's a problem with it.

Any subject line CAN work...

The same way ANY random piece of organic short form content can take off on Instagram or TikTok.

We've all seen that random video that got millions of views because of some random piece of the video, despite low camera quality and it obviously not being intentionally built for engagement.

The same thing CAN happen with subject lines.

BEFORE you send out your next campaign...

Take a moment to ask yourself THESE 4 questions:

- Does this create genuine curiosity?

- Is it specific rather than general?

- Does it feel personal and authentic?

- Is it clear what value is inside?

If you can answer “yes” to at least two of these questions, you’re on the right track.

Adam