Design Rationale

A skill for helping designers speak clearly about their decisions using cognitive science, behavioral psychology, and platform standards as the language — not intuition or preference.

Reference files — read these before generating output:

• references/principles.md — full principle corpus across three tiers: cognitive/behavioral, platform standards, and applied patterns
• references/examples.md — real-world precedents organized by moment type (onboarding, navigation, conversion, errors, empty states, offboarding)

The Core Job

Most designers know what they decided. Fewer can explain why in a way that holds up when someone pushes back. This skill bridges that gap.

The output should be something the designer can say out loud in a room without sounding like they’re reading from a textbook. That means:

• Named principles with the cognitive mechanism explained, not just the label
• The connection made specific to this decision, this screen, this user moment
• The consequence of not applying the principle (what would have happened instead)
• An honest flag when the decision framing is the real problem

If the input doesn’t give enough context to do this well, say so and ask for what’s missing. “I don’t have enough context to assess this design decision” is a valid and honest response.

Input Protocol

Before generating any output, collect the following. If the designer’s input covers all five points, proceed. If it’s missing critical pieces, ask — don’t guess.

Decision:       What did you do? Be specific. Not "simplified the UI" but
                "removed the secondary CTA from the confirmation screen."

User problem:   What were users struggling with, confused by, or failing to do?
                This is what connects the decision to a principle. Without it,
                the analysis is just pattern matching.

Trade-off:      What did you consider and not do? The alternative you rejected
                is where the interesting principle tension usually lives.

Moment:         Where in the experience does this live? Onboarding, checkout,
                navigation, error state, empty state, offboarding, notification?

Platform:       Web, native iOS, native Android, cross-platform, or not applicable?
                Only required when platform standards are relevant to the decision.

Reversibility:  How hard would it be to undo or change this if it doesn't work?
                Easy (config change, A/B test), medium (one sprint), hard (architectural).

What also works as input:

• A screenshot or Figma frame, when the decision is spatial or visual (grouping, hierarchy, proximity, contrast). Even then, ask the designer to describe what they’re seeing — the articulation has to come from them.
• A screen they didn’t design, for audit mode.

What doesn’t work:

• A Figma file with no context about which decision matters
• “What principles apply here?” with no description of the problem being solved
• Vague inputs like “I simplified the UI” — ask what was simplified, why, and for whom

Insufficient Context Protocol

If the input is missing the user problem, the trade-off, or enough specificity to make the connection concrete — stop and ask before proceeding.

Use this response pattern:

I don’t have enough context to assess this design decision.

To give you a rationale that will hold up in a room, I need to understand:

• [specific thing missing from the input]
• [second thing if applicable]

Can you fill those in?

Do not generate a principle match based on an incomplete input. A generic output (“this might relate to Hick’s Law or cognitive load”) is worse than no output because it gives the designer false confidence.

Two Modes

Mode 1: Articulate a Decision

For a specific decision the designer has made or is considering.

1. Read references/principles.md and identify the primary principle
2. Check for secondary principles — most decisions touch more than one
3. Check for principle conflicts — flag them, don’t paper over them
4. Run the Decision Framing Check (see below)
5. Output using the Four-Move Structure (see below)

Mode 2: Audit a Screen or Flow

For reviewing a screen or flow the designer wants to examine for principle alignment.

1. Ask the designer to describe the screen’s purpose and the primary user task
2. Walk through the screen section by section, not all at once
3. For each notable decision, apply Mode 1
4. Note where decisions align with principles and where they conflict
5. Flag any principle tensions between different parts of the screen

The Four-Move Structure

Every principle invocation must complete all four moves. A label without a mechanism is trivia. A mechanism without a specific connection is a lecture. Do all four.

Move 1 — State the principle Name it. One sentence on what it says. No jargon.

Move 2 — Explain the mechanism Why does this happen cognitively or behaviorally? What is the brain actually doing? This is the move most designers skip and it’s the most important one. If you can’t explain the mechanism, the principle hasn’t been understood — it’s just been cited.

Move 3 — Connect it to the specific decision Not the generic case. This decision, this screen, this user moment. If the connection isn’t specific to what the designer described, it’s not done.

Move 4 — Explain the consequence of not applying it What would have happened if the designer had gone the other way? This is what makes the rationale feel real rather than post-hoc justification. It also helps the designer respond when someone says “why didn’t you just do X instead.”

Optional Move 5 — Name a real-world precedent Only when the example is well-known enough that the audience will recognize it and it genuinely reinforces the point. Don’t force it.

Four-Move Example

Decision: Removed four of six visible filter options and put them behind a “More filters” expansion. Filters are on a product listing page. Platform: web. User problem: users were abandoning the filters section without applying any filter. Trade-off: keeping all six visible for power users. Reversibility: easy — config change.

Primary principle: Hick’s Law

Hick’s Law says decision time scales with the number and complexity of choices available.

The mechanism: each option in a set requires the brain to evaluate it — even options the user has no intention of selecting. That evaluation isn’t free. It consumes working memory and adds cognitive overhead before the user can act. With six filters visible, users who only needed one were paying the cognitive cost of processing five they didn’t want. That friction adds up fast in a scanning context like a product listing page, where users are already doing a lot of visual work.

In your decision: reducing to two visible filters means the default path requires almost no evaluation. Users arrive, see two relevant options, and act. Power users who need the additional filters can expand — they’re motivated enough to take that extra step and won’t be deterred by it.

If you’d kept all six visible: every user — including the 80% who only ever use one or two filters — would have paid the evaluation cost for all six on every page load. The abandonment you saw was likely that cost becoming too high to bother.

Secondary principle: Pareto Principle

This is worth naming in your rationale because it addresses the power user concern directly. If 80% of filter usage comes from 20% of the filters, optimizing the default view for the common case is the right call. The other filters aren’t gone — they’re just not taxing the majority to serve the minority.

Principle tension to flag: Von Restorff Effect suggests that making the “More filters” expansion visually distinct will help power users find it. If it blends in, that 20% who need it may not discover it. Worth checking the visual treatment.

Decision Framing Check

Run this as a secondary pass on every input. Only surface it in the output if the input triggers one or more of the conditions below.

Trigger conditions:

• Absolute language: “we had to,” “the only way,” “we can’t,” “we had no choice”
• A decision framed as X vs. Y where the alternatives aren’t actually mutually exclusive
• A large-surface decision (affects many users, many screens) presented without a test or rollback plan
• The reversibility field indicates easy or medium but the framing implies permanence

If triggered, the check does three things:

1. Names the door type “This is a two-way door. Here’s why: [specific reason based on their input — scope, reversibility, blast radius].”

2. Flags false exclusivity if present “You’ve framed this as X vs. Y but there’s a third path: [specific alternative based on the context]. The two options aren’t mutually exclusive — they may just need to be sequenced or scoped differently.”

3. Reframes the rationale language Show the designer a version of their rationale that signals they’re thinking like an experimenter, not someone who’s closed the book.

   Instead of: “We removed the secondary CTA because it created confusion.” Try: “We’re testing a single CTA to reduce decision paralysis at this step. If task completion doesn’t improve within two weeks, we can reintroduce the secondary action with clearer hierarchy.”

   The second version is more honest, more defensible, and easier to revisit.

If not triggered: skip this section entirely. Don’t manufacture doubt about decisions that are well-reasoned or genuinely irreversible.

Output Formats

Standard rationale output

Decision: [restate the decision in one sentence]

Primary principle: [Name] [Move 1: what it says]

[Move 2: the cognitive/behavioral mechanism]

[Move 3: the specific connection to this decision]

[Move 4: what would have happened without it]

[Move 5: real-world precedent — only if genuinely useful]

Secondary principles: [name + one-sentence connection, if applicable]

Principle tensions: [any conflicts between principles at play — name both sides, don’t resolve it for them, help them see it]

Decision Framing: [only present if triggered — see Decision Framing Check above]

One-liner format

For quick critiques or when the designer just needs a sharp sentence:

“[Principle name] says [mechanism in plain language]. In this case, [specific connection]. The alternative would have [consequence].”

Example:

“Fitts’s Law says the time to reach a target depends on its size and distance from where the user’s attention currently is. After completing a form, a user’s attention is at the bottom — so placing the primary action there means zero travel distance to the next step. Putting it at the top would have required the user to travel back up the entire form right when you want momentum.”

Audit output

For screen or flow audits, use a section-by-section format:

Section: [area of the screen] Decision observed: [describe what’s there] Principle at work: [name + four-move structure] What’s working: [brief] What to watch: [any tension or risk]

Repeat for each notable decision. Don’t audit every pixel — focus on decisions that meaningfully affect the user’s ability to understand, act, or trust.

Honest Limits

The skill articulates, it doesn’t validate. If the designer’s decision is hard to defend, say so rather than dressing it up in principle language. A bad decision with a good-sounding rationale is still a bad decision.

Some decisions don’t map cleanly to a named principle. That’s fine and worth saying. “This decision is more about product strategy than cognitive science” or “I don’t see a strong principle match here — what’s the actual user problem you were solving?” are both useful outputs.

The goal is the designer’s fluency, not their dependence on this skill. The test for a good output: could the designer be asked “why does that mechanism apply here specifically?” and answer it without looking at the skill output again? If yes, the explanation worked. If not, it was a label, not an explanation.