Don’t invert established UX mental models

In the browser, to zoom in, to enlarge the text, you press Command-+ if you’re on a Mac or Control-+ if you’re on Microsoft Windows. Sometimes it makes for easier reading. As I am aging, I find it useful to use this feature, as my near vision isn’t as sharp as it once was. So to zoom in, I use Command-+, and to zoom out I use Command--.

I recently had to read this paper on the ACM’s website.

It opened as what looked like a PDF, and I thought it would be easier to zoom in. I pressed Command-+, and, lo and behold, it zoomed out instead of zooming in. It decreased the font size; I was expecting it to increase it. Pressing Command-- zoomed in, which is normally used to zoom out. The usage is inverted on ACM’s website.

The keyboard shortcuts for zooming are an established mental model in the browser. This made me think. Why invert it? Why introduce a custom keyboard shortcut that is the opposite of what the medium you are displaying your text on already uses?

This is not just a preference issue. It is an error and trust issue. Established shortcuts turn into procedural memory, or muscle memory. When you invert them, you create slips: the user performs a well-learned action and gets the opposite result. In practice, this means more errors, and more time spent correcting them. The interface starts to feel unpredictable. This goes beyond “don’t make users think” as this feels like an avoidable user error.

There is also an accessibility angle here. For many users, zoom is a primary way of accessing text. People with presbyopia or low vision often rely on Command or Control plus and minus repeatedly while reading. Inverting these keys is effectively an accessibility regression.

I believe one of the core pillars of user experience is not to confuse users and not to make them think unnecessarily.