What's easy to process appeals to us more.
The easier something is to process (a scent, a piece of text, a decision), the more you like it, and that changes what you choose next. Processing fluency is how easy it is for the mind to process what's in front of it. Something can have many options and still feel simple.
A study modified a store's environment at different times using two scents that had previously been rated equally pleasant, but that differed in complexity. One was simple (orange), the other was complex (basil, green tea, and orange), plus a no-scent condition. Customers spent significantly more money when the scent was simple than when it was complex or absent, even though both scents were rated equally pleasant when evaluated separately. The only real difference was how easy each one was to process.
Greater processing ease associated with a stimulus (a scent, a piece of text, an interface) tends to increase how much we like that stimulus, and positively influences the decisions that follow. It's not about having fewer things to choose from. It's about how little mental effort it takes to understand it.
Source: Herrmann, A., Zidansek, M., Sprott, D. E. & Spangenberg, E. R. (2013). The Power of Simplicity: Processing Fluency and the Effects of Olfactory Cues on Retail Sales. Journal of Retailing, 89(1), 30-43.
Simplifying a message, a form, or an interface doesn't always mean removing actual content or options. Sometimes it's enough to make it easier to process. Clearer language, an obvious visual hierarchy, or fewer steps to reach the same result, without eliminating a single alternative.
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