The Willat Effect is the hedonic change caused by side-by-side comparison of similar things. Your hedonic response to the things compared (e.g., two or more dark chocolates) expands in both directions. The “better” things become more pleasant and the “worse” things become less pleasant. In my experience, it’s a big change, easy to notice.
I discovered the Willat Effect when my friend Carl Willat offered me five different limoncellos side by side. Knowing that he likes it, his friends had given them to him. Perhaps three were homemade, two store-bought. I’d had plenty of limoncello before that, but always one version at a time. Within seconds of tasting the five versions side by side, I came to like two of them (with more complex flavors) more than the rest. One or two of them I started to dislike. When you put two similar things next to each other, of course you see their differences more clearly. What’s impressive is the hedonic change.
The Willat Effect supports my ideas about human evolution because it pushes people toward connoisseurship. (I predict it won’t occur with animals.) The fact that repeating elements are found in so many decorating schemes and patterns meant to be pretty (e.g., wallpapers, textile patterns, rugs, choreography) suggests that we get pleasure from putting similar things side by side — the very state that produces the Willat Effect. According to my theory of human evolution, connoisseurship evolved because it created demand for hard-to-make goods, which helped the most skilled artisans make a living. Carl’s limoncello tasting made me a mini-connoisseur of limoncello. I started buying it much more often and bought more expensive brands, thus helping the best limoncello makers make a living. Connoisseurs turn surplus into innovation by giving the most skilled artisans more time and freedom to innovate.