Our 2 eyes are positioned horizontally, side by side. That means that we have a wider horizontal view. And research conducted by Xiaoyan Deng, Barbara E. Kahn, H. Rao Unnava, and Hyojin Lee* confirmed that most of the time, horizontal blocking is the better option.
The researchers analysed how horizontal versus vertical displays of alternative products influenced range processing, perceived variety, and subsequent choice. They concluded that horizontal (vs. vertical) displays are easier to process because of a match between the human binocular vision field (which is horizontal in direction) and the dominant direction of eye movements required for processing horizontal displays.
They also identified that that horizontal blocking allows people to browse information more efficiently, which improves mental processing fluency, increases their perception of a wide assortment and ultimately leads to more products being bought.
In summary, when shoppers see variety in a horizontal (vs. vertical) display, they browse and mentally process the range more efficiently and extensively. But this is only a benefit when more variety is a positive category attribute. When this is the case, shoppers find choosing easier and have a higher level of satisfaction and confidence about their purchase decisions.
But when more variety is not necessarily positive, for example, when a shopper is just seeking out their favourite brand of ketchup, [or insert your category name here] the benefits of horizontal blocking disappear.
In fact, a vertical block becomes more preferable. Why? Because as shoppers scan the category horizontally, the visibility of their preferred brand increases by way of it being a vertical, visual ‘barrier’. So blocking definitely isn't a case of one size fits all.
Categories that are a destination and Grab & Go by nature should be merchandised with vertical blocking. But the categories that account for more Considered purchases should be horizontally blocked.