Designing for Touch Devices

I’ve been reading Smashing Magazine’s “The Mobile Book” and in the chapter about Designing for Touch, there’s a section about hybrid devices that sparked my interest. Here’s the quote:

“One layout has to win, though, and as with every other touch device, the winner should always be the thumbs. As it turns out, hybrid users begin to prefer thumb use over time, with expert users going nearly all thumbs, reaching them in and out of the screen from the edges to drive interaction. Once again, thumbs are the primary utility pointer.”
I’m not sure what kind of studies have been done on this, but if it’s true, I think I know why. It makes sense, but took a moment for me to step back and pay attention to what I was doing subconsciously. If you have a tablet handy (maybe you’re reading this on one!), consider how you’re holding it. I typically hold my iPad Mini with my right hand, in portrait orientation, and I hold it in the lower part of the iPad. This leaves my thumb free to comfortably reach and interact with the most of the screen without too much effort. However, if I hold the tablet in my left hand and use my index finger, I technically have more freedom to interact with the screen with much less effort.
So why do thumbs win out over using our index fingers? If we have more range of motion with our index fingers, why do we prefer our thumbs?
I think it has to do with the fact when we’re gripping a tablet with the same hand that we’re using to touch the screen with, we unconsciously provide resistance against the screen. As we tap with our thumb, the other fingers behind the device are responding with support. It’s the whole “opposable thumb” thing. Pretty ingenious. Whereas, when we use our index fingers, the device is propped up on something or in our lap. The simultaneous support that happens when we use our thumbs is not there. If we hold the device with our other hand, the support might be there, but it’s making our brain do extra work to coordinate those muscles to sync with the actions of our index fingers. The littlest details matter apparently!
Understanding why we prefer using our thumbs over our index fingers will help us both device interfaces more intuitively as well as be able to communicate with clients why we want to put buttons or navigation elements in the areas that are more natural for thumbs versus what may be more natural for index fingers.
That’s my take on trying to understand the conclusion from the quote above. Do you have any other insights as to why we generally resort to using our thumbs over our index fingers on touch devices?

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