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Really? Why would that be? Is there some psychological or physical reason for it?
There is an argument for the level of concentration one must engage in to hold a phone conversation vs with someone in physical presence. This is even outside of the typical issues of phone reception or whatever. Half of normal human conversation is actually visual (just ask an Italian!

), so when you talk on the phone, you miss half the cues that you normally get, so you literally have to concentrate more. There are piles and piles of studies that show how we know someone is smiling just by their eyes (very classic), how certain mannerisms influence our perception of the individual, and how this influences our interpretation of the conversation. Since dialogue/conversation is rarely, if ever, done with complete sentences, visual cues are used and interpreted, subconsciously or not, to fill in bits of information that we probably don't even realize is missing.
So, yeah, when you talk to someone on the phone, you are missing half of the information that you normally get, so the brain is forced to concentrate harder to interpret such conversational information.
When you throw in the fact that you are trying to drive on top of that, the distraction is compounded even more. I certainly find this to be true.
The funny thing, is that all of the studies that I have seen talking about cell phones while driving mention this--the problem is not that you're using one hand--I mean, seriously...think about how stupid this is--it's that you are having a conversation on the phone. No one recommends hands-free as a safer way to drive. The data says only that talking on the phone is dangerous--not holding a phone.
I mean..if that were true, then we should be fined for driving with one hand, right? it's obviously the problem?
I've always rolled my eyes at this notion that hand-free devices are somehow safer, b/c it doesn't take much thinking to realize how ridiculous of a notion that is.
but anyway, texting...way worse.
I'll mention something, though I don't think it's related to the same psychological/physical reasons. Another study suggested why we are so annoyed when in public, and we hear someone else on a cell phone. It's less because of them being loud and obnoxious (while true...), but an issue of how our brain demands symmetry. We know this to be true with visual patterns, music, all sorts of things. When you overhear someone talking on a phone in public, you only hear
half the conversation. While you're likely annoyed that the person is talking very loudly, you're brain is a bit pissed that you're missing half of what's going on--you don't get the full story, so subconsciously, you are quite frustrated.