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Emotions run high
Well, that was unexpected. The OED has just announced its word of the year for 2015 - and it's not a word. In fact, the 'word' of the year is the emoji or pictograph 😂 (Face With Tears of Joy).
The emoji is not unrelated to the emoticon which is described as a 'metacommunicative pictorial representation of a facial expression', i.e. one of those keyboard 'expressions' one can add to one's messaging, such as :-) or :-(. The emoji takes the idea of the emoticon and adds art.
So, it's easy to see how these emoticons and emoji developed. Just add human imagination to a set of symbols, sit back, and wait for something fun and creative to emerge.
But why have they become so widespread? Well, basically because, while we can all write a sensible sentence and usually communicate the facts of our situation, it's amazingly difficult to communicate tone and emotion in a short, written piece. This would be why it's so much harder than one thinks to write a decent novel, let alone writing poetry.
From day to day, we rely on seeing and hearing the person to whom we're speaking. Seeing is good - all those lovely, tiny movements of the face that we can interpret almost without being conscious of them. Then there's the voice. Face-to-face we get visual and audio cues to a person's mood. Even the telephone is remarkable for allowing nuances in conversation - a laugh, a sympathetic sigh.
However, the text-based communication of the SMS and the blog comment compels us all to do this in a sentence. And that's where the emoticons and emoji enter.
So add the emoticons and emoji to your sentence - the dictionary approves and they're an efficient addition to communication when we can't see each other.