Many people with autism find it difficult to understand facial expressions, tone of voice, jokes and sarcasm – often interpreting language very literally.
Source: Sweepstake poster @ The National Autistic Society
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Image "Emotions3" by allyaubry☆ - http://www.flickr.com/photos/allyaubryphotography/2535453766/lightbox/. Licensed under CC BY 2.0 via Wikimedia Commons.