Look at this guy.
He is half-smiley, half-frowny. I drew the mouth carefully to make it equal parts sad and happy.
But when you look at him — take him in whole — would you say he's having a good day or a bad day?
Most people would say: good day. He seems a little more smiley than not.
That's because, says science writer Sam Kean, when we look at somebody, the left side of that person's face is more emotionally powerful and "determines the overall emotional tenor."