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Horses prefer to look at negative and threatening sights with their left eye, and positive social stimuli with their right eye. In the study, when they saw a model they had seen frowning earlier, they spent more time looking with their left eye. They also exhibited more stress-related behaviours, like scratching and floor sniffing. In contrast, when they saw a model they had seen smiling earlier, they spent more time looking with their right eye.

https://www.newscientist.com/article/2167423-horses-remember-if-you-smiled-or-frowned-when-they-last-saw-you/

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