When looking at the angry faces, the horses were more likely to turn their heads so they were using their left eyes - something they are known to do in response to negative stimuli.
Their heart rates also increased and they displayed more signs of stress than when they were shown happy faces, the journal Biology Letters reported.
Amy Smith, who co-led the study, said the horses' stronger reaction to negative expressions could be a mechanism that helps them respond to threats.
The scientists said the ability to read human faces could be an evolutionary trait, or something that individual horses learn over their lifetime.
- Daily Mail