Some get depressed. Some get aggressive. And some just “turn off” completely and have no reaction at all.

We don’t need science to tell us that horses have a variety of behaviors with which to express negative emotions. But a recent review of behavior studies by a French behavior researcher confirms that these negative emotions—and the way horses express them—can be a direct result of the way the animals are trained and managed.

“Although more research is needed, the elements we already have clearly indicate that we should consider equine welfare to be critical not only in the horse’s health and physiological state, but in his relationship with humans as well,” said Clémence Lesimple, PhD, researcher at the University of Rennes. Lesimple presented her findings at the 2014 French Equine Research Day held March 18 in Paris.

In her review of hundreds of horses in various behavior studies, Lesimple found that horses housed in individual box stalls with little or no access to other horses tended to be more aggressive toward humans, she said. This was also true of horses fed diets high in concentrated starch feeds and low in forage—probably because of the “frustration” of not being able to chew all day long as well as potential gastric pain, she said