horses and therapy and research

What is it about horses that is so healing for humans?

Maybe something about the facial geometry or symmetry of a horse mimics a particular human expression, maybe one of acceptance? Forgiveness? Love? - something we need or crave, anyway.

It's not necessarily a quality of or action by the horse that is healing; wouldn't it really be more about our perception of their qualities or actions? -our perceptions affect even physical (brain) healing (by?) directing our emotions.

Research in equine-based/aided therapy has faced many challenges, resulting in a lot of inconclusive or non-generalizable results. But maybe we're asking the wrong questions by focusing on the relationship; maybe it's all about perception.