What is the connection between a horse's behavior problems and the quality of the human-horse relationship?

The connection between behavior problems and relationship quality is a theme that Warwick Schiller has explored most deeply, and his position — supported by the observations of Pat Parelli and to a degree by Clinton Anderson — is that the vast majority of horse behavior problems have a relationship component, even when they appear to be purely technical or training issues. Schiller's framework identifies two primary relationship deficits that drive most behavior problems. The first is insufficient trust — the horse does not have enough confidence in the human as a reliable, safe, non-threatening presence, and its behavior problems are expressions of anxiety, defensiveness, or self-protection in response to perceived threat. Spookiness, biting when handled, refusing to be caught, and buddy sourness all fit this pattern. The second deficit is insufficient leadership — the horse does not have a confident, consistent human directing its energy and decisions, so it defaults to directing its own energy, which in horses translates to pushy, dominant, testing behavior. Clinton Anderson's approach addresses primarily the leadership deficit — his Fundamentals program is designed to establish the handler as the clear director of the horse's feet and decisions, which resolves most pushy, dominant, and testing behavior. His observation is consistent with Schiller's: horses without clear human leadership fill the leadership vacuum themselves, and a horse that has appointed itself leader will not listen to the handler the way a horse that has accepted human leadership will. Parelli's Horsenality system identifies which relationship deficit is dominant for a specific horse, because the treatment differs: a horse that is anxious needs confidence building through Friendly Game work and gradual exposure, while a horse that is dominant needs leadership establishment through the driving and disengaging exercises. Applying the leadership approach to an anxious horse, or the confidence approach to a dominant horse, typically makes the problem worse.