This is a link to an article at TheHorse.com about Rider and Handler Effect on Horse Behavior:
http://www.thehorse.com/ViewArticle.aspx?ID=19162
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The study "Equine behaviour and heart rate in temperament tests with or without rider or handler" was published in Physiology and Behavior in May 2011.
The abstract can be found here:
http://www.ncbi.nlm.nih.gov/pubmed/21616087
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Some key findings in the study include:

* Riding was linked with the strongest or medium reactions from horses, while leading was linked with the weakest reactions;
* Rider level influenced the horses' emotional expressions; horses ridden by professionals earned lower scores (showing less reaction) than those ridden by amateurs; and
* Horses' heart rate and behavior did not always coincide (i.e., the horse could appear calm but still have an elevated heart rate), leading researchers to believe that riders or handlers might have more influence over the horse's behavior than his physiological reaction.

Quote: "The present study shows that a rider or handler influences, but not completely masks, the horses' innate behavior in a temperament test," concluded the team."