Poor study design.
"If they took the left branch of the Y, they would work two circles before the rider dismounted. If they took the right branch of the Y, they would work only one circle before the rider dismounted."
The study admitted it:
"Overall, the experimental setup did not seem to be appropriate to answer the research question. It is likely that the repeated mounting and dismounting caused discomfort or confused many of the horses to an extent that they did not actively select a treatment but rather searched for ways to evade further mounting (and riding). In conclusion, horses did not show a clear preference for either shorter or longer riding bouts, but their behavioral reactions indicate that they perceived mounting as uncomfortable and that their motivation to rejoin their herd-mates and/or to obtain feed in the barn was greater than their motivation to being ridden at all. Nevertheless, pronounced individual differences also seem to exist, with some horses showing little aversion to, and perhaps enjoyment of, being ridden, whereas others clearly preferring not to be ridden."Elsevier
Yep, Mia would head for the exit ASAP and go back to hanging out with other horses.
However, supposed they took jumpers who loved to jump, or cow horses who loved to work cattle, and tried it then? Or how about trail riding, either alone or with another horse?
Over the last 5 years, I think I've gotten to know Mia fairly well. She prefers hanging out in the corral, but going along a trail with another horse
isn't that far behind. Cowboy can be lazy, but he gets excited about getting to go on a trail ride with the other two horses instead of being left behind.
It is true that when I go jogging, none of my horses are going to jump the corral panels to come offer to carry me instead...probably a bonding problem...
...or maybe I suck as a leader.