Current Research

On the Scary Side: lateralization of the horse


If you handle enough horses, you are certain to encounter a few that are quite sensitive about you being on the off side. They are nervous about movement on that side and if you should try to mount on that side, they are likely to act like they had never been ridden before. You might think that no one had spent enough time on that side of the horse; that its training was incomplete. But this sidedness is evident in unhandled horses as well. It's not about one side being trained; it's innate asymmetrical behavior.

Laterality Demonstration

In the documentary film Wild Horse Redemption, one of the horses followed through his training is named Sammy. His trainer, Jon Peterson, is challenged by Sammys' refusal to run clockwise in the roundpen. Sammy wants to only look at Jon with his left eye. For Sammy to run clockwise, Jon would be on his scary side. Here is a film clip of what this looks like.

In the film, Jon eventually gets Sammy to reverse his direction by running him with another horse.

As horse trainers, we experience it as the animal not being able to look at the scary object with one of his eyes. The scary side saw it, and off the animal ran. But it turns out that it is probably more an effect of needing to keep the scary object in view of the safer eye.

I train mustangs in preparation for adoption. A significant percentage of those animals are lateralized and the lateralized animals are much harder to train. I started wanting to know how fear and this one-sidedness were related. Asking this question took me into the scientific literature of brain functions, ocular neurology, emotional chemistry, --- all deep science and hard reading. But none of them told me how to fix a one-sided mustang. I kept reading, and then I started contacting the scientists who have studied the subject. I invited several to think about extending their work to mustangs. The result has been a fruitful think-tank.

Folk-Wisdom and Science try to explain why it happens.

I've asked a lot of horse trainers and scientists why they think it happens.

    Some ideas:
  • Incomplete Training theory: a horse that is one sided has been trained too much on one side compared to the other. We can discount this theory because this phenomenon is found even in wild animals that have had no training. However, this is not a reason to discourage training both sides of the horse.
  • Protecting the Strong Side theory: a horse instinctively protects its strongest and most valuable side, just as a right-handed person might protect himself by turning the left side of his body out to take an impending impact.

  • Vision theory: sidedness reflects poor vision on the side protected. They keep you where they can see you best. Horses that don't see well are more one-sided. This theory is challenged by the training of left-eye-blind horses, which often prefer to keep the trainer on the blind side.

  • Foal on Mothers side theory: Foals grow up with their mothers on one side which becomes the safe side, and the horse organizes its subsequent world to keep things that don't need to be watched on the safe side out of habit.

  • Perception Reception theory: the 2 sides of the brain are hardwired to process different kinds of information and the horse is trying to understand and process its world in a specific way by its orientation to stimuli.

  • Emotional theory : the 2 sides of the brain are hardwired for different emotions, and the emotional state of the animal plays into the need for a specific orientation (eg. The animal is tense so it orients with its safe eye on the stimulus. If it weren't tense it could orient differently.)

Here is the science:

Asymmetry of the brain is the physical reality.

The majority of the nerves coming out of the eyes are wired to the opposite side of the brain. So the left eye activates the right hemisphere of the brain. This makes it a little confusing to read the scientific literature. I find it useful to think of it this way:

RE=LHem and LE=RHem.

Lateralization was first noticed in humans that had damage to one side of their brain. It was discovered that in most people damage to the left side of the brain affected the right side of the body and the ability to use and understand language. Damage to the right side of the brain affected the left side of the body and spatial perceptions especially of space on the left side of the body. This was not true for all people though because for some fraction of people the language/spatial perception sides are flipped.

This asymmetry of behavior has been studied in many kinds of animals. Here is a summary table of results from the scientific literature. The actual experiments used to test for these effects range from studying eye-preference to destruction of parts of the brain.

RE or LHem LE or RHem
Prey stalking and catching (fish, toads) Predator detection (fish, chicks)
Food choice and manipulation (birds) Predator escape ( frog tadpoles, fish, toads)
Approach and manipulation of objects (birds, monkeys, apes) Avoidance/withdrawal (monkeys, apes, humans)
Regulation and expression of aggression (chicks, monkeys, humans) Aggression (toads, lizards, chicks, monkeys, horses)
Learning (dolphins) Fear (chicks, rats)
Recognition of categories/attention to large changes (birds, rats) Interaction between members of a social unit (fish, tadpoles)
Recognition of species-typical vocalizations (birds, mice, monkeys, humans) Recognition of friends (chicks, sheep, monkeys, humans)
Attention to landmarks (birds) Spatial cognition (birds, rats, humans)
Attention to local cues (birds, monkeys, humans) Attention to global cues (chicks, monkeys, humans)
Assessment of unfamiliar thing (horses, dolphins)

From all of these examples of various animals and behaviors, it is evident that this asymmetry of behaviors is common across the vertebrate world.


This topic has received some interest in Europe and Australia. Scientists in France found that mares preferred to look at novel stimuli with their right eyes and were slightly more likely to look at the white shirt of a veterinarian with their left eye.

My colleagues Kate Farmer, Konstanze Krueger, and Nichole Austin have published papers on the laterality of horses that have significance for training mustangs. Kate and Konstanze found that most domestic horses are left lateralized (scary side on the right). This sidedness is slightly stronger when horses are being handled by unknown persons rather than known persons, which means their emotional state affects the behavior. Finding that horses trained to be handled and trained from both sides showed less lateralization than horses only trained from the left means it can be changed by training.

Nichole documented that horses seem to attack each other from the left side more often than from the right. From the list of behaviors associated with each side we see that aggression and defense are typically associated with the left-eye/right-brain functions. It may be that when a horse shows a left-eye preference to look at a person it may be trying to communicate its willingness to defend itself.

What can we do about it.

The protocol for training lateralized mustangs at Mustang Camp is based on reducing their fears, eliminating their need for aggression or self-defense. We attempt to engage the other side of their brains by offering them the opportunity to engage with us to earn food rewards.

We start at the face of the mustang and start asking it to touch us with the scary side. We do not reach out to touch it, but give the horse an opportunity for complete control. If the horse is highly reactive, we sometimes work through a fence panel for safety.

Our horses are trained to hold their nose (NOT THEIR LIPS) to our fingers for five seconds or more for a food reward. We then start requiring the horse to touch with the cheekbone under the scary side eye. This first departure from the safe side can take a very long time, but once the horse gets that far, the rest is relatively easy. From the cheekbone, we go to the jaw, then to the neck, the shoulder, the withers, the ribs, the hip, the flank, and finally even the tail. We name each part for the horse and help by pointing to the part we want the horse to touch us with. The horse, in an amazing short time, learns the names of its body parts and willingly moves to touch us with parts it wouldn't even let us look at previously. If you have to train through a panel, be aware that forcing the horse (even just with food reinforcement) to let you on the scary side may open the door to aggression at first. Use extreme caution. This procedure will totally overcome the problem with practice. It may not be the best or most effective method and that is why we continue to search for a new solution.

About a third of the lateralized mustangs are lateralized with the left as the scary side. These horses are much more difficult to train but we are not sure if this reflects a difference in the horse trainability or the fact that horse training in general is geared towards training from the left.

Call for research

There are many questions we would like to investigate about this phenomenon.

  1. What is the most effective way to overcome this problem?
  2. Is a horse lateralized in the wild?
  3. Does training on the safe side make training on the scary side easier or harder?
  4. Can you train enough on the safe side to eliminate the underlying fear?
  5. What are the other behavioral differences in right and left lateralized horses?

First year research results

Here is a video of the presentation we made at the Animal Behavior Society Meeting in Boulder, CO. in August of 2013.

ABS presentation

A copy of our paper can be found in the files below.

Patricia Barlow-Irick,
Dec 19, 2013, 5:59 AM