That look Jolene gives me?
It’s not one of stubbornness, it’s one of smarts.
Imagine my delight in reading the recent findings of a British study with horses, donkeys, and mules.
To summarize in three words:
Mules reign supreme.
[Jolene, if you’re reading this, I’m sorry I ever doubted what you were trying to tell me.] Read more on Jolene.
Dr. Britta Osthaus of Canterbury Christ Church University and her colleagues tested six donkeys, six horses, and six 
mules. Each animal was shown sets of two food buckets, each marked with a different symbol. Read more.
In order to gain access to the food, the animals had to pick the correct bucket. The mules learned to discriminate between more pairs of symbols than the horses or donkeys, and did so more consistently.
Said Osthaus:
“The mules’ performance was significantly better than that of either of the parent species and got faster over a period of time. Hopefully, our findings will ensure that people change their attitudes towards mules, which frequently have a bad reputation because they often are mentally under-stimulated and therefore might turn against humans or become destructive. This study provides the first empirical evidence that the improved characteristics of mules may be extended from physical attributes to cognitive function.”
Gotta say, those mules sound like the really bright kids who act out while bored and in a classroom full of less intelligent peers.
More on this research soon!