In an ethics course in college I once told the professor that emotions are not rational, and before I could explain my meaning he jumped on it and made a caricature, “oh, women are so emotional they're irrational blah blah blah”.
But that was not what I meant at all.
It only now (thirteen years later) occurred to me that the word I was looking for was arational. It's perplexing that over 36 years I don't recall ever hearing any discussion of the concept of “arational”. At some point I learned about amorality as opposed to morality or immorality, which is why, while taking a shower earlier tonight, I realized there should be a concept of arationality as well, and there is. And analogous to amorality, arationality refers to a thing being neither rational nor irrational.
The philosopher A. C. Grayling pointed out that the word rational has the word ratio in it because it means “to proportion your belief to the evidence,” that is, the stronger the evidence for it, the more confident you should be that it is the case; conversely, the weaker the evidence, the more doubt you should reserve. As a child I was taught this by Bill Nye as “extraordinary claims require extraordinary evidence”. And he got it from Carl Sagan, who in turn got it from Marcello Truzzi. But the concept predates all of them by centuries, with brilliant minds like Laplace and Hume making similar statements, all of them essentially describing the meaning of rationality. Rational thinking means assessing the evidence to make a decision about what to believe and how much to believe it. Irrational means the decision is in direct conflict with the evidence, it means drawing the wrong conclusion about what the evidence is telling you.
But emotions are neither of those things, because emotions aren't decisions, no one decides to be angry, or happy, or in love, or heartbroken, or stressed.
Which is why I'd say emotions are arational.
This isn't a bad thing either, or a criticism. Emotions are a product of evolution, like instinct, they're evolutionary shortcuts to heuristically “good” behavior (as in fitness improving rather than morally good behavior). In organisms that reproduce sexually, some degree of tolerance of the opposite sex is required to produce offspring. When those offspring require a great deal of nurturing to mature into adolescents that can survive on their own, a greater degree of cooperation from the parents is incredibly beneficial to their survival. As the behavior of the organisms gets more complicated, and there is more interaction between individuals (as is the case in highly social species, like humans), the panoply of emotions plays a big role in their success. I'm getting off the rails here.