Humans are emotional animals. The degree of this “emotionality” varies from person to person. From childhood, most of us are raised in a family, a society, a country. Basically with other living beings, animals, ideology and beliefs. We tend to emotionally relate with others based on similarities or common beliefs and preferences. We are naturally, emotionally attached to our parents, siblings. Some develop emotional attachments to pets, animals, trees, environment. And there are some, who develops attachments to even objects (kids are emotionally attached to their toys). And then, there are cases of emotional attachment of masses. Example: Sachin Tendulkar for Indians and cricket fans.

There is another side of emotionality. While in the previous para, I described some of the examples where a person is emotionally positively related to a person, belief or even a thing. There is this other version, where people are emotionally negatively related to a person, belief or thing. A classic example is Manchester United and Liverpool. Both sets of fans have a strong disliking for each other and the clubs. Other examples: Marvel and DC, Windows and Mac, Narendra Modi and Arvind Kejriwal (+Mamta + Mayawati+ Left + Congress + Lalu, etc.), India and Pakistan.

Before describing the relation between emotions and stupidity, I would like to describe a very basic, fundamental fact that is most often overlooked due to the emotional nature of human beings. This simple fact is: nothing is absolutely black or absolutely white, absolutely good or absolutely evil, absolutely correct or absolutely wrong. Things, people, ideology, beliefs are shades of black and white (except in real world, there are way more combinations than black and white). So, a person necessarily considered evil may have some good qualities (Hitler started an anti-tobacco movement in Nazi government). Similarly, a person widely considered correct or good may have some shortcomings and may have done something wrong (Mahatma Gandhi didn’t quite put a strong effort/movement to oppose the hangings of Bhagat Singh, Sukhdev, Rajguru just because they followed a different ideology to achieve a common goal).

Now coming to the point. My simple theory is:
“A person who’s emotionally attached (positively or negatively) to any subject (living, non-living, belief, organisation, etc.), his/her ability to make rational judgements about the subject is lesser than when he/she is emotionally not attached to that subject; provided in both cases, the person is equally knowledgeable about the subject.” An emotional person struggles to see the different shades of subjects. Too strong emotions can lead to black and white judgement (he may consider a belief absolutely correct and or absolutely wrong, e.g. ISIS members).
A good anti-example: Sherlock Holmes

Though fictitious, a perfect example of emotionally blunt, highly rational individual.
Also note that stupidity doesn’t necessarily mean the person is emotional. Many times, people are just stupid.

Does this all mean we should stop being emotional at all and try to pretend as if we are more rationals? Absolutely Not. Emotionality is one of the key characteristics of our evolved (evolving) brain. Emotionally attached people are more passionate about their subjects. Think about the people with whom you’re emotionally attached. Without emotions, life may seem meaningless and boring.
However, to avoid irrational judgements, we have to simply increase our overall ability to make rational judgements. This will automatically compensate for the lack of rationality in emotional subjects. And sometimes yes, we should learn to control our emotions.

Inspired by teachings of Lord Krishna in Bhagwad-Gita on Nishkam Karmayog.