There has been much controversy on forums about the personality type of Sheldon, thus we decided to address this. Before we start our analysis of Sheldon Cooper from the Big Bang Theory, view Principles of MBTI and functional stack of MBTI types.
Sheldon Cooper-ISTJ
Sheldon has Introverted Sensing(Si) as his dominant function. Despite advancing the field of physics, he is fixated on the past. He rarely talks about the future and instead tries to hold on to the past. He views the past with nostalgia, and is reluctant of change. This is seen as he is unhappy with his best friend Leonard moves out to stay with his fiancee Penny, and has fond memories of his meemaw.
Moreover, he behaves like a child, needing to be sang bedtime songs and loving to play with trains that he played with as a child, accentuating his desire to seek comfort in the past.
Sheldon also has an eidetic memory, being able to remember all details and facts. His detail oriented nature is especially seen when he wants to organize everything in his house in a very particular way. He also takes extreme comfort in his routines and even plans his bathroom breaks.
Moreover, he values social convention highly, always talking about the importance of social contracts.
A popular opinion is that Sheldon is an INTP. Even though INTPs have Si as their tertiary function, the frequency and intensity in which Sheldon displays Si throughout the movie makes it clear that Si is his dominant function.
Sheldon has Extraverted Thinking(Te) as his auxiliary function. In contrast to Ti, which is most often kept private, extraverted thinkers tend to speak their mind, criticizing the ideas of others. This is apparent throughout the movie as Sheldon does not hesitate to correct others. Another reason why he is ISTJ, not INTP
Furthermore, Sheldon attempts to impose an external order on the people and objects around him, requiring them to act in ways according to his expectations, such as wanting them to always take care of him when sick, drive him to work, or do not sit in his spot on the sofa.
Since he requires external data in order to back up his beliefs, he does not believe in any religion due to the lack of empirical evidence.
Sheldon has Introverted Feeling(Fi) as his tertiary function. INTP has Fe, not Fi. Fe users usually exude warmth and care about maintaining harmony within the group. Even though INTPs have Fe as their inferior function, they still care about the feelings of their friends to some extent. This is clearly not the case when it comes to Sheldon, as he often has no awareness or regard for how others feel.
Moreover, Sheldon keeps his emotions to himself. His depth of emotion is only displayed when he overreacts to certain stimuli and behaves irrationally.
He has a pretty good sense of what is right and what is wrong. He trusts his internal moral compass when he refuses to exclude Amy from being nominated for the nobel prize as he knows it is wrong to take all the credit himself.
Sheldon has Extraverted Intuition(Ne) as his inferior function. As this is his least developed function, he rarely uses it in a productive way, and instead always worries constantly about how everything can go wrong, and is afraid to take any risks. However, he does uses it for optimistic ideation when thinking about possibilities in scientific theory.