His commitment to his friends is often called into question, as he is known for constantly and consistently being condescending and rude, many times even without being intentionally hurtful.

Included among them are parents of the main characters, their dates and their coworkers.

Celebrities such as Stephen Hawking occasionally appear in cameo roles as themselves.

A running gag has him always knocking on doors in a pattern of three knocks and one call repeated three times, which he has the urge to complete no matter what.

Sheldon has major difficulties in social settings; he shows great difficulty in identifying sarcasm and irony, and a recurring topic is his efforts to acquire those skills.

Sheldon refrains from any form of physical contact if possible and exhibits a strict adherence to routines to the point of exhibiting signs of obsessive–compulsive personality disorder.

Any disruption to routines distresses him enormously.

He shares an apartment in Pasadena with friend and fellow physicist Dr. Sheldon has a deep love and interest for science fiction and comic books, to the point of an obsession, and shows particular affinity for Spock, a fictional character from the Star Trek franchise.

He usually wears clothing emblazoned with logos for these interests as well as science-related designs.

Leonard's family includes other accomplished scientists: his mother, Dr.

Beverly Hofstadter, is a neuroscientist and world-renowned psychiatrist, his sister is a medical researcher, and his father, Dr. Also, his brother Michael is a Harvard law professor.

The two attempted a long-distance relationship after Priya moved back to India, but Leonard eventually broke up with her in season five after she admitted to cheating on him with her former boyfriend.