Throughout the course of the series, Brennan appears to be a brilliant anthropologist who lacks social skills. Her social ineptitude is especially apparent when it comes to pop culture jokes, sarcasm, and metaphors which she often interprets very literally. An example of this is when she mistakes Will Ferrell for Colin Farrell.
She had a difficult adolescence, and it is implied that her withdrawn social tendencies are a defence mechanism. She also sometimes struggles in identifying and explaining her emotions, and takes comfort in the rationality of her anthropologic discipline. Although it has been stated that Brennan was based on a person with Asperger syndrome, this has never been confirmed in the plot of the series.
The creator of the series has stated that the character was never labelled as having the syndrome in order to increase the appeal of the show on network television. This influence on her character also helps to explain her extreme rationality in early seasons, as well as some of her social difficulties. Brennan is a self-proclaimed atheist and often points out what she believes to be the irrationality of religious and spiritual beliefs. This has led to more than one argument with Booth, who is a devout Roman Catholic; he becomes particularly irate when she compares less common religions, such as voodoo, to Christianity.
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