According to Sigmund Freud, what we do and why we do it, who we are and how we became this way are all related to our sexual drive. Differences in personalities originate in differences in childhood sexual experiences. In the Freudian psychoanalytical model, child personality development is discussed in terms of "psychosexual stages". In his "Three Essays on Sexuality" (1915), Freud outlined five stages of manifestations of the sexual drive: Oral, Anal, Phallic, Latency, and Genital. At each stage, different areas of the child's body become the focus of his pleasure and the dominant source of sexual arousal. Differences in satisfying the sexual urges at each stage will inevitably lead to differences in adult personalities. Conflicts between the sex drive and rules of society are present at every stage. A proper resolution of the conflicts will lead the child to progress past one stage and move on to the next. Failure to achieve a proper resolution, however, will make the child fixated in the present stage. The latter is believed to be the cause of many personality and behavioral disorders.
1. Oral Stage (Age 0 - 1.5)
Erogenous Zone in Focus: Mouth
Gratifying Activities: Nursing - eating, as well as mouth movement, including sucking, gumming, biting and swallowing, [and possibly oral "self injury..?].
Interaction with the Environment: To the infant, the mother's breast not only is the source of food and drink, but also represents her love. Because the child's personality is controlled by the id and therefore demands immediate gratification, responsive nurturing is key. Both insufficient and forceful feeding can result in fixation in this stage.
Symptoms of Oral Fixation:
Constant chewing on gum, pens, pencils, etc.
Sarcasm ("the biting personality") and verbal hostility
[Possibly "oral self-injury....?]
Also, see http://peace.saumag.edu/faculty/Kardas/Courses/GPWeiten/C12Personality/FreudStages.html
For more about oral fixation and Freud's stages of development.