Final answer:
Fixation in the oral stage occurs due to being weaned too early or breast-fed for too long, resulting in tendencies such as smoking or overeating. Freud's psychosexual development theory is controversial but still influences various fields.
Step-by-step explanation:
Fixation in the oral stage occurs because of being weaned too early or breast-fed for too long. According to Freud, the oral stage of psychosexual development occurs from birth to 1 year, where pleasure is focused on the mouth. Weaning from the bottle or breast at around 1 year of age can create conflict if not handled properly, resulting in fixation tendencies, such as smoking, overeating, or nail-biting, in adulthood.
It is important to note that Freud's psychosexual development theory is controversial and not supported by empirical evidence. However, his ideas continue to contribute to the work of scholars in various disciplines.