Frustration or over-gratification was said to result in an oral fixation and in an oral type of character, characterized by feeling dependent on others for nurturance and by behaviours representative of the oral stage. Later psychoanalytic theories shifted the focus from a drive-based approach of dependency to the recognition of the importance of early relationships and establishing separation from these early caregivers, in which the exchanges between the caregiver and the child become internalized, and the nature of these interactions becomes part of the concepts of the self and of others. References
Sources
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They are often anxious and tend to feel guilty. These people have difficulty acknowledging and expressing anger and struggle to get their own needs and goals met. Unable to soothe or comfort themselves when distressed, they require involvement of another person to help regulate their emotions. Psychodynamic Diagnostic Manual
The Psychodynamic Diagnostic Manual (PDM) approaches dependent personality disorder in a descriptive, rather than prescriptive sense and has received empirical support. The Psychodynamic Diagnostic Manual includes two different types of dependent personality disorder:
Passive-aggressive
Counter-dependent The PDM-2 adopts and applies a prototypic approach, using empirical measures like the SWAP-200. It was influenced by a developmental and empirically grounded perspective, as proposed by Sidney Blatt. This model is of particular interest when focusing on dependent personality disorder, claiming that psychopathology comes from distortions of two main coordinates of psychological development:

The anaclitic/introjective dimension. The relatedness/self-definition dimension.The anaclitic personality organization in individuals exhibits difficulties in interpersonal relatedness, exhibiting the following behaviours:

Preoccupation with relationships
Fear of abandonment and of rejection
Seeking closeness and intimacy
Difficulty managing interpersonal boundaries
Tend to have an anxious-preoccupied attachment style.Introjective personality style is associated with problems in self-definition. Differential diagnosis
There are similarities between individuals with dependent personality disorder and individuals with borderline personality disorder, in that they both have a fear of abandonment. Those with dependent personality disorder do not exhibit impulsive behaviour, unstable affect, and poor self-image experienced by those with borderline personality disorder, differentiating the two disorders. Treatment
People who have DPD are generally treated with psychotherapy.