Many of us have or have had friends who will enter into abusive relationships and then stay with their abusive partner even after they have openly admitted they are unhappy. We will wonder, "If you are so unhappy, why don't you leave him/her?" There is a portion of psychological literature that tells this story and will perhaps answer the aforementioned question.
In this story, I'll have the man as the abuser and the woman as the victim, not because it can only happen this way but because men are more often the abuser. It starts with Bandura's idea of vicarious conditioning: the idea that you can receive reinforcement for a behavior by observing it. If a boy watches his father verbally or physically abuse his mother and does not see the father receive punishment for his misbehavior, he will be vicariously conditioned to be more likely to perform this behavior in the future. The child will identify with the parent of the same gender so girls will be conditioned similarly if they observe their mother being abused by their father. The boy will observe cowering cues from his mother and then learn to identify these cowering cues in women he will meet as an adult. He will also understand the feeling of power that his father had and consciously or subconsciously realize that these cowering cues present an opportunity for him to enter into a relationship with someone he can assert power over. The girl will similarly seek out men that display aggression since she observed her father not being punished or even being rewarded for his abusive behavior.
Once these two people find each other and enter into a relationship, the woman will realize over a period of time that she is suffering at the hands of this abusive man but will have difficulty leaving him. Why is this? According to Swan's Self Verification Theory, people strive for both consistency and self-enhancement. However, the drive for consistency is ultimately more powerful. This idea was proven in a study where one group of students with low self-esteem and another group of students with high-self esteem were given a test and then received fake results. The high self-esteem students were told that they did poorly on the test and were quick to attribute the poor results on the test to the situation and not themselves. The low self-esteem students were told that they performed excellently on the test and briefly reported a feeling of self-enhancement. However, they quickly disregarded their boost in self-esteem and claimed that the test had nothing to do with their intelligence. Ultimately, the drive for consistency was so powerful that they were willing to disregard the feeling of increased self-esteem in order to maintain consistency (the technical term for this is a "cognitive affective crossfire").
I believe the results of this study can be applied and used to explain the woman's difficulty in leaving this abusive relationship. Women in abusive relationships recognize that they are in pain but see the idea of abandoning consistency as even more frightening. They may have grown up with abuse to the point where it is comfortable; to the point where the discomfort of leaving what they know is powerful enough to keep them around for further doses of abuse. Some women even experience what I believe is a form of cognitive affective crossfire when they briefly leave their abusive partner for a non-abusive man and experience self-enhancement. At this point, the woman can either overcome the discomfort or will experience the cognitive affective crossfire by dumping the newly found/non-abusive man and running back into the arms of the abuser in order to maintain consistency.
This vicious cycle is difficult to avoid for people who were raised in abusive households. However, I believe that something can be done to moderate or even mediate the abuse. Seeing a therapist who can bring to light these tendencies to abuse or to seek abusers is a wonderful form of intervention. By using collaborative empiricism in order to identify the roots of maladaptive behaviors, the patient can learn to identify abusive people they are likely to end up with and consciously choose to avoid them or perhaps finally realize why they are having trouble leaving an abusive relationship. Being abused as a child is unfortunate but does not doom one to add chapters onto the history of abuse if the pattern and logic of abusive behavior can be recognized and presented to the abuser and/or victim.