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What is Stockholm syndrome and explain in detail?
Stockholm syndrome is a theorized condition in which hostages develop a psychological bond with their captors during captivity. It is supposed to result from a rather specific set of circumstances, namely the power imbalances contained in hostage-taking, kidnapping, and abusive relationships. Therefore, it is difficult to find a large number of people who experience Stockholm syndrome to conduct studies with any sort of power. This makes it hard to determine trends in the development and effects of the condition— and, in fact, it is a "contested illness" due to doubt about the legitimacy of the condition.Emotional bonds may be formed between captors and captives, during intimate time together, but these are generally considered irrational in light of the danger or risk endured by the victims. Stockholm syndrome has never been included in the Diagnostic and Statistical Manual of Mental Disorders or DSM, the standard tool for diagnosis of psychiatric illnesses and disorders in the US, mainly due to the lack of a consistent body of academic research.  The syndrome is rare: according to data from the FBI, about 8% of hostage victims show evidence of Stockholm syndrome.This term was first used by the media in 1973 when four hostages were taken during a bank robbery in Stockholm, Sweden. The hostages defended their captors after being released and would not agree to testify in court against them. It was noted that in this case, however, the police were perceived to have acted with little care for the hostages safety, providing an alternative reason for their unwillingness to testify. Stockholm syndrome is paradoxical because the sympathetic sentiments that captives feel towards their captors are the opposite of the fear and disdain which an onlooker might feel towards the captors.
There are four key components that characterize Stockholm syndrome: 

A hostages development of positive feelings towards the captor
No previous relationship between hostage and captor
A refusal by hostages to cooperate with police and other government authorities
A hostages belief in the humanity of the captor, ceasing to perceive them as a threat, when the victim holds the same values as the aggressor.The term "Stockholm syndrome" has also been used to describe the reactions of some abuse victims beyond the context of kidnappings or hostage-taking. Actions and attitudes similar to those with Stockholm syndrome have also been found in victims of sexual abuse, human trafficking, extremism, terrorism, economic oppression, financial repression,  political repression and religious persecution.
 This is because Stockholm syndrome can be argued as "another method of coping with the stress and danger...similar to some forms of coping in that the participants do not directly address the problem but find a way to cope with the situation by identifying with the aggressor. Coping mechanisms such as these can have a large impact on PTSD."Helsinki syndrome is a term sometimes used incorrectly instead of Stockholm syndrome. The confusion is often deliberate and used for ironic effect. 
It originates in the substitution of one Nordic capital (Stockholm, Sweden) for another (Helsinki, Finland). It entered popular culture when used in the Bruce Willis film Die Hard, by a doctor appearing on a television show and describing the phenomenon. The bumbling host says this refers to "Helsinki, Sweden", and the doctor corrects him, saying "Finland".

History
Stockholm bank robbery
In 1973, Jan-Erik Olsson, a convict on parole, took four employees (three women and one man) of Kreditbanken, one of the largest banks in Stockholm, Sweden, hostage during a failed bank robbery. He negotiated the release from prison of his friend Clark Olofsson to assist him. They held the hostages captive for six days (23–28 August) in one of the banks vaults. When the hostages were released, none of them would testify against either captor in court; instead, they began raising money for their defense.Nils Bejerot, a Swedish criminologist and psychiatrist coined the term after the Stockholm police asked him for assistance with analyzing the victims reactions to the 1973 bank robbery and their status as hostages. As the idea of brainwashing was not a new concept, Bejerot, speaking on "a news cast after the captives release" described the hostages reactions as a result of being brainwashed by their captors. He called it Norrmalmstorgssyndromet (after Norrmalmstorg Square where the attempted robbery took place), meaning "the Norrmalmstorg syndrome"; it later became known outside Sweden as Stockholm syndrome. It was originally defined by psychiatrist Frank Ochberg to aid the management of hostage situations.This analysis was provided by Nils Bejerot after he was criticized on Swedish radio by Kristin Enmark, one of the hostages. Enmark claims she had strategically established a rapport with the captors. She had criticized Bejerot for endangering their lives by behaving aggressively and agitating the captors. She had criticized the police for pointing guns at the convicts while the hostages were in the line of fire and she had told news outlets that one of the captors tried to protect the hostages from being caught in the crossfire. She was also critical of prime minister Olof Palme, as she had negotiated with the captors for freedom, but the prime minister told her that she would have to content herself to die at her post rather than give in to the captors demands.
Olsson later said in an interview:It was the hostages fault. They did everything I told them to. If they hadnt, I might not be here now. Why didnt any of them attack me? They made it hard to kill. They made us go on living together day after day, like goats, in that filth. There was nothing to do but get to know each other.

Mary McElroy
Mary McElroy was abducted from her home in 1933 at age 25 by four men who held a gun to her, demanded her compliance, took her to an abandoned farmhouse and chained her to a wall. She defended her kidnappers when she was released, explaining that they were only businessmen. She then continued to visit her captors while they were in jail. She eventually committed suicide and left the following note: “My four kidnappers are probably the only people on Earth who dont consider me an utter fool. You have your death penalty now – so, please, give them a chance."

Patty Hearst
Patty Hearst, the granddaughter of publisher William Randolph Hearst, was taken and held hostage by the Symbionese Liberation Army, "an urban guerilla group", in 1974. She was recorded denouncing her family as well as the police under her new name, "Tania", and was later seen working with the SLA to rob banks in San Francisco. She publicly asserted her "sympathetic feelings" toward the SLA and their pursuits as well. After her 1975 arrest, pleading Stockholm syndrome (although the term was not used then, due to the recency of the event) did not work as a proper defense in court, much to the chagrin of her defense lawyer F. Lee Bailey. Her seven-year prison sentence was later commuted, and she was eventually pardoned by President Bill Clinton, who was informed that she was not acting under her own free will.

Sexual abuse victims
There is evidence that some victims of childhood sexual abuse come to feel a connection with their abuser. They often feel flattered by adult attention or are afraid that disclosure will create family disruption. In adulthood, they resist disclosure for emotional and personal reasons.

Lima syndrome
An inversion of Stockholm syndrome, called Lima syndrome, has been proposed, in which abductors develop sympathy for their hostages. An abductor may also have second thoughts or experience empathy towards their victims.Lima syndrome was named after an abduction at the Japanese embassy in Lima, Peru, in 1996, when members of a militant movement took hostage hundreds of people attending a party at the official residence of Japans ambassador.

Symptoms and behaviors
Victims of the formal definition of Stockholm syndrome develop "positive feelings toward their captors and sympathy for their causes and goals, and negative feelings toward the police or authorities". These symptoms often follow escaped victims back into their previously ordinary lives.

Physical and psychological effects
Cognitive: confusion, blurred memory, delusion, and recurring flashbacks.
Emotional: lack of feeling, fear, helplessness, hopelessness, aggression, depression, guilt, dependence on captor, and development of post-traumatic stress disorder (PTSD).
Social: anxiety, irritability, cautiousness, and estrangement.
Physical: increase in effects of pre-existing conditions; development of health conditions due to possible restriction from food, sleep, and exposure to outdoors.

Ronald Fairbairns object relations theory of attachment to the abuser
Ronald Fairbairn wrote a complete psychoanalytic model in a series of papers (1940, 1941, 1943, 1944)  which are collected in his 1952 text Psychoanalytic Studies of the Personality. His model explains the surprising psychological reality that abused children become deeply attached to their abusers. He saw that lack of love, chronic indifference and abuse led to a counter-intuitive emotional attachment to the very parent who was abusing them.  The childs unmet dependency needs from chronic emotional deprivation, as well as the complete lack of other human alternatives in their environment, leaves the child stuck at an earlier emotional age, as they have not been able to continue their developmental progress in the absence of parental help and support. Thus the child may be 12, but emotionally and developmentally they may experience the world like a six-year-old, as their increasingly developmental needs force them to focus on the abuser, waiting for any hint of developmental support. The child becomes concerned for the abusers welfare because their developmental progress hinges upon on the whims, moods and emotional state of the abusive parent. In addition to the pressure from unmet developmental needs, the child is also aware of the potential danger that can emerge from the volatile and aggressive parent, and anything that they can do to placate, please or draw praise from the abuser increases their chance of survival.
The neglected or abused childs utter helplessness and absolute dependency upon the goodwill of their parents prevents them from "seeing" or remembering those interpersonal events in which they have faced indifference or physical abuse, as this awareness would overwhelm them and submerge them in a torrent of dread. This feeling of dread is most often experienced as a massive abandonment panic during those moments when the child realizes that they are living in constant danger with no one to help them to survive. The solution to this enormous problem is for the child to encase themself within a thick psychological cocoon of denial and fantasy that creates a false reality in which they believe that they are living in a loving and caring family.
The first way that the child protects themself is by using the greatest reality-altering defense that humans have at their disposal, which is the defense of dissociation. The dissociative defense mechanism is seen in adults who have suffered a life-threatening trauma, and dissociation prevents them from fully realizing what has happened. In children, the same defense protects the child by forcing intolerable memories of neglect, abuse, or total indifference that they suffered at the hands of their parents into their unconscious, where these memories will not disturb the childs illusion that they live in a safe and loving family. The dissociative defense is the basis of what is commonly called denial. The more frequent the abuse, the more frequently dissociation is required and the larger and larger the number of intolerable memories are forced into the unconscious. Once lodged in their unconscious, the child cannot remember the horrifying incidents that they previously experienced.

Splitting defense
The child not only dissociates memories of the abusive parent, but also memories of themselves in those anxiety-filled encounters with the rejecting parent. Their memory of themselves in these situations is one of being a frightened, impotent, and vulnerable child who is overwhelmed and deeply ashamed because they are unable to protect themselves when confronted by the aggressive parent. If they had access to these memories of themselves, they would inform their conscious ego that they were in a dire, life-threatening situation, information that is too catastrophic to accept. Over time, these memories of themselves in relationship to their rejecting parent coalesce and form internal representations. The process of dissociation of memories of the self and of the parent is called "splitting of the ego" or simply "splitting", because part of the childs original conscious ego (or self) is "split off" from the rest of their normal view of themselves and hidden in their unconscious. Similarly, the memories of that part of the angry, enraged and irritated parent are split off from the "normal" aspects of the parent and held in the unconscious as well. The memories of the angry parent are appropriately called the "Rejecting Object" in Fairbairns model. "Object" is an awkward term used in psychoanalytic theory to designate a person outside the self. So both the terrified memory of themselves and the abusive aspect of the parent (the object) are split off from the conscious self and they become "part selves" and "part objects", The terrified part of the self (called the "Antilibidinal Ego" in Fairbairns model) and the terrorizing part of the object are cut off from consciousness and are no longer associated with the conscious representation of either the self or the object. This affords the child a (false) sense of security that prevents them from feeling anxious about their fate from moment to moment.
Now that the abused child has split off memories of abuse, they have a second equally significant problem, which is to create an illusion for themselves that they are living in a safe environment. Splitting is the perfect defense for the abused child because not only is it able to isolate the unacceptable aspects of the parents in the unconscious, but, equally importantly, it is able to create a fantasy-based view of the parent out of their neglectful, indifferent or abusive parent(s). This psychological mechanism begins when the child selectively takes those few moments of attention or tenderness that has been shown to them by their parent and magnifying them and creating a “better parent” . The process is the same, in that the few positive incidents from the real parent are split off from the actual parent, and are forced into their unconscious as well. This view of the parent (which is unrealistic) is "enhanced by the child’s unmet needs and [their] use of fantasy." The child holds a view that somewhere in their parents heart there is a hidden storehouse of love, if they only knew how to reach it. This fantasy-based view of the parent is called "the Exciting Object" in Fairbairns model, as the child feels excitement when they fantasize that they have a loving parent. The childs part-ego (or self) that relates to the Exciting Object is called "the Libidinal Ego". In Fairbairns model, Libidinal means loving. Fairbairn had seen children with libidinal fantasies in the orphanage where he worked from 1927 to 1935. For a full discussion of the splitting defense, and Fairbairns structural theory see Celani, 2010. The two pairs of unconscious structures do not know about each other, which allows the child "to relate to the parent as if [they] were two different people." The splitting defense prevents the integration of good and bad object images into a single ambivalent object, which is a key developmental milestone.

Literature is filled with real examples of children creating fantasy parents out of their failed actual parents, as the following one-page essay by the writer Junot Diaz, who was born in Santo Domingo, describes. Unlike many neglected children, Diazs fantasy was more conscious than unconscious and based on the "promise" that his father was going to take the whole family to the United States to join him. He added the hope that his father would save him and the family in the process. But my earliest exposure to television was a Spider-Man cartoon - one of the flipped out Ralph Bakshi episodes from the late sixties...A little context: I had a father in New York City whom I did not remember, and who (it was promised) would one day deliver my family to the States. And here was my first television and my first cartoon and my first superhero — a hero who like my father, was in America—and somehow it all came together for me in a lightning bolt of longing and imagination. My fathers’ absence made perfect sense. He couldnt come back right away because he was busy fighting crime in NYC...as Spider–Man. The diasporic imagination really is its own superpower...I believed I had seen my father on that TV, and if I paid close enough attention it would show him to me again...For the record: my father did eventually return and take us to the States...My father was the worst shock of all. He had no problem laying hands on us kids for the slightest infraction. Beatings like he was making up for lost time. Like he was mad that he had a family...Are you surprised, then, that I was drawn back to the television? Because I was lost, because I wanted help with my English, because my father was a nightmare. And because I was convinced, foolish little fantasist that I was, that somehow my family and I had ended up in the wrong America and that the country and the father I’d first glimpsed on TV in Santo Domingo, the country and father I’d been promised, was still out there somewhere. I just had to find them. Never did. (Diaz, 2017, p.42 )
This essay demonstrates just how strong the need for a “good object” parent is, and how it motivates children to hold on to illusions, despite the overwhelming crush of reality. A "Good Object" is a parent or parent-like figure who fulfills the parenting role, including being interested in, and respectful of, the childs developmental needs. When the writers first elaborate fantasy was disproved, he did not give up fantasizing, because his need for a parent continued to be great, so he assumed that there was a second America where his good father resided. For a full description of the libidinal ego and the exciting object see Celani, 2010, pp. 58–115.

Intense relationships between the ego structures
The relationship between the two split off part-selves and their respective part-objects is intense because they were created out of enormous need, pain and desire. The intense need of the child for a good, loving object cannot be described in a more powerful way that the preceding quote by Diaz. He notes that his desperation was fueled because he was lost, he needed help learning English and needed an escape from his violent father. He was seeking a new father that would right all the wrongs that he had suffered.

On the other side of the split is the childs antilibidinal ego, which is intensely motivated to force the rejecting object parent to become a good object, and own up to the mistakes they have made by rejecting their child. Conversely, the rejecting internalized parent (who is an internalization of the original parent)  holds its ground and endlessly argues that the child was deserving of their condemnation. This dialogue continues in the unconscious, as described in the following quote by Odgen (2010)Neither the rejecting object nor the internal saboteur (the antilibidinal ego) is willing or able to think about, much less relinquish, that tie. In fact, there is no desire on the part of either to change. The power of that bond is impossible to overestimate. The rejecting object and the internal saboteur are determined to nurse their feelings of having been deeply wronged, cheated, humiliated, betrayed, exploited, treated unfairly, discriminated against, and so on. The mistreatment at the hands of the other is felt to be unforgivable. An apology is forever expected by each, but never offered by either (Odgen, 2010, p. 109).The “tie” that Odgen mentions is the emotional investment that each part-ego, or part-object structure, has in fighting with the other. The combination of the libidinal egos tie to finding love in the elusive and ever-shifting exciting object, and the equally motivated antilibidinal egos desire to force the rejecting object to apologize and see his/her value as a human being constitute what Fairbairn called "The Attachment to the Bad Object". The "Bad Object" is a parent or other significant caretaker who has failed the child, but is still cherished by the libidinal ego and fought against by the antilibidinal ego. This model, of separate ego states, that see different “parts” of the other (the object) explains the extraordinary attachment between the battered woman and her abuser (see Celani, 1995).

Model of attachment to the bad object
Fairbairn saw his model of human behavior as universal, that is, he assumed that all children, no matter how benevolent their family environment was, had to dissociate a few intensely frustrating events and, at other times, had to fantasize that their parents had hidden love that they were not displaying; that is, they used the same psychological mechanisms as did children from abusive families, but to a lesser extent. The following analysis is not based on interviews of the four victims, but rather is the result of applying Fairbairns model to the reported behavior of the four individuals.

Antilibidinal ego-rejecting object side of the split
When the bank robber and  his accomplice, who was released from prison and allowed to join him, began their six-day hostage taking, the four adult prisoners faced the same environment as do abused children; that is, their lives were absolutely dependent upon the good will of their captors, who had unlimited power over their lives. Their captors were far more important to them than were the police, who were a threat to all of them, captives and criminals alike. Fairbairns model assumes that the captives used the splitting defense to abolish the most terrorizing aspects of their captivity, in order to keep from breaking down into an absolute state of anxiety. This initial dissociation of the most terrifying events they experienced with their captors prevented the four victims from facing the disintegration of their ego structures. Once freed, the most frightening and toxic actual events they experienced are assumed to be still held out of awareness, as revisiting those events are likely to bring up overwhelming emotions.  Fairbairn noted that one of the primary reasons for keeping horrifying memories in the unconscious was because of the emotional disruption caused when they are re-experienced.There is little doubt in my mind, in conjunction with another factor to be mentioned later, that the deepest source of resistance is fear of the release of bad objects from the unconscious: for when such bad objects are released, "the world around the patient becomes peopled with devils which are too terrifying to face" (Fairbairn, 1952, p.69-70).
This quote graphically describes the results of suddenly remembering those memories of interpersonal events between the captives and captors that were saturated with fear, dread and hopelessness. There is no reason now, given the fact that the captivity is long over, for the four victims to remember the horrifying details.

Libidinal ego-exciting object side of the split
The other side of the split is abundantly obvious. All four victims refused to testify against their captors, and in fact raised money for their defense. Thus, given Fairbairns theory, they continue to see their captors through their libidinal egos as if the captors have a hidden storehouse of goodness somewhere in them. This view of reality could not continue, in Fairbairns theory, if the four captives were able to access the fear, terror and, indeed, rage assumed to be held in their Antilibidinal Ego-Rejecting Object structures. The depth of their fear and rage at being abused, would clash with their split-off, opposite view of the hidden “goodness” in the captors. As mentioned, the splitting defense allows the user to see others as if they are two different people.
This offers a second possible reason that the terrifying memories of events remain dissociated (in the Antilibidinal Ego-Rejecting Object structures in the unconscious). If one or more of the captives were able to experience these feelings directly (including the impotent rage), during the six days when they were held captive in the presence of the captors, they might have been killed for being disruptive and threatening. This ultimate terror, of being killed for experiencing the fear/rage and humiliation that is assumed to have been dissociated into the unconscious, may be the motivation that promotes the libidinal egos view of the two captors to continue, and simultaneously avoiding the enormously toxic memories of their six days in captivity. Thus, Fairbairns model offers a sound psychological explanation for attachment to abusers (Celani, 1995).

Possible evolutionary explanations
Evolutionarily speaking, research evidence exists to support the genuine scientific nature of Stockholm syndrome. Responses similar to those in human captives have been detected in some reptiles and mammals, primates in particular. Abuse and subsequent submission and appeasement by the victim have been observed among chimpanzees, leading to the theory that the Stockholm syndrome may have its roots in evolutionary needs.Life in the "environment of evolutionary adaptedness" (EEA) is thought by researchers such as Israeli military historian Azar Gat to be similar to that of the few remaining hunter-gatherer societies. Gat asserts that war and abductions were typical of human pre-history. Being captured by neighbouring tribes was a relatively common event for women. In some of those tribes (the Yanomamo, for instance), practically everyone in the tribe is descended from a captive within the last three generations. As high as one in ten of females were abducted and incorporated into the tribe that captured them. Being captured and having their children killed may have been common; women who resisted capture risked being killed. When selection is intense and persistent, adaptive traits (such as capture-bonding) become universal to the population or species.

Loving to survive
First published in 1994, author Dee Graham uses the Stockholm syndrome label to describe group or collective responses to trauma, rather than individual reactions. Graham focuses specifically on the impact of Stockholm syndrome on battered and abused women as a community. She claimed that in both the psychological and societal senses, these women are defined by their sense of fear surrounding the threat of male violence. This constant fear is what drives these women to perform actions that they know will be pleasing to men in order to avoid emotional, physical, or sexual assault as a result of male anger. Graham draws parallels between women and kidnapping victims in the sense that these women bond to men to survive, as captives bond to their captors to survive. In 1995, Graham developed a 49 item scale for use is determining Stockholm syndrome.

Recovery
Recovering from Stockholm syndrome ordinarily involves "psychiatric or psychological counseling", in which the patient is helped to realize that their actions and feelings stemmed from inherent human survival techniques. The process of recovery includes reinstating normalcy into the lives of victims, including helping the victim learn how to decrease their survival-driven behaviors.

Criticism
Diagnostic and Statistical Manual (DSM 5, 2013)
The DSM-5 is widely used as the "classification system for psychological disorders" by the American Psychiatric Association. Stockholm syndrome has not historically appeared in the manual, as many believe it falls under trauma bonding or post-traumatic stress disorder (PTSD) and there is no consensus about the correct clarification. In addition, there is no extensive body of research or consensus to help solve the argument, although before the fifth edition (DSM 5) was released, Stockholm syndrome was under consideration to be included under Disorders of Extreme Stress, Not Otherwise Specified. The work was updated in 2013, but Stockholm syndrome was not present.

Namnyak et al. (2008)
A research group led by Namnyak has found that although there is a lot of media coverage of Stockholm syndrome, there has not been a lot of research into the phenomenon. What little research has been done is often contradictory and does not always agree on what Stockholm syndrome is. The term has grown beyond kidnappings to all definitions of abuse. It stated that there is no clear definition of symptoms to diagnose the syndrome.

FBI law enforcement bulletin (1999)
A 1998 report by the FBI containing over 1,200 hostage incidents found that only 8% of kidnapping victims showed signs of Stockholm syndrome