There is much debate as to whether or not Codependency is actually a psychological disease (much less a progressive one).
The Codependency movement was born out of the link between the behaviour of those who suffered from Alcoholism (the Dependent) and the behaviour of those who took care of them. These carers (often seen as enablers) would adapt their behaviour to take care of the Dependent through means such as over protection, assumption of responsibility for financial and emotional obligations, lying to cover up drunken behaviour etc. As a result of providing long term care these carers became overly responsible for their partners and their own mental and physical health disintegrated. The carers were named as "Codependents".
After some time psychologists saw similarities between carers of alcoholics and the carers of those who suffered from compulsive disorders, mental disorders etc. Thus more and more carers were seen to be "Codependents".
Critics such as Katz and Liu feel that the characteristics used to describe a Codependent are so contradictory and all encompassing that they could apply to anyone. They also argue that the Codependency movement seeks to demonstrate that any person who wants to take care of someone else (a friend, family member, partner, spouse or stranger) for whatever reason (alcoholism, gambling, other addictions such as food or sex, mental illness, hospitalisation etc.) over variable periods of time is a "Codependent". That all carers essentially have a problem with control. As such they argue that the Codependency movement has made it almost seem wrong to want to care of others.
They do not feel it is a disease and that most people who are labeled as "Codependent" are in fact normal people who at times struggle with life circumstances, feelings and behaviours. That often in caretaking for others who are unwell they are experience a normal reaction to abnormal people.
In her book Beattie selects quotes from various experts who define Codependency as:
"A emotional, psychological, and behavioural condition that develops as a result of an individual's prolonged exposure to, and practice of, a set of oppressive rules - rules which prevent the open expression of feeling as well as the direct discussion of personal and interpersonal problems" pg. 32 (taken from Robert Subby)
"spouse or child or lover of someone who was chemically dependent was seen as having developed a pattern of coping with life that was not healthy, as a reaction to someone else's drug or alcohol abuse" pg. 33 (taken from Robert Subby and John Friel)
"Those self-defeating, learned behaviours or character defects that result in a diminished capacity to initiate or to participate in loving relationships." pg. 32 (taken from Earnie Larsen)
She then goes on to personally define "A co-dependent person is one who has let another person's behaviour affect him or her, and who is obsessed with controlling that person's behaviour" p.g.36.
Beattie herself doesn't label Codependency as a disease and she doesn't know if it is or is not an illness but that "it can make you sick. And, it can help the people around you stay sick." pg. 38
Many appear to grow up in dysfunctional families, they learn their behaviours through childhood exposure to dependent parents or family members. Children of Alcoholics usually develop codependent patterns of behaving but in general any long term exposure to a parent or family member with a compulsive, addictive or mental disorder can lead to the development of a Codependent.
Some may not have come from dysfunctional backgrounds but have developed a relationship with someone who is compulsive, addictive or suffering from a mental disorder in adulthood. Again long term exposure can result in Codependency.
Their own biology can to some degree dictate their behaviour. Psychologists seek to show that a persons temperament from birth determines how they deal with events in the future. Possibly Codependents have a more difficult time in dealing with negative changes in their life and accepting other people are not capable of providing them with happiness as a result.
Education in society through the school system, repressive religions etc. teaches that to be "good" a person must think of others first. Certain institutions often praise martyrdom. The mass media does it's bit through the portrayal of the women who's love and devotion helps their partner to recover, grow and find happiness. They imply that you should always "stand by your man" regardless of level of suffering you have to go through to get to the good times.
The Codependent has adapted their behaviour in order to get their needs met in a setting where someone they cared about was unable or unwilling to take care of themselves and their own problems. After progressively taking on the care taking role the Codependent somehow forgets to look after their own needs and deal with their own problems. As a result even if they physically break free from the person who is dependent on them they take their Codependency forward into future relationships.
Beattie has produced a very long list of Characteristics on pg. 42 to 52 of her book. They are catergorised in the following groups, Care taking, Low Self-Worth, Repression, Obsession, Controlling, Denial, Dependency, Poor Communication, Weak Boundaries, Lack of Trust, Anger, Sex Problems, Miscellaneous and Progressive. Below is a selection of some of the behaviours and feelings she describes. They outline that Codependents may:
Beattie believes that this behaviour is habitual and self-destructive, that it can "lead us into, or keep us in, destructive relations, relationships that don't work. These behaviours sabotage relationships that may otherwise have worked." pg. 38-37 She also explains that Codependents unconsciously pick troubled partners in order to have purpose, be needed and feel fulfilled.