The question, "Why Do Adults Stay In Abusive Relationships?" is somewhat complex to understand. Partners in abusive relationships have varying reasons for remaining in them.
A first layer of the reasons for staying in an abusive relationship is practical, even if they are not always rational. Some abused people feel they cannot leave their relationships because they are economically dependent on them. For instance, an abused stay-at-home mother may feel that she cannot leave her abusive relationship because if she did, she would have no way of providing for her children.
Other abused people stay because they believe that is the proper thing to do, given their religious or cultural background. Some practicing religious people, for example, believe that divorce is a bad thing to be avoided at most all costs. They may be motivated to put up with a lot of spousal abuse because the alternative is to go against the teachings of their faith.
Still other abused people may rationalize staying in abusive relationships because they think it is the right thing to do for their children. They might say to themselves, "If it was just me, I'd leave this marriage, but my children will be better off coming from an intact home than from a divorced one". This may not be a rational position to take in all cases; the children may be in fact far more damaged by staying in proximity to an abusive father than they would be by being raised by a single mother. However, regardless of the truth of any of these rationalizations, the believe that they are true is more powerful than whether or not they are really true.
A second layer of reasons for why people stay in abusive relationships is uncovered by learning about the so-called "cycle of abuse." In a typical instance of domestic abuse (where one partner is abusive towards the other), abuse tends to occur periodically (cyclically), rather than constantly (all the time). There is no clear beginning to the cycle of abuse, but for purposes of describing it, we can start at an arbitrary stage along its progression. Something event occurs, whether real or only imagined by the abuser, that generates feelings of anger or even rage. These feelings then lead to the second stage of the cycle, which is where the actual abusive behavior occurs. Such behavior may be verbal, physical, emotional/mental, or sexual in nature. If the cycle stopped here and stayed constant, most victims would find it very easy to leave and not endure abuse for long periods of time. However, shortly after the abusive event occurs, the abuser frequently expresses remorse or guilt and wants to apologize. The abuser will swear, "It will never happen again" and may shower the victim with gifts and demands that the victim forgive him or her. There may be so-called "makeup sex" which can be quite pleasurable and provide the victim with a sense that he or she is valued, and really loved. In a parent/child abusive relationship, guilt over abuse may be expressed as special privileges or gifts for the child victim. Following the guilt and making up stage comes a "honeymoon" or latency period during which things are good for a while between the partners. Inevitably, in truly abusive relationships, the latency period ends with the beginning of another abuse episode; the abuser again feels angry, disrespected or treated poorly in some way and the cycle starts all over again.
Though such cyclical abuse is repetitive and predictable, it is also intermittent, and the rest of the relationship might be perceived as good enough or even loving. In this context, victims often rationalize that they aren't really being abused, that their partner really loves them despite being abusive and that makes it okay, that the abuse really isn't all that bad, and other similar statements. Victims are motivated to generate excuses their abuser, to think of each abuse episode as a "one time" thing (even when it isn't), and to focus on the good aspects of the relationship (particularly those positive things that during the guilt/latency phase of the abuse cycle) and convince themselves that the relationship is really a good one and that everyone has some problems in a relationship, i.e., my partner just occasionally loses his/her temper when really stressed at work, etc. Or for those with poor self-esteem, the rationalizations may be thoughts such as “I don’t deserve any better” or “this is the best relationship I’ve had in my life.”
Victims may have any number of low-self-esteem type beliefs that also keep them paralyzed and willing to accept something that is merely "good enough." They may believe that they will be alone forever if they go out on their own. They may believe that they are so damaged that they would only pick another abusive partner anyway so why not stay with this one? They may believe that they don't deserve any better than to be beaten or raped on a semi-regular basis. Abusers may reinforce this lack of self-worth by saying that abuse is normal, that they are over-reacting, etc.
Victims that do try to break away from abusive partners may find that abuse escalates to dangerous proportions. Abusive partners may stalk victims who try to leave them, beat them severely, or otherwise attempt to control their ability to exit the relationship. If they don't threaten to kill or harm the victim or the children, they may threaten to harm themselves, and by so doing, guilt the victim into feeling sympathy for them and then staying to prevent the threatened suicide from happening.
The combination of internal self-esteem deficit, intermittent actual abuse, makeup sex or other positive attention obtained in the wake of abuse episodes, and escalating threats when the victim tries to get away is enough to convince many victims to stay put. Every time a victim forgives an abuser, that abuser is reinforced for being abusive, and it becomes that much more likely that the abuser will become abusive again in the future. The net effect is that the abuse tends to continue forever until the victim finds the courage to leave or is abused to death (e.g., murdered, in the most serious, violent cases). This truth is frequently lost on both the abuser and the victim, however.