Motivational internalism holds that if a person sincerely believes that X is wrong, then he is motivated not to do X. On some versions of internalism, this motivation may be overridden by other concerns, but it is always there. In other versions, the agent with a sincere belief that X is wrong will not do X.
Desirism holds that an act is wrong if a person with good desires would not perform that act - where good desires are those that people have reason to promote through social tools such as praise and condemnation. It is possible that a person can sincerely believe that people have many and strong reasons to condemn somebody who does X, but still shrug his shoulders with indifference at the fact.
Consequently, desirism is inconsistent with motivational internalism.
The justification for motivational internalism comes from the observation that we never see a person change their moral judgment without changing their behavior. A person who changes her mind on capital punishment - who goes from thinking that capital punishment is obligatory to thinking that it is wrong - is expected to undergo a change in behavior as well. In fact, she is required to undergo a change in behavior - otherwise we doubt that she has actually changed her mind. At best, we may look for signs that she is pretending that she still holds her former views - perhaps to keep from offending former friends.
Motivational internalism explains this my making motivation a part of the meaning of moral terms. A person who says 'X is wrong' when not motivated to refrain from doing X has not learned to use moral terms correctly.
So, how can desirism explain the observations that seem to support internalism - the near perfect match between moral judgment and motivation?
First, when we are talking about something that people generally have many and strong reasons to condemn, we are almost always talking about something that the agent has many and strong reasons to condemn. The hit man still has reasons to encourage society to use condemnation to build in people generally an aversion to killing. The rapist who has any female friends whatsoever (or male friends who themselves have female friends) has reasons to promote condemnation of those who force sex on women without consent. He more generally has reason to promote an aversion to people forcing themselves on others - insofar as he has reason not to have others force their interests on him. We all have reasons to promote a general aversion to lying or to taking the property of others without consent.
Second, many of us have a desire to do the right thing. One way to get a person to refrain from lying is to give him an aversion to lying. Another way is to give him an aversion to doing that which is wrong and a belief that lying is wrong. We use both methods. If "wrong" means "that which a person with good desires would not do", then our claim of wrongness can actually be true. Giving him a belief that lying is wrong is not sufficient to motivate him. However, it will motivate anybody with an aversion to doing that which is wrong - and the vast majority of us have that aversion.
Third, people generally have reason to avoid condemnation and punishment. Desirism identifies what is wrong with what people have reason to condemn or punish. Condemnation and punishment play on the reward system - they are inherently things that those condemned or punished have reasons to avoid. You cannot punish a kid by denying him permission to watch a television show he would never want to watch. Consequently, even the person who lacks the desires of a good person, including an aversion to doing that which is wrong, probably has reasons to avoid condemnation and punishment.
Fourth, humans are great rationalizers. They tend to adopt propositions that they want to believe are true. Being motivated to perform an action implies being motivated to believe that the state that the action brings about or the act itself is good. This brings about a correspondence between what a person is motivated to do and what he is motivated to believe that does not depend on any type of necessary connection between the two.
These elements point to a pernicious implication of internalism - it's use in deflecting guilt.
Under intenalism, 'I have an obligation to do X' implies 'I am motivated to do X'
This invites people to respond to a claim that they have an obligation to perform a particular act by seeking within themselves a motivation to perform that act. They are tempted to respond, 'I am not motivated to do X' implies 'I have no obligation to do X'.
Technically, the internalist is claiming that the person would be motivated if they knew all of the relevant facts. If the agent is not yet motivated, this can be explained by claiming that the agent does not recognize all of the relevant facts.
Still, the motivational internalist would have to admit that if the agent knew all of the relevant facts - as the agent claims, and the agent did not feel motivated to perform the action, then the agent must not have an obligation to perform the act. The person who knew all there is to know about a poor and starving community and was still indifferent to their fate would not actually have any moral obligation to help them. This further means that if a person can cultivate an indifference to the plight of the poor in the face of knowing all of the facts, then he can rid himself of any obligation to help the poor as well.
Desirism holds that claim that one has an obligation would still be true, even if the agent had no interest in living up to that obligation. Desirism would says, "It may be the case that you do not care, but you should care. By this we mean that people generally have many and strong reasons to make people care by praising those who care and condemning those who do not."
In other words, with desirism, moral claims are not concerned with what an agent is motivated to do or would do if he was fully informed. It concerns what he should be motivated to do - what people have reason to cause him to be motivated to do - and what they may employ social tools such as praise and condemnation to promote.
Motivational internalism has a lot of people measuring what they should and should not do by measuring what the want to do or not do. They are concluding, in effect, "I am obligated to do what I want to do - what I find myself motivated to do - and prohibited from doing what I have no interest in doing."
This is a very convenient moral theory. This also probably makes a major contribution to our observation that changes in behavior track changes in moral judgments. However, the fact that people are doing this does not imply that the behavior is in any way justified. It also does not change the fact that people generally have many and strong reasons to use praise and condemnation to modify the motivations of others independently of what those agents are currently motivated to do.