Much of  Ajjzen and Fishbein's classic  work in social psych on theory of planned behavior use intentions, based on correlations close to 1.0 between these.

But  rules/restrictions, as well as the wishes of  one's allies and enemies kick in..

Dear H. Serkan.

You ask whether it is meaningful to study intentions rather than behaviors. What follows is a short answer to your polemic and highly debated issue. An answer to your question also depends on what kind of behaviors you are thinking about. In the cognitive and moral domains, for example, is highly important to take into account intentions or reasons that underlie one's effective behavior. It can even be the case that we only grasp the true meaning of certain behaviors when we become aware of reasons behind such behaviors.

Let me start with the cognitive domain. If you are, for example, acquainted with Piaget's theory of cognitive development, you also know that the correctness of a response or solution to a given cognitive problem or Piagetian task has more to do with the reasons behind the response or solution than with the solution or response in itself. Consider the case of a typical Piagetian class-inclusion task: " On this table -- the interviewer (I) says to a typical 5-year-old child ) -- we have 10 flowers, 8 being roses, and 2 daffodils. Here, the I continues -- are there more flowers than daffodils, more daffodils than flowers, or is there 'the same thing' of flowers and daffodils? Why is this so?" As you certainly know the focal child tends to say that on the table there are more flowers than daffodils (child's cognitive performance). At first glance this seems to be a correct answer because there are in fact more flowers (10) than daffodils (2). However, when the focal child is asked to explain why this is so (intentions or cognitive reasons behind his/her performance) the child tends to say that on the table there are more flowers that daffodils because there are many roses (8) and only few daffodils (2). So, what appears to be a correct answer is really an incorrect performance when we take into account the child's reasons or intentions underlying his/her initial answer. Note that for a differential or psychometric psychologist -- who generally does not ask the child to justify his/her answers on a mental test -- the above mentioned child's answer would be correct, even though it was based on incorrect reasoning. This also shows that an answer to your question depends on what theoretical framework one espouse. In other words, for differential or behavioral psychologists behaviors matter more than intentions. Note also that I could present many other cognitive examples where intentions or reasons behind actions matter more than, say, external behaviors.

Let me pass now to the moral domain. Think of the case where, for instance, you hurt physically your brother or sister without intending it. You certainly agree that in this case, your action is not morally wrong because you had no intention to hurt your relative. If you are acquainted with Kohlberg's theory of moral stages and moral development, you also know that it is individuals' reasons or intentions behind their answers on a hypothetical moral dilemma, not their answers in themselves that count for them to be classified as Stage 1, 2, 3, 4, or 5-oriented answers: " Would/would you not steal to save a human life? "Why/Why not?"

Consider the following response. "I would not steal in this situation because I would be caught up and put in jail". This would be a stage 1-oriented action, not because of the course of action to be taken (I would not steal), but because of the intentions behind it (I would be put in jail.). " I would steal in this situation because the life at issue is the life of a famous person." This would be also a stage 1-oriented action, not because of the course of action to be taken (I would steal), but because of the intentions behind it (the life at issue is the life of a famous person)..

Consider the following response. "I would not steal in this situation because if I stole, I would have problems with the police." This would be a stage 2-oriented action, not because of the course of action to be taken (I would not steal), but because of the intentions behind it (to have problems with the police).

Note now the following answer " I would steal in this situation because I want to appear to others as a decent and nice guy."This would be a stage 3-oriented action, not because of the course of action to be taken (I would steal), but because of the intentions behind it (to appear as a decent guy).

Note now the following answer " I would not steal in this situation because I have to respect others' right to their property. This would be a stage 4-oriented action, not because of the course of action to be taken (I would not steal), but because of the intentions behind it ( to respect others' right to their property)

Note now the following answer " I would steal in this situations because the right to live is priori to one's right to property." This would be a stage 5 or 6-oriented action,  not because of the course of action to be taken (I would steal), but because of the intentions behind it (the right to live is priori to one's right to property). I say a stage 5 or 6-oriented action because the late Kohlberg gave up his stage 6 as a hard structural stage, not as a moral ideal or telos of one's moral development. 

As the  foregoing considerations show there are occasions where it is meaningful to   study intentions rather than behaviors. However, this also depends upon the theoretical framework we espouse and the domain of knowledge at issue. Even so, I also admit that on some occasions it may be more meaningful to study behaviors rather than intentions.

I hope that I have got your questions and that this helps.

Best regards,

Orlando

YES, it is definitely possible to study intentions independently of acts/behaviors through a Libetus Interruptus (Libet's Interruption) paradigm (see Schurger, Sitt, and Dehaene, 2012). As shown by the Libet-type experiments done using EEG, fMRI, and microelectrode recordings, an unnoticeable delay of around 200 ms occur between intention- or decision-making and the act/behavior itself. What the participant performing the interruptus paradigm/task does pertains to withholding his/her motor movements (i.e., flexing the index finger) when he/she felt an urge to make the motor act; this amounts to exercising Libet's "conscious veto" (see my in press theoretical review, Zhong, 2017, click on the link below or click on my name).

Article What does neuroscience research tell us about human consciou...

Yes. Certainly!

If you want to assess the behavior then you have to go for that. Intention is just prior to behavior not all intentions can be reflected in behaviors because in between then external factors can block them. Go through a book please for clear understanding about TPB .

Another thing is that it's like steps of decision-making towards behavior. If you cut one step then you can not get your appropriate outcome sure.

The two should be studied. But most of what is studied is apparent behavior. As for intentions and internal things, it is very difficult to measure them.

Yes, the two should be studied. A possible way to study the intentions behind a certain overt behavior is to perform a semi-structured interview in which the beholder is solicited to indicate why s/he behaved as s/he did. Jean Piaget was a prominent psychologist that is known for his amazing ability to access to the underlying processes of an overt intellectual behavior.

You can see only decision-making that's might be enough for the study. Lots of study on intention only. Not necessary always to see the behavior.