I am pretty sure that under the standard battery of IQ tests, jats very like other strata of population will have a score much in comparison with other "castes". I do not think that intelligence is the problem.
I believe to do well in a competition, you have to be against so many other people who not only are atleast of your own intelligence, but also focussed and hard working. It is like an intellectual war, where along with intelligence, what counts is hard work, motivation and perseverance.
If there is a particular slab of population (e.g. if you define it by caste as in this case Jats) which is performing under par, it indicates to a condition like attitude towards these exams which leads to factors like motivation and hard work being under par relative to other slabs of populations which are performing well.
The upbringing environement of a child/ student, motivation not only from parents but also peers and general social circle around him/ her are very important. With lack of motivation, is lack of hard work. Definitely motivation will depend on factors within the same social circle. For e.g. a student say in Bihar might have a motivating factor of his chacha, bhai etc clearing the same exam, this will give him confidence and motivation to repeat a similar feat.
Some people argue about the "luck" or destiny factor, which no doubt is important. However it is more valid for individual cases, for a whole slab of population, it is not important, because it is balanced out....(e.g. no one can say that all jats do not do well because they are unlucky!!)
So with intelligence and luck factors ruled out, the clear writing on the wall is Hard Work!! Right motivation and right channelization of energy is the only way out.
I believe that more or less factors like schools, teachers in these schools etc are very important. However, arugably if students in Bihar can clear such exams easily, it shows that these factors, though improtant are not excluding. Individual effort and motivation is finally what pays off, not only in competition but in all spheres of life.
Just to emphasize this point, I have a very short example. While in my 4th yr in DCE in Delhi, I had many friends in IIT too. Due to involvement in many technical projects in IIT, I was constantly in IIT campus, sometimes for days at a stretch and thus made friends there very much like in DCE itself. Both these colleges are ranked among top ten in engineering with intelligent, motivated and focussed students. Now many people were preparing for GRE (MS or Phd abroad), many for CAT (for IIM's and other MBA institutes). The trend was more towards GRE in IIT and CAT in DCE. I know many people in DCE working very very hard with themotivation being mutual discussion and competition. This spirit was lacking in IIT towards CAT, though many people were giving the exam.
After the results were out, 74 people from DCE got calls from IIM's that year and 13 from IIT. It was remarkable. (TOI, Indianexpress etc. termed DCE to have the highest density of students with extremely high scores in CAT). 27 were from just Mechanical branch itself!! more than double than the whole IIT!!
What I wish to stress is that the IITians have both the intelligence and means to do well as the DCE'ites, however the difference was motivation. Similarly GRE was comparably better in IIT than DCE.
cheers,
~vivek