It is in horses and other animals, so there is no reason to suppose it is not a valid construct in those with intellectual disabilities, and certainly no reason to doubt its validity in learning disabilities like dyslexia.

There is quite a bit of research in this area now. Please see this website maintained by my supervisor Dr Regi Alexander: http://personalitydisordersinlearningdisability.com/page4.html; a recent systematic review in this area: http://www.emeraldinsight.com/doi/abs/10.1108/AMHID-04-2014-0007; and an older review article: http://bjp.rcpsych.org/content/182/44/s28

I am curious why you asked this question and wonder if you actually meant to say Intellectual Disabilities rather than Learning Disabilities.  I don't want to assume you feel it might not be a valid construct. Learning Disabilities and personality are presented in many different ways; the same is true for Intellectual Disabilities. Keep in mind that just because a person has certain traits that are not often seen, it is still personality. Thank you for the question.