Some explanations for this pattern posit that differences in the syntactic usages of Desire and Belief verbs underlie the observed developmental lag of the latter (de Villiers, 2005;Pascual et al., 2008).
desire verbs occur mostly with an infinitival SC (as in I want (her) to leave), while Belief verbs occur mostly with a finite SC (a full tensed embedded clause, as in I think (that) she left).