Parent MLU and children with language disabilities

You have a child with a language disability who has a low mean length of utterance (MLU).
Do you coach the child’s parents to talk to their child with unaltered sentences, full of grammatical input, or suggest that they reduce their utterances to just above the child’s current language level?
Previous research suggests that typically-developing children do best with full grammatical input. In this meta-analysis (257 participants), the authors look at parents' MLU in interactions with their children with disabilities. Results showed that for each subgroup of children with language disabilities other than autism, the association between parent and child MLU was “weakly positive at best”, so not strong enough to impact clinical decision making either way. 
But for the subgroup of 47 children with autism there was a strong association between parent and child MLU. Does this mean that children with autism may have better language when their parents use longer utterances with them? Possibly! And that could mean a lot for clinical practice. Remember, though—correlation does not imply causation. It’s possible that children with autism affect their parents’ language, not the other way around. 

See: Sandbank, M. & Yoder, P. (2016). The Association Between Parental Mean Length of Utterance and Language Outcomes in Children With Disabilities: A Correlational Meta-Analysis. American Journal of Speech-Language Pathology. Advance online publication. doi:10.1044/2015_AJSLP-15-0003.