I agree with the observation that reinforcing behaviors are deliberate. My wife and I discussed exactly how to handle these issues on an ongoing basis. It was also my habit to spend a lot of time with both daughters, walking around and observing things, pointing out, explaining, and reading every street sign or anything else that presented an opportunity. Language is incredibly important. Beyond this is the part where I disagree -- "short sentences" are *not* obvious. I actually have clear memories of learning to speak and read, and I *hated* it when people spoke down to me. So I did not do that with my daughters. No baby talk. Use complete sentences. Use the correct word, even if it is not common vocabulary, etc. Of course, if my kids had not been capable of learning from this I am sure I would have reverted to tradition, but that did not happen. Both began speaking very early, and learned rapidly.
I don't recall specifics around 18 months, but I do know that my older daughter was reading well before she turned 3, because we read together every night and I would write the date that she first read a book on her own inside the front cover. I remember the exact book that convinced me that this wasn't just a good memory: http://www.amazon.com/dp/0679832696/?tag=googhydr-20&hvadid=3878470371&ref=pd_sl_17n0twlwjr_e#reader_0679832696 (P.J. FunnyBunny Camps Out). She read that out loud to me about 3 months before her third birthday, without any help from me, and without anyone having read it to her first. After a year of preschool at age 3, we were convinced that a second year of it would be a waste of time, especially when she would recognize misspellings on paperwork the administrator was sending home to parents. We found a private kindergarten that would take her if she passed an interview and made it through a class (this while she was still 3, in order to register for the fall classes that would start a few weeks before she turned 4). During her class time, she took to walking around the class to help the other kids with their work, including writing names for those kids that did not know how to write their own yet. I remember the kindergarten teacher being shocked that she had correctly spelled "Christopher" for one of those kids.
Daughter two has always been a little more difficult to assess. I can't tell you when she learned to read. Before she was 3, she confirmed with me that she had read the phrases on two signs in a parking lot correctly, after we took her to get glasses to correct for her "lazy eye". This was in the parking lot of the optometrist, when I was putting her into the car seat while her mother was still inside paying the bill. I believe she was just validating that her glasses were working properly. After that, she refused to perform -- would not even read the same signs to her mother. I would read to her every night just like I had with her sister, but she refused to ever read for me. I was convinced that she understood how to do it, she just did not want to. When she went to preschool, we would hear gushing reports from her teachers about how well she read, but never got any demonstration of it at home. Likewise it went, on up through the other grades. She is in second grade now, and I am still reading to her every night and she still refuses to read out loud for me, except when it is a homework assignment. My wife helps out with the school, doing math and reading assessments, and daughter two is in the "advanced readers" group. My wife says she is so far ahead of the other "advanced readers" that it is like night and day. She does read voraciously on her own now, having just completed several of the Harry Potter novels. I had recently read a few of the "Illustrated Classics" versions of some of Jack London's books to her, including White Fang and Call of the Wild. Tonight she told me she doesn't like the simplified language, and asked me if she could read the "real books". I pointed her to my collection of Jack London in our family library, and also the Jules Verne, since she is interested in that too. I believe she's about on pace with where I was at, at that age. I distinctly remember checking White Fang out from the school library when I was in second grade, loving it, then reading everything I could by Jack London from the public library, including my favorite, "The Sea Wolf".
I do think this is one area that is really difficult to research meaningfully. I think a child's brain is so amazingly good at learning that a whole variety of techniques will be successful, and that differences come down to individual preferences on the part of the child as much as anything else. There are far too many uncontrolled and uncontrollable variables involved to draw any absolute conclusions.