What the research is showing (and the reason behind the significant increase in expectations of the Common Core State Standards) is that students are not improving their ability to tackle complex text. 

In fact, research has proven that the complexity of text has DECREASED over the past ten years.  The typical response to “this text is too tough” is “give them easier text”…what we are learning through recent studies is that students are learning to tackle tough text by tackling tough text…perhaps we are giving up on students’ ability (or lack thereof) to tackle that complex text. 

Bottom line?  We are not going to teach students to tackle complex text unless we teacher them to tackle complex text.