Dr Dabrowska and research student James Street then tested a range of adults, some of whom were postgraduate students, and others who had left school at the age of 16. All participants were asked to identify the meaning of a number of simple active and passive sentences, as well as sentences which contained the universal qualifier "every."
As the test progressed, the two groups performed very differently. A high proportion of those who had left school at 16 began to make mistakes. Some speakers were not able to perform any better than chance, scoring no better than if they had been guessing.After looking at things like working memory and testing ability, which turned out to have no influence, they taught the grammar to the bad speakers, and they picked it up quickly. "(The researcher) speculates that this could be because their attention was not drawn to sentence construction by parents or teachers when they were children."
The authors point out that this means public information should be written without passive sentences, not because it's weak writing but because some people just won't understand it.
It also means we should be teaching grammar in school, which I guess we don't do anymore. I have fond memories of 8th grade English, were I learned to diagram sentences. I still do it in my head when I come across a complicated sentence.