1991'5, p.47
Bulohov V. Ya.
On the psychology of spelling mistakes in words - predicates

Pupils of the 3-10th forms make much more mistakes in words-predicates than in any other words. The greater orthographical vulnerability of words-predicates can be explained by two psychological reasons: 1) the "stronger" component of the sentence (inner speech which is word-predicate) in writing turns out to be more "weak" and as a result is satiated with spelling mistakes, 2) predicates are more often situated in the postposition (more difficult for the operative memory part of the simple sentence) which also weakens the orthographical "immunity" of words-predicates.