Reasonable assumptions are not necessarily correct

From Shakespeare The World as Stage by Bill Bryson. Page 32.

Shakespeare’s father is often said (particularly by those who wish to portray William Shakespeare as too deprived of stimulus and education to have written the plays attributed to him) to have been illiterate. Illiteracy was the usual condition in sixteenth-century England, to be sure. According to one estimate at least 70 percent of men and 90 percent of women of the period couldn’t even sign their names. But as one moved up the social scale, literacy rates rose appreciably. Among skilled craftsmen—a category that included John Shakespeare—some 60 percent could read, a clearly respectable proportion.

The conclusion of illiteracy with regard to Shakespeare’s father is based on the knowledge that he signed his surviving papers with a mark. But lots of Elizabethans, particularly those who liked to think themselves busy men, did likewise even when they could read, rather as busy executives might today scribble their initials in the margins of memos. As Samuel Schoenbaum points out, Adrian Quiney, a Stratford contemporary of the Shakespeares, signed all his known Stratford documents with a cross and would certainly be considered illiterate except that we also happen to have an eloquent letter in his own hand written to William Shakespeare in 1598.