We investigate how not-at-issue content is integrated into a sentence by
comparing the online attachment of parenthetical relative clauses to that
of better-studied restrictive relative clauses. In an eye-tracking
experiment, we report an early penalty for high attachment in restrictive
relative clauses (as predicted by processing principles such as late
closure or recency) but not for parenthetical relative clauses. These
findings suggest that the processor delays structural integration of
not-at-issue material, even when attachment is disambiguated by verb
agreement. A preliminary or underspecified parse may initially be computed
for parenthetical clauses during comprehension.