We tested four predictions based on the assumption that optional infinitives can be attributed to properties of the input whereby children inappropriately extract non-finite subject-verb sequences (e.g. "the girl run") from larger input utterances (e.g. "Does the girl run?" "Let's watch the girl run"). Thirty children with specific language impairment (SLI) and thirty typically developing children heard novel and familiar verbs that appeared exclusively either in utterances containing non-finite subject-verb sequences or in simple sentences with the verb inflected for third person singular "-s." Subsequent testing showed strong input effects, especially for the SLI group. The results provide support for input-based factors as significant contributors not only to the optional infinitive period in typical development, but also to the especially protracted optional infinitive period seen in SLI.