Paper-marking test for chimpanzee: simple control for social cues.

A series of paper-marking tests, modeled after tests widely applied to humans, was administered to Sarah, a "language"-trained chimpanzee (Pan troglodytes). The test format was simple, economical, and controlled for social cues. The ape successfully made same-similar-different judgments on pictures of familiar objects when up to four questions were presented at the same time. Performance remained satisfactory on same-different judgments of novel pictures of alphabetic characters. Throughout the series of tests, the subject showed a superiority on same judgments.