Assessing the validity of ape-human comparisons: a reply to Boesch (2007).

Boesch (2007) criticizes research comparing ape and human cognition on the basis of both internal and external validity. The authors show here that most of those criticisms are not valid because: (i) most threats to internal validity (e.g., conspecific experimenters for humans but not apes) are controlled for experimentally; (ii) externally, there is no empirical evidence that captive apes have fewer cognitive skills than wild apes and indeed some evidence (especially from human-raised apes) that they have more; and (iii) externally, there is no empirical evidence that Western middle-class children have different cognitive skills from other children at very early ages in basic cognitive domains. Although difficult, with appropriate methodological care, experimental cross-species comparisons may be validly made.