"The overwhelming number of incorrect alerts [by drug and/or explosive detection dogs] identified across conditions confirms that handler beliefs affect performance. Further, the directed pattern of alerts in conditions containing a marker compared with the pattern of alerts in the condition with unmarked decoy scent suggests that human influence on handler beliefs affects alerts to a greater degree than dog influence on handler beliefs."
"In conclusion, these findings confirm that handler beliefs affect working dog outcomes, and human indication of scent location affects distribution of alerts more than dog interest in a particular location."
Lit, Lisa; Schweitzer, Julie B.; Oberbauer, Anita M., "Handler beliefs affect scent detection dog outcomes," Animal Cognition (Heidelberg, Germany: January 2011), Volume 8, Number 3, pp. 202 and 204.