"The overwhelming number of incorrect alerts 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. That is, total number of alerts identified by handlers did not differ across conditions. However, distribution of these alerts did differ across conditions; more alerts were identified on target locations indicated by human suggestion (paper marker) than on locations indicated by increased dog interest (hidden sausage and tennis balls)."
Lit, L., Schweitzer, J.B. & Oberbauer, A.M. Handler beliefs affect scent detection dog outcomes. Anim Cogn 14, 387–394 (2011). doi.org/10.1007/s10071-010-0373-2