IN THE early 20th century, a horse named Clever Hans was believed capable of counting and other impressive mental tasks. After years of great performance, psychologists put the ruse to rest by demonstrating that though Hans was certainly clever, he was not clever in the way that everyone expected. The horse was cunningly picking up on tiny, unintentional bodily and facial cues given out not only by his trainer, but also by the audience. Aware of the “Clever Hans” effect, Lisa Lit at the University of California, Davis, and her colleagues, wondered whether the beliefs of professional dog handlers might similarly affect the outcomes of searches for drugs and explosives. Remarkably, Dr Lit found, they do.

Dr Lit asked 18 professional dog handlers and their mutts to complete two sets of four brief searches. Thirteen of those who participated worked in drug detection, three in explosives detection, and two worked in both. The dogs had been trained to use one of two signals to indicate to their handlers that they had detected something. Some would bark, others would sit.

The experimental searches took places in the rooms of a church, and each team of dog and human had five minutes allocated to each of the eight searches. Before the searches, the handlers were informed that some of the search areas might contain up to three target scents, and also that in two cases those scents would be marked by pieces of red paper.

What the handlers were not told was that two of the targets contained decoy scents, in the form of unwrapped, hidden sausages, to encourage the dogs' interest in a false location. Moreover, none of the search areas contained the scents of either drugs or explosives. Any “detections” made by the teams thus had to be false. Recorders, who were blind to the study, noted where handlers indicated that their dogs had raised alerts.

The findings, which Dr Lit reports in Animal Cognition, reveal that of 144 searches, only 21 were clean (no alerts). All the others raised one alert or more. In total, the teams raised 225 alerts, all of them false. While the sheer number of false alerts struck Dr Lit as fascinating, it was where they took place that was of greatest interest.

When handlers could see a red piece of paper, allegedly marking a location of interest, they were much more likely to say that their dogs signalled an alert. Indeed, in the two rooms where red paper was present and sausages were not, 32 of a possible 36 alerts were raised. In the two where both red paper and sausages were present that figure was 30–not significantly different. In contrast, in search areas where a sausage was hidden but no red piece of paper was there for handlers to see, it was only 17.

The dogs, in other words, were distracted only about half the time by the stimulus aimed at them. The human handlers were not only distracted on almost every occasion by the stimulus aimed at them, but also transmitted that distraction to their animals–who responded accordingly. To mix metaphors, the dogs were crying “wolf” at the unconscious behest of their handlers.

How much that matters in the real world is unclear. But it might. If a handler, for example, unconsciously “profiled” people being sniffed by a drug- or explosive-detecting dog at an airport, false positives could abound. That is not only bad for innocent travellers, but might distract the team from catching the guilty. Handlers' expectations may be stopping sniffer dogs doing their jobs properly.