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Pioneering work into experimenter effects demonstrated that several aspects of the experimenter can have significant influence. Scientists such as Robert Rosenthal laid the groundwork for this understanding, revealing the importance of experimenter expectations in relation to participant performance and, among other things, the importance of experimenter gender (5). Since these initial investigations, the field has grown: From intelligence testing to pain sensitivity, participants demonstrate robust responses to manipulation of experimenter gender (6, 7). The range of effects is troubling because it is broad enough to influence many fields of scientific inquiry that are not accustomed to controlling for experimenter effects.


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Variance in such prominent mental and physical variables could potentially encourage reporting of illusory effects in clinical biomedical trials, inducing potentially serious consequences for patient treatment. For instance, when testing the efficacy of antinociceptive drugs, males report less pain to nociceptive stimulation when supervised by a female experimenter, as demonstrated by Alabas et al. (8). If, when testing an antinociceptive drug, a disproportionate number of treatment trials with male participants are supervised by female experimenters, then this could result in overestimations of drug efficacy. Putting aside the possibility of false positives, false negatives could be holding back progress. If scientists have difficulty replicating findings because of excessive null results, then the resulting noise makes any broader analysis less conclusive and more likely to induce further inquiry and delays. For instance, the collaborators in the Open Science Collaboration unsuccessfully attempted to replicate the findings of Epley et al. (9). The original study had shown that lonely participants were more likely to restore their sense of belonging through increased belief in supernatural agents and events (9). Meanwhile, the replication failed to find significance. Surprisingly, in both the original and the replicated studies, the authors failed to report experimenter gender.

Looking speculatively toward the future of policy changes intended to improve replicability, there are several key players involved in the process that could promote change. For instance, universities or research institutes could take a top-down approach to the issue: It is not uncommon for universities to disseminate policy changes directly to laboratories under their umbrella. Because of the weight that universities have in setting the trajectory of individual scientists and ethical scientific standards, any guidance from them to report experimenter gender could be impactful.

NOTE: If you're interested in transmitting differential signals, please see the sfp-breakout board instead. It's like the SFP experimenter board, but contains no differential transmitter + receivers.

Figure 1. Overview of the experimental design. The behavioral testing was carried out using two batches of eight weeks old C57BL/6J (n = 12) and DBA/2J (n = 12) mice. All the behavioral procedures were applied by a 25-year-old female experimenter (M) and a 49-year-old male experimenter (V) in two different laboratories: the Institute of Anatomy in Zürich (Z) and the Laboratory Animal Center in Helsinki (H). An adaptation phase of four weeks was followed by the elevated O-maze, open field with object, rotarod and Barnes maze tests.

Figure 2. Results of the behavioral battery of tests. (A) Body weight (g) during the behavioral testing (ANOVA: strain F1,88 = 57.46, p p post-hoc test: *p p (B) Lingering (m/s) defined as sum of resting and any deceleration and walking velocity (m/s) in the open field (ANOVA: strain F1,88 = 74.48, p p post-hoc test: ***p (C) Tortuosity index defined as sum of unsigned direction changes divided by total distance moved in the openfield (ANOVA: strain F1,88 = 36.52, p post-hoc test: ***p p (D) Time to fall (s) as measure of motor learning and coordination ability in the rotarod (ANOVA: strain F1,88 = 18.44, p p p post-hoc test: *p p p

Figure 3. Results of the behavioral battery of tests. (A) Preference for sector (%) in the elevated O-maze (ANOVA: strain sector F1,184 = 28.02, p post-hoc-test: p (B) Object scanning (m/min) as measure of exploratory activity in the open field with object (ANOVA: strain F1,88 = 48.45, p post-hoc-test: *p p p (C) Percentage of time (%) state as measure of the motor profile in the open field (ANOVA: strain state F1,184 = 12.79, p = 0.0004, ω2 = 0.06). DBA/2J mice displayed higher resting time percentage and lower percentage of walking time in the open field. This was detected in 2 of 4 individual experiments: missed in MZ and MH (post-hoc test: p p (D) Distance moved (m) as measure of spatial learning abilities in the Barnes maze (ANOVA: strain F1,88 = 51.44, p p post-hoc test: *p p

Figure 6. Results showing the experimenter contribution to overall variance. Behavioral measures assigned to 3 sections according to strain effect in pseudo population: evidence for, inconclusive, evidence against (false positives). Based on partial omega squared of the interaction, 30 variable subsets with overall strain effect and large vs small person strain interactions, were extracted. A modest enrichment of anxiety measures in the subset with large person strain interaction is observed. Additionally, motor related measures seem relatively resistant against person strain interaction.

Now, an international team of pain researchers led by scientists at McGill University in Montreal may have uncovered one important factor behind this vexing problem: the gender of the experimenters has a big impact on the stress levels of rodents, which are widely used in preclinical studies.

In research published online April 28 in Nature Methods, the scientists report that the presence of male experimenters produced a stress response in mice and rats equivalent to that caused by restraining the rodents for 15 minutes in a tube or forcing them to swim for three minutes. This stress-induced reaction made mice and rats of both sexes less sensitive to pain.

The research team, which included pain experts from Haverford College and the Karolinska Institutet in Sweden and a chemosensory expert from Université de Montreal, found that the effect of male experimenters on the rodents' stress levels was due to smell. This was shown by placing cotton T shirts, worn the previous night by male or female experimenters, alongside the mice; the effects were identical to those caused by the presence of the experimenters, themselves.

"Our findings suggest that one major reason for lack of replication of animal studies is the gender of the experimenter -- a factor that's not currently stated in the methods sections of published papers," says Robert Sorge, a psychology professor at the University of Alabama, Birmingham. Sorge led the study as a postdoctoral fellow at McGill.

The good news, Mogil says, is that "the problem is easily solved by simple changes to experimental procedures. For example, since the effect of males' presence diminishes over time, the male experimenter can stay in the room with the animals before starting testing. At the very least, published papers should state the gender of the experimenter who performed the behavioral testing."

A particular type of experimenter effect in which the expectations of the experimenter as to the likely outcome of the experiment acts as a self-fulfilling prophecy, biasing the results in the direction of the expectation. In a series of classic experiments carried out by the German-born US psychologist Robert Rosenthal (born 1933) in the early 1960s, the effect was induced by telling experimenters what kind of behaviour to expect from laboratory animals. In one experiment published in 1963, 12 psychology students were each given five laboratory rats of the same strain, but six were told that the rats had been bred for maze brightness and the other six that they had been bred for maze dullness. The students were given the assignment of running the rats in a maze-learning experiment, and the rats believed to be bright performed significantly better than those believed to be dull. Later research suggested that in such experiments the effect is due to subtle differences in the way experimenters handle the animals. Also called the Rosenthal effect. See also demand characteristics, Oedipus effect, Pygmalion effect. 041b061a72


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