: Confirmation bias is a tendency of people to interpret information in a way that confirms their expectations. A long recognized phenomenon in human psychology, confirmation bias can distort the results of a study and thus reduce its reliability. While confirmation bias can be avoided by conducting studies blind to treatment groups, this practice is not always used. Surprisingly, this is true of research in animal behaviour, and the extent to which confirmation bias influences research outcomes in this field is rarely investigated. Here we conducted a meta-analysis, using studies on nestmate recognition in ants, to compare the outcomes of studies that were conducted blind with those that were not. Nestmate recognition studies typically perform intra- and inter colony aggression assays, with the a priori expectation that there should be little or no aggression among nestmates. Aggressive interactions between ants can include subtle behaviours such as mandible flaring and recoil, which can be hard to quantify, making these types of assays prone to confirmation bias. Our survey revealed that only 29% of our sample of 79 studies were conducted blind. These studies were more likely to report aggression among nestmates if they were conducted blind (73%) than if they were not (21%). Moreover, we found that the effect size between nestmate and non-nestmate treatment means is significantly lower in experiments conducted blind than those in which colony identity is known (1.38 versus 2.76). We discuss the implications of the impact of confirmation bias for research that attempts to obtain quantitative synthesises of data from different studies.