Rats are highly social animals whose welfare in laboratory settings depends critically on appropriate social housing, and single housing causes documented psychological harm that also affects research validity.
Social isolation is one of the most significant and preventable welfare harms in laboratory rat research. Rats housed alone lack the social interaction they are strongly motivated to seek, and develop measurable psychological harm that includes anxiety, depression-like states, and physiological stress responses. This matters for welfare and for science: isolated rats produce more variable and stress-confounded data, reducing experimental reproducibility. The case for social housing is therefore both ethical and scientific. Barriers to implementation include concerns about disease transmission, aggression, and experimental confounds from social interaction. These concerns are manageable with appropriate protocols. The default should be social housing with single housing reserved for specific experimental necessities and minimized in duration.