Rats are highly social animals that suffer significantly when housed alone in research settings. Social housing dramatically improves welfare without compromising most research.
Rat social isolation in research settings is a widespread and scientifically unjustified welfare harm for the majority of experiments. The evidence for rat sociality and the welfare cost of isolation is robust, while evidence that social housing compromises research is weak for most experimental designs. Transitioning to social housing as the default is an achievable and impactful welfare improvement.