Whether or not the covid-19 pandemic was responsible for a secondary pandemic of poor mental health is a question posed repeatedly by scientists, the media, and the public. The answer has been elusive, despite a high volume of research—sometimes of inconsistent quality that encourages the cherry picking of findings to match preconceived hypotheses.

The linked systematic review by Sun and colleagues (doi:10.1136/bmj-2022-074224) will help address these issues.1 The authors included only studies that assessed participants’ mental health both before and after the start of the pandemic. This means that the studies were not prone to selection biases related to the pandemic, at least at baseline assessments. The authors included results from an impressive 134 cohorts, mostly from high or upper middle income countries, looking at changes in general mental health, depression, and anxiety.

So, what did we learn? The authors found no evidence in the general (not high risk) population of changes in general mental health, except for a slight deterioration in symptoms of depression. The word “slight” is important here. Sun and colleagues used a metric called the standardised mean difference, or SMD, and found a deterioration in depression symptoms of 0.12 SMD after the onset of the pandemic. Formally, this means a deterioration of 0.12 standard deviations. Cohen, who invented the metric, developed it for randomised trials and estimated that SMD values less than 0.2 indicated a minimal effect, 0.2-0.5 a small effect, 0.5-0.8 a moderate effect, and 0.8 or more a large effect.2 This may be too simplistic, however, and values between 0.24 and 0.5 have generally been suggested to correspond to a minimal clinically relevant difference for trials in people with depression.3 Whether these cut-offs can be directly applied to general population studies, such as those in Sun and colleagues’ systematic review, is not yet clear.