Reasons for differences in the effect sizes of studies on complex interventions, such as assertive outreach, between the US and the UK are a much debated topic. One possible explanation was suggested to be the potentially different quality of standard care in the two countries.

We aimed to

• (i) empirically establish the comparability of research results on complex interventions for people with severe mental illness (SMI) from the UK and the US, and

• (ii) explore developments over time in standard care in both countries by comparing studies that use “treatment as usual” (TAU) as the control intervention.

We conducted a systematic review and meta-analysis of RCTs conducted in the UK or the US

• (i) involving people with SMI,

• (ii) comparing complex interventions with TAU, and

• (iii) using the outcome relapse or readmission to hospital.

The Risk Ratios for relapse/readmission were very similar and favouring experimental treatment both in the UK (RR 0.80, CI 0.73–0.88) and the US (RR 0.87, CI 0.79–0.95). The development of effects resulting from experimental interventions relative to those from TAU over time shows a slightly different pattern for the two countries.

The broadly similar total RR for relapse/readmission in both countries confirms the comparability of studies conducted in the UK and the US and suggests no significant overall difference in the quality of standard care. The chronological development of effects, however, reflects developments in TAU over time which differ between the two countries.