Despite the prevalence of the ethnic gap in attainment in medicine, medical educationalists have struggled to explain it, and there is scant evidence to support the use of any practical measures to ameliorate it. Some researchers have suggested the effect may be partially due to subtle linguistic differences between candidates and examiners [4,14]; however that does not explain differences on machine-marked written assessments [1-3]. Only a small part of the ethnic disparity in medical students can be explained in terms of prior educational underachievement or differences in other background variables [19].

Social psychologists in America have proposed that people from ethnic minority groups underachieve academically due to a psychological phenomenon called stereotype threat [20,21]. According to stereotype threat theory, in test situations members of negatively-stereotyped groups (e.g. black students) can feel sufficient anxiety at the prospect of fulfilling a negative stereotype about their group that they subsequently underperform (see [21] and [22] for reviews). Although much of the research on stereotype threat has been done with African American students, the negatively stereotyped group does not have to be black for stereotype threat to occur. Stereotype threat has shown to negatively affect general academic performance in Latinos in the USA [23], mathematics scores in women [24] and sporting performance in white (W) men [25].

Evidence suggests that the negative effects of stereotype threat can be reduced by changing individuals' perceptions of themselves, their ability and their potential. [26,27]. In a recent US study [28] psychologists Geoffrey Cohen and his colleagues randomly allocated adolescent white and black students to self-affirmation intervention and control conditions. In the self-affirmation condition students wrote a short reflective piece about a value which was most important to them; in the control condition students wrote a short reflective piece about a value which was not important to them but which might be important to someone else. black students in the intervention condition did significantly better in post-intervention assessments. No change was observed in the white students. The pre-intervention ethnic gap in attainment was thus narrowed by almost 40%. The self-affirmation task was theorised to bolster students' self-esteem and self-worth, thus protecting black students against stereotype threat and improving their grades. White students' lack of improvement was explained by their hypothesised lack of stereotype threat.

The positive effects of self-affirmation have been shown in university students as well as the school children in Cohen et al.'s study [26,27]. It therefore seemed appropriate to attempt to replicate Cohen et al.'s study in the different context of EM underperformance at a UK medical school, where the majority of the EM group is of Asian (Indian, Pakistani or Bangladeshi) ethnicity – a group which has previously been found to underperform in medical school assessments [2,3] – [see Additional File 1 and Additional File 2].

We carried out a prospective cluster randomised controlled trial to assess the effects of including a brief self-affirmation intervention in the medical school curriculum, using high stakes machine-marked written assessments and OSCE (Objective Structured Clinical Examination) assessments as the outcome measures. Our research question was "can a brief self-affirmation task reduce ethnic differences in attainment in medical school examinations?".

Figure ​Figure11 shows the trial profile. Data from 335/352 students were analysed (intervention condition n = 174; control condition n = 161): four students were not on the MBBS course, and 13 were lost to follow up (six with no August examination data and seven with no ethnicity data). All clusters were included in the analyses.

Data were analysed on an intention to treat basis, and we were aware of no important adverse events in the intervention group. The coefficient of intracluster correlation was found to be zero (95% CI: 0.00–0.03). The 95% confidence interval for the design effect was 1.00–1.82, which was smaller than 2 and therefore negligible. All subsequent analyses were therefore undertaken discounting the effects of the cluster or "nested" design. [33]. [see Additional file 1 for the effects of the intervention on the primary outcome measure presented by individual tutor group]. Mean scores with standard deviations for each group are given in Table ​Table33.

The study was designed, as far as possible given the somewhat different context of medical school undergraduates, as a direct replication of the study by Cohen et al., with a clear a priori expectation of an ethnicity by intervention interaction in the same direction. This is indeed what we found on the main outcome measure of the written assessment. The implication being that ethnic differences in performance could in some way be mediated via social perceptions, and as a result might be altered by social interventions, and perhaps indeed by social interventions which are surprisingly minimal.

However, detailed post hoc comparisons of the means of the groups showed that the decrease in the ethnic gap was not due to increased performance of the ethnic minority students as hypothesised, but instead was due to a decreased performance of the white students in the intervention condition. The finding that the intervention reduced white students' performance was completely unexpected. The intervention was designed to build self-confidence and therefore should not have reduced performance in any group. These results also defy interpretation in terms of stereotype threat, particularly as white students generally tend to overperform in assessments [see Additional file 1]. In a further twist, the intervention improved the results of both ethnic groups on the secondary outcome measure of the OSCE.

The study benefited from a strong experimental design and theoretical underpinning – features that medical education research is sometimes accused of lacking [35]. The random allocation of individuals to clusters, and of clusters to conditions, increased confidence in the validity of the results, and ensured that the results were not due to differences on academic, demographic or psychological factors at baseline (as an additional check, baseline academic performance was adjusted for statistically). The results were probably not due to the clustered or "nested" design, as the design effect was calculated as negligible; and Figure 2 in the Additional material shows that the effect on the primary outcome measure was not due to tutor differences [see Additional file 3]. Neither were they likely to be due to demand characteristics [36] as the participants were blinded, and the word analysis provided further evidence that the students completed their exercises as instructed.

The unexpected results may relate to the characteristics of the study population. Most of the ethnic minority participants were Asian Indian, Pakistani or Bangladeshi ("South Asian") medical students, whereas those in the original Cohen et al. study were black African American teenagers. These two populations differ enormously on a great number of factors and it is therefore important to question how much, or indeed whether, stereotype threat applied to the ethnic minority students in this study.

Although pervasive negative stereotypes exist about the intelligence of people from black backgrounds [22,37,38], stereotypes about South Asians in educational contexts are perhaps less well known. Recent qualitative research has shown that a negative stereotype of Asian medical students may exist [39] which is similar to reported stereotypes of South Asian people as hard-working, rote learning, and apparently unwilling to mix with people who are not South Asian.[38,40,41] Moreover, although studies of UK higher education have shown that Asian Indian students tend to have a higher level of attainment at university than other ethnic minority groups, including blacks [17,18], they still has a lower record of achievement than whites throughout higher education, as well as specifically in undergraduate and postgraduate medical education.

This relative underachievement of Asian medical students, together with the existence of the negative stereotype together, mean that the ethnic minority group in this study might reasonably be expected to have suffered from stereotype threat. The degree of stereotype threat they might have been experiencing is however not known and cannot reliably be predicted. Future research could incorporate a measure of implicit stereotype activation both pre- and post-intervention to gain greater insight into the levels of stereotype threat in UK medical students.

The effect of the intervention on OSCE results may partially reflect the format of the examination. Unlike the written examinations, the OSCE is conducted face-to-face with the examiner, and scoring may be influenced by the way in which a candidate comes across both to the examiner and to the patients (simulated or real). Self-affirmations can increase positive feelings towards others such as love and connection [42] so students who reaffirmed their self-worth may have related better to examiners and patients and thus achieved higher scores.

The present study raises serious questions for medical educators (as well as social psychologists). The study was in many ways a success: the intervention was small and the effects were significant. And yet the outcomes were unexpected and difficult to explain. If the effects we had found were the results of a pharmacological or surgical intervention in patients, then a host of questions would have to be answered. We believe they also have to be answered here, not least by further replications with more and better controls, which would enable a meta-analytic review of the effects of this type of intervention on medical students' examination performance. If the examination behaviour of a robust group such as medical students is so sensitive to such tiny interventions then that is something that medical educators have to understand. In a commentary published with the Cohen et al. study, Wilson asked:

"Without the experimental results ... who would have thought that a 15-min exercise would have had such long-lasting effects"? [43]