Collective intelligence refers to the ability of groups to outperform individual decision makers when solving complex cognitive problems. Despite its potential to revolutionize decision making in a wide range of domains, including medical, economic, and political decision making, at present, little is known about the conditions underlying collective intelligence in real-world contexts. We here focus on two key areas of medical diagnostics, breast and skin cancer detection. Using a simulation study that draws on large real-world datasets, involving more than 140 doctors making more than 20,000 diagnoses, we investigate when combining the independent judgments of multiple doctors outperforms the best doctor in a group. We find that similarity in diagnostic accuracy is a key condition for collective intelligence: Aggregating the independent judgments of doctors outperforms the best doctor in a group whenever the diagnostic accuracy of doctors is relatively similar, but not when doctors' diagnostic accuracy differs too much. This intriguingly simple result is highly robust and holds across different group sizes, performance levels of the best doctor, and collective intelligence rules. The enabling role of similarity, in turn, is explained by its systematic effects on the number of correct and incorrect decisions of the best doctor that are overruled by the collective. By identifying a key factor underlying collective intelligence in two important real-world contexts, our findings pave the way for innovative and more effective approaches to complex real-world decision making, and to the scientific analyses of those approaches.collective intelligence | groups | medical diagnostics | dermatology | mammography C ollective intelligence, that is, the ability of groups to outperform individual decision makers when solving complex cognitive problems, is a powerful approach for boosting decision accuracy (1-7). However, despite its potential to boost accuracy in a wide range of contexts, including lie detection, political forecasting, investment decisions, and medical decision making (8-14), little is known about the conditions that underlie the emergence of collective intelligence in real-world domains. Which features of decision makers and decision contexts favor the emergence of collective intelligence? Which decision-making rules permit this potential to be harnessed? We here provide answers to these important questions in the domain of medical diagnostics.Our work builds on recent findings on combining decisions, a research paradigm known as "two heads better than one" (15)(16)(17)(18)(19)(20). In their seminal study, Bahrami et al. (15) showed that two individuals permitted to communicate freely while engaging in a visual perception task, achieved better results than the better of the two did alone. Koriat (17) subsequently demonstrated that this collective intelligence effect also emerges in the absence of communication when the "maximum-confidence slating algorithm" (hereafter called confidence rule) is used and th...