The motto of the European Union, “united in diversity” signifies how Europeans from many different cultures, traditions and languages have come together to work for peace and prosperity. It is the leading example of the central sense of the term as the situation resulting from many different types of people being included in a whole. More generally, the term may refer to the inclusion of people with different ethnic heritages, socio-economic origins, genders, sexual orientations, geographical locations, demographic characteristics or other socio-cultural markers of difference. Diversity may be a consideration in considering both research participation and research teams.

Where the aims of a research project include drawing conclusions from a sample that will be relevant for other populations, evidence of the extent and nature of diversity in the sample population will be an important consideration. This will require recognition that traditional inclusion criteria have often excluded children, women, women of child-bearing age, prisoners, undocumented immigrants and people with physical, intellectual or emotional disabilities (Mertens and Ginsberg 2009). In some circumstances, of course, diversity will not be essential, as in case control studies, or where the focus is on problems, like thalassemia or sickle cell anaemia, that only affect specific population groups.

Diversity may also be a consideration in determining whether a research team is appropriately constituted for the task, particularly where it is necessary to engage with minority communities or other groups who may be underrepresented within the researcher workforce.